diff --git a/Documentation/config.txt b/Documentation/config.txt
index ed59853190..5f4d7939ed 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2030,6 +2030,10 @@ receive.updateserverinfo::
 	If set to true, git-receive-pack will run git-update-server-info
 	after receiving data from git-push and updating refs.
 
+receive.shallowupdate::
+	If set to true, .git/shallow can be updated when new refs
+	require new shallow roots. Otherwise those refs are rejected.
+
 remote.pushdefault::
 	The remote to push to by default.  Overrides
 	`branch.<name>.remote` for all branches, and is overridden by
diff --git a/Documentation/fetch-options.txt b/Documentation/fetch-options.txt
index f0ef7d02a5..92c68c3fda 100644
--- a/Documentation/fetch-options.txt
+++ b/Documentation/fetch-options.txt
@@ -14,8 +14,18 @@
 	branch history. Tags for the deepened commits are not fetched.
 
 --unshallow::
-	Convert a shallow repository to a complete one, removing all
-	the limitations imposed by shallow repositories.
+	If the source repository is complete, convert a shallow
+	repository to a complete one, removing all the limitations
+	imposed by shallow repositories.
++
+If the source repository is shallow, fetch as much as possible so that
+the current repository has the same history as the source repository.
+
+--update-shallow::
+	By default when fetching from a shallow repository,
+	`git fetch` refuses refs that require updating
+	.git/shallow. This option updates .git/shallow and accept such
+	refs.
 
 ifndef::git-pull[]
 --dry-run::
diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index 450f158779..49878570bf 100644
--- a/Documentation/git-clone.txt
+++ b/Documentation/git-clone.txt
@@ -181,12 +181,7 @@ objects from the source repository into a pack in the cloned repository.
 
 --depth <depth>::
 	Create a 'shallow' clone with a history truncated to the
-	specified number of revisions.  A shallow repository has a
-	number of limitations (you cannot clone or fetch from
-	it, nor push from nor into it), but is adequate if you
-	are only interested in the recent history of a large project
-	with a long history, and would want to send in fixes
-	as patches.
+	specified number of revisions.
 
 --[no-]single-branch::
 	Clone only the history leading to the tip of a single branch,
diff --git a/Documentation/git-prune.txt b/Documentation/git-prune.txt
index bf824108c1..058ac0dc85 100644
--- a/Documentation/git-prune.txt
+++ b/Documentation/git-prune.txt
@@ -24,6 +24,8 @@ objects unreachable from any of these head objects from the object database.
 In addition, it
 prunes the unpacked objects that are also found in packs by
 running 'git prune-packed'.
+It also removes entries from .git/shallow that are not reachable by
+any ref.
 
 Note that unreachable, packed objects will remain.  If this is
 not desired, see linkgit:git-repack[1].
diff --git a/Documentation/gitremote-helpers.txt b/Documentation/gitremote-helpers.txt
index f1f4ca9727..c2908db763 100644
--- a/Documentation/gitremote-helpers.txt
+++ b/Documentation/gitremote-helpers.txt
@@ -437,6 +437,13 @@ set by Git if the remote helper has the 'option' capability.
 'option check-connectivity' \{'true'|'false'\}::
 	Request the helper to check connectivity of a clone.
 
+'option cloning \{'true'|'false'\}::
+	Notify the helper this is a clone request (i.e. the current
+	repository is guaranteed empty).
+
+'option update-shallow \{'true'|'false'\}::
+	Allow to extend .git/shallow if the new refs require it.
+
 SEE ALSO
 --------
 linkgit:git-remote[1]
diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index b898e97988..c73b62f5e1 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -161,6 +161,7 @@ MUST peel the ref if it's an annotated tag.
 
 ----
   advertised-refs  =  (no-refs / list-of-refs)
+		      *shallow
 		      flush-pkt
 
   no-refs          =  PKT-LINE(zero-id SP "capabilities^{}"
@@ -174,6 +175,8 @@ MUST peel the ref if it's an annotated tag.
   other-tip        =  obj-id SP refname LF
   other-peeled     =  obj-id SP refname "^{}" LF
 
+  shallow          =  PKT-LINE("shallow" SP obj-id)
+
   capability-list  =  capability *(SP capability)
   capability       =  1*(LC_ALPHA / DIGIT / "-" / "_")
   LC_ALPHA         =  %x61-7A
@@ -461,7 +464,9 @@ contain all the objects that the server will need to complete the new
 references.
 
 ----
-  update-request    =  command-list [pack-file]
+  update-request    =  *shallow command-list [pack-file]
+
+  shallow           =  PKT-LINE("shallow" SP obj-id)
 
   command-list      =  PKT-LINE(command NUL capability-list LF)
 		       *PKT-LINE(command LF)
diff --git a/builtin/clone.c b/builtin/clone.c
index f98f52980d..43e772ccdb 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -252,6 +252,12 @@ static int add_one_reference(struct string_list_item *item, void *cb_data)
 		die(_("reference repository '%s' is not a local repository."),
 		    item->string);
 
+	if (!access(mkpath("%s/shallow", ref_git), F_OK))
+		die(_("reference repository '%s' is shallow"), item->string);
+
+	if (!access(mkpath("%s/info/grafts", ref_git), F_OK))
+		die(_("reference repository '%s' is grafted"), item->string);
+
 	strbuf_addf(&alternate, "%s/objects", ref_git);
 	add_to_alternates_file(alternate.buf);
 	strbuf_release(&alternate);
@@ -791,8 +797,15 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	else
 		repo = repo_name;
 	is_local = option_local != 0 && path && !is_bundle;
-	if (is_local && option_depth)
-		warning(_("--depth is ignored in local clones; use file:// instead."));
+	if (is_local) {
+		if (option_depth)
+			warning(_("--depth is ignored in local clones; use file:// instead."));
+		if (!access(mkpath("%s/shallow", path), F_OK)) {
+			if (option_local > 0)
+				warning(_("source repository is shallow, ignoring --local"));
+			is_local = 0;
+		}
+	}
 	if (option_local > 0 && !is_local)
 		warning(_("--local is ignored"));
 
@@ -887,6 +900,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 
 	remote = remote_get(option_origin);
 	transport = transport_get(remote, remote->url[0]);
+	transport->cloning = 1;
 
 	if (!transport->get_refs_list || (!is_local && !transport->fetch))
 		die(_("Don't know how to clone %s"), transport->url);
diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 8b8978a252..1262b405f8 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -3,6 +3,7 @@
 #include "fetch-pack.h"
 #include "remote.h"
 #include "connect.h"
+#include "sha1-array.h"
 
 static const char fetch_pack_usage[] =
 "git fetch-pack [--all] [--stdin] [--quiet|-q] [--keep|-k] [--thin] "
@@ -13,6 +14,13 @@ static void add_sought_entry_mem(struct ref ***sought, int *nr, int *alloc,
 				 const char *name, int namelen)
 {
 	struct ref *ref = xcalloc(1, sizeof(*ref) + namelen + 1);
+	unsigned char sha1[20];
+
+	if (namelen > 41 && name[40] == ' ' && !get_sha1_hex(name, sha1)) {
+		hashcpy(ref->old_sha1, sha1);
+		name += 41;
+		namelen -= 41;
+	}
 
 	memcpy(ref->name, name, namelen);
 	ref->name[namelen] = '\0';
@@ -39,6 +47,7 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 	char **pack_lockfile_ptr = NULL;
 	struct child_process *conn;
 	struct fetch_pack_args args;
+	struct sha1_array shallow = SHA1_ARRAY_INIT;
 
 	packet_trace_identity("fetch-pack");
 
@@ -110,6 +119,14 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 			args.check_self_contained_and_connected = 1;
 			continue;
 		}
+		if (!strcmp("--cloning", arg)) {
+			args.cloning = 1;
+			continue;
+		}
+		if (!strcmp("--update-shallow", arg)) {
+			args.update_shallow = 1;
+			continue;
+		}
 		usage(fetch_pack_usage);
 	}
 
@@ -158,10 +175,10 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 		if (!conn)
 			return args.diag_url ? 0 : 1;
 	}
-	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL);
+	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL, &shallow);
 
-	ref = fetch_pack(&args, fd, conn, ref, dest,
-			 sought, nr_sought, pack_lockfile_ptr);
+	ref = fetch_pack(&args, fd, conn, ref, dest, sought, nr_sought,
+			 &shallow, pack_lockfile_ptr);
 	if (pack_lockfile) {
 		printf("lock %s\n", pack_lockfile);
 		fflush(stdout);
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 09825c84d7..025bc3e38d 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -36,7 +36,7 @@ static int prune = -1; /* unspecified */
 
 static int all, append, dry_run, force, keep, multiple, update_head_ok, verbosity;
 static int progress = -1, recurse_submodules = RECURSE_SUBMODULES_DEFAULT;
-static int tags = TAGS_DEFAULT, unshallow;
+static int tags = TAGS_DEFAULT, unshallow, update_shallow;
 static const char *depth;
 static const char *upload_pack;
 static struct strbuf default_rla = STRBUF_INIT;
@@ -105,6 +105,8 @@ static struct option builtin_fetch_options[] = {
 	{ OPTION_STRING, 0, "recurse-submodules-default",
 		   &recurse_submodules_default, NULL,
 		   N_("default mode for recursion"), PARSE_OPT_HIDDEN },
+	OPT_BOOL(0, "update-shallow", &update_shallow,
+		 N_("accept refs that update .git/shallow")),
 	OPT_END()
 };
 
@@ -524,6 +526,8 @@ static int iterate_ref_map(void *cb_data, unsigned char sha1[20])
 	struct ref **rm = cb_data;
 	struct ref *ref = *rm;
 
+	while (ref && ref->status == REF_STATUS_REJECT_SHALLOW)
+		ref = ref->next;
 	if (!ref)
 		return -1; /* end of the list */
 	*rm = ref->next;
@@ -570,6 +574,13 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 			struct ref *ref = NULL;
 			const char *merge_status_marker = "";
 
+			if (rm->status == REF_STATUS_REJECT_SHALLOW) {
+				if (want_status == FETCH_HEAD_MERGE)
+					warning(_("reject %s because shallow roots are not allowed to be updated"),
+						rm->peer_ref ? rm->peer_ref->name : rm->name);
+				continue;
+			}
+
 			commit = lookup_commit_reference_gently(rm->old_sha1, 1);
 			if (!commit)
 				rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
@@ -798,6 +809,8 @@ static struct transport *prepare_transport(struct remote *remote)
 		set_option(transport, TRANS_OPT_KEEP, "yes");
 	if (depth)
 		set_option(transport, TRANS_OPT_DEPTH, depth);
+	if (update_shallow)
+		set_option(transport, TRANS_OPT_UPDATE_SHALLOW, "yes");
 	return transport;
 }
 
diff --git a/builtin/gc.c b/builtin/gc.c
index 25f2237c08..c19545d49e 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -16,6 +16,7 @@
 #include "run-command.h"
 #include "sigchain.h"
 #include "argv-array.h"
+#include "commit.h"
 
 #define FAILED_RUN "failed to run %s"
 
diff --git a/builtin/prune.c b/builtin/prune.c
index beb0dc942c..de43b26cfe 100644
--- a/builtin/prune.c
+++ b/builtin/prune.c
@@ -180,5 +180,9 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 	s = mkpathdup("%s/pack", get_object_directory());
 	remove_temporary_files(s);
 	free(s);
+
+	if (is_repository_shallow())
+		prune_shallow(show_only);
+
 	return 0;
 }
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index e09994f4df..85bba356fa 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -13,6 +13,7 @@
 #include "string-list.h"
 #include "sha1-array.h"
 #include "connected.h"
+#include "argv-array.h"
 #include "version.h"
 
 static const char receive_pack_usage[] = "git receive-pack <git-dir>";
@@ -43,6 +44,8 @@ static int fix_thin = 1;
 static const char *head_name;
 static void *head_name_to_free;
 static int sent_capabilities;
+static int shallow_update;
+static const char *alt_shallow_file;
 
 static enum deny_action parse_deny_action(const char *var, const char *value)
 {
@@ -121,6 +124,11 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "receive.shallowupdate") == 0) {
+		shallow_update = git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -178,6 +186,8 @@ static void write_head_info(void)
 	if (!sent_capabilities)
 		show_ref("capabilities^{}", null_sha1);
 
+	advertise_shallow_grafts(1);
+
 	/* EOF */
 	packet_flush(1);
 }
@@ -187,6 +197,7 @@ struct command {
 	const char *error_string;
 	unsigned int skip_update:1,
 		     did_not_exist:1;
+	int index;
 	unsigned char old_sha1[20];
 	unsigned char new_sha1[20];
 	char ref_name[FLEX_ARRAY]; /* more */
@@ -418,7 +429,46 @@ static void refuse_unconfigured_deny_delete_current(void)
 		rp_error("%s", refuse_unconfigured_deny_delete_current_msg[i]);
 }
 
-static const char *update(struct command *cmd)
+static int command_singleton_iterator(void *cb_data, unsigned char sha1[20]);
+static int update_shallow_ref(struct command *cmd, struct shallow_info *si)
+{
+	static struct lock_file shallow_lock;
+	struct sha1_array extra = SHA1_ARRAY_INIT;
+	const char *alt_file;
+	uint32_t mask = 1 << (cmd->index % 32);
+	int i;
+
+	trace_printf_key("GIT_TRACE_SHALLOW",
+			 "shallow: update_shallow_ref %s\n", cmd->ref_name);
+	for (i = 0; i < si->shallow->nr; i++)
+		if (si->used_shallow[i] &&
+		    (si->used_shallow[i][cmd->index / 32] & mask) &&
+		    !delayed_reachability_test(si, i))
+			sha1_array_append(&extra, si->shallow->sha1[i]);
+
+	setup_alternate_shallow(&shallow_lock, &alt_file, &extra);
+	if (check_shallow_connected(command_singleton_iterator,
+				    0, cmd, alt_file)) {
+		rollback_lock_file(&shallow_lock);
+		sha1_array_clear(&extra);
+		return -1;
+	}
+
+	commit_lock_file(&shallow_lock);
+
+	/*
+	 * Make sure setup_alternate_shallow() for the next ref does
+	 * not lose these new roots..
+	 */
+	for (i = 0; i < extra.nr; i++)
+		register_shallow(extra.sha1[i]);
+
+	si->shallow_ref[cmd->index] = 0;
+	sha1_array_clear(&extra);
+	return 0;
+}
+
+static const char *update(struct command *cmd, struct shallow_info *si)
 {
 	const char *name = cmd->ref_name;
 	struct strbuf namespaced_name_buf = STRBUF_INIT;
@@ -526,6 +576,10 @@ static const char *update(struct command *cmd)
 		return NULL; /* good */
 	}
 	else {
+		if (shallow_update && si->shallow_ref[cmd->index] &&
+		    update_shallow_ref(cmd, si))
+			return "shallow error";
+
 		lock = lock_any_ref_for_update(namespaced_name, old_sha1,
 					       0, NULL);
 		if (!lock) {
@@ -666,12 +720,16 @@ static int command_singleton_iterator(void *cb_data, unsigned char sha1[20])
 	return 0;
 }
 
-static void set_connectivity_errors(struct command *commands)
+static void set_connectivity_errors(struct command *commands,
+				    struct shallow_info *si)
 {
 	struct command *cmd;
 
 	for (cmd = commands; cmd; cmd = cmd->next) {
 		struct command *singleton = cmd;
+		if (shallow_update && si->shallow_ref[cmd->index])
+			/* to be checked in update_shallow_ref() */
+			continue;
 		if (!check_everything_connected(command_singleton_iterator,
 						0, &singleton))
 			continue;
@@ -679,18 +737,26 @@ static void set_connectivity_errors(struct command *commands)
 	}
 }
 
+struct iterate_data {
+	struct command *cmds;
+	struct shallow_info *si;
+};
+
 static int iterate_receive_command_list(void *cb_data, unsigned char sha1[20])
 {
-	struct command **cmd_list = cb_data;
+	struct iterate_data *data = cb_data;
+	struct command **cmd_list = &data->cmds;
 	struct command *cmd = *cmd_list;
 
-	while (cmd) {
-		if (!is_null_sha1(cmd->new_sha1)) {
+	for (; cmd; cmd = cmd->next) {
+		if (shallow_update && data->si->shallow_ref[cmd->index])
+			/* to be checked in update_shallow_ref() */
+			continue;
+		if (!is_null_sha1(cmd->new_sha1) && !cmd->skip_update) {
 			hashcpy(sha1, cmd->new_sha1);
 			*cmd_list = cmd->next;
 			return 0;
 		}
-		cmd = cmd->next;
 	}
 	*cmd_list = NULL;
 	return -1; /* end of list */
@@ -710,10 +776,14 @@ static void reject_updates_to_hidden(struct command *commands)
 	}
 }
 
-static void execute_commands(struct command *commands, const char *unpacker_error)
+static void execute_commands(struct command *commands,
+			     const char *unpacker_error,
+			     struct shallow_info *si)
 {
+	int checked_connectivity;
 	struct command *cmd;
 	unsigned char sha1[20];
+	struct iterate_data data;
 
 	if (unpacker_error) {
 		for (cmd = commands; cmd; cmd = cmd->next)
@@ -721,10 +791,10 @@ static void execute_commands(struct command *commands, const char *unpacker_erro
 		return;
 	}
 
-	cmd = commands;
-	if (check_everything_connected(iterate_receive_command_list,
-				       0, &cmd))
-		set_connectivity_errors(commands);
+	data.cmds = commands;
+	data.si = si;
+	if (check_everything_connected(iterate_receive_command_list, 0, &data))
+		set_connectivity_errors(commands, si);
 
 	reject_updates_to_hidden(commands);
 
@@ -741,6 +811,7 @@ static void execute_commands(struct command *commands, const char *unpacker_erro
 	free(head_name_to_free);
 	head_name = head_name_to_free = resolve_refdup("HEAD", sha1, 0, NULL);
 
+	checked_connectivity = 1;
 	for (cmd = commands; cmd; cmd = cmd->next) {
 		if (cmd->error_string)
 			continue;
@@ -748,11 +819,26 @@ static void execute_commands(struct command *commands, const char *unpacker_erro
 		if (cmd->skip_update)
 			continue;
 
-		cmd->error_string = update(cmd);
+		cmd->error_string = update(cmd, si);
+		if (shallow_update && !cmd->error_string &&
+		    si->shallow_ref[cmd->index]) {
+			error("BUG: connectivity check has not been run on ref %s",
+			      cmd->ref_name);
+			checked_connectivity = 0;
+		}
+	}
+
+	if (shallow_update) {
+		if (!checked_connectivity)
+			error("BUG: run 'git fsck' for safety.\n"
+			      "If there are errors, try to remove "
+			      "the reported refs above");
+		if (alt_shallow_file && *alt_shallow_file)
+			unlink(alt_shallow_file);
 	}
 }
 
-static struct command *read_head_info(void)
+static struct command *read_head_info(struct sha1_array *shallow)
 {
 	struct command *commands = NULL;
 	struct command **p = &commands;
@@ -766,6 +852,14 @@ static struct command *read_head_info(void)
 		line = packet_read_line(0, &len);
 		if (!line)
 			break;
+
+		if (len == 48 && starts_with(line, "shallow ")) {
+			if (get_sha1_hex(line + 8, old_sha1))
+				die("protocol error: expected shallow sha, got '%s'", line + 8);
+			sha1_array_append(shallow, old_sha1);
+			continue;
+		}
+
 		if (len < 83 ||
 		    line[40] != ' ' ||
 		    line[81] != ' ' ||
@@ -817,11 +911,14 @@ static const char *parse_pack_header(struct pack_header *hdr)
 
 static const char *pack_lockfile;
 
-static const char *unpack(int err_fd)
+static const char *unpack(int err_fd, struct shallow_info *si)
 {
 	struct pack_header hdr;
+	struct argv_array av = ARGV_ARRAY_INIT;
 	const char *hdr_err;
+	int status;
 	char hdr_arg[38];
+	struct child_process child;
 	int fsck_objects = (receive_fsck_objects >= 0
 			    ? receive_fsck_objects
 			    : transfer_fsck_objects >= 0
@@ -838,72 +935,63 @@ static const char *unpack(int err_fd)
 			"--pack_header=%"PRIu32",%"PRIu32,
 			ntohl(hdr.hdr_version), ntohl(hdr.hdr_entries));
 
+	if (si->nr_ours || si->nr_theirs) {
+		alt_shallow_file = setup_temporary_shallow(si->shallow);
+		argv_array_pushl(&av, "--shallow-file", alt_shallow_file, NULL);
+	}
+
+	memset(&child, 0, sizeof(child));
 	if (ntohl(hdr.hdr_entries) < unpack_limit) {
-		int code, i = 0;
-		struct child_process child;
-		const char *unpacker[5];
-		unpacker[i++] = "unpack-objects";
+		argv_array_pushl(&av, "unpack-objects", hdr_arg, NULL);
 		if (quiet)
-			unpacker[i++] = "-q";
+			argv_array_push(&av, "-q");
 		if (fsck_objects)
-			unpacker[i++] = "--strict";
-		unpacker[i++] = hdr_arg;
-		unpacker[i++] = NULL;
-		memset(&child, 0, sizeof(child));
-		child.argv = unpacker;
+			argv_array_push(&av, "--strict");
+		child.argv = av.argv;
 		child.no_stdout = 1;
 		child.err = err_fd;
 		child.git_cmd = 1;
-		code = run_command(&child);
-		if (!code)
-			return NULL;
-		return "unpack-objects abnormal exit";
+		status = run_command(&child);
+		if (status)
+			return "unpack-objects abnormal exit";
 	} else {
-		const char *keeper[7];
-		int s, status, i = 0;
+		int s;
 		char keep_arg[256];
-		struct child_process ip;
 
 		s = sprintf(keep_arg, "--keep=receive-pack %"PRIuMAX" on ", (uintmax_t) getpid());
 		if (gethostname(keep_arg + s, sizeof(keep_arg) - s))
 			strcpy(keep_arg + s, "localhost");
 
-		keeper[i++] = "index-pack";
-		keeper[i++] = "--stdin";
+		argv_array_pushl(&av, "index-pack",
+				 "--stdin", hdr_arg, keep_arg, NULL);
 		if (fsck_objects)
-			keeper[i++] = "--strict";
+			argv_array_push(&av, "--strict");
 		if (fix_thin)
-			keeper[i++] = "--fix-thin";
-		keeper[i++] = hdr_arg;
-		keeper[i++] = keep_arg;
-		keeper[i++] = NULL;
-		memset(&ip, 0, sizeof(ip));
-		ip.argv = keeper;
-		ip.out = -1;
-		ip.err = err_fd;
-		ip.git_cmd = 1;
-		status = start_command(&ip);
-		if (status) {
+			argv_array_push(&av, "--fix-thin");
+		child.argv = av.argv;
+		child.out = -1;
+		child.err = err_fd;
+		child.git_cmd = 1;
+		status = start_command(&child);
+		if (status)
 			return "index-pack fork failed";
-		}
-		pack_lockfile = index_pack_lockfile(ip.out);
-		close(ip.out);
-		status = finish_command(&ip);
-		if (!status) {
-			reprepare_packed_git();
-			return NULL;
-		}
-		return "index-pack abnormal exit";
+		pack_lockfile = index_pack_lockfile(child.out);
+		close(child.out);
+		status = finish_command(&child);
+		if (status)
+			return "index-pack abnormal exit";
+		reprepare_packed_git();
 	}
+	return NULL;
 }
 
-static const char *unpack_with_sideband(void)
+static const char *unpack_with_sideband(struct shallow_info *si)
 {
 	struct async muxer;
 	const char *ret;
 
 	if (!use_sideband)
-		return unpack(0);
+		return unpack(0, si);
 
 	memset(&muxer, 0, sizeof(muxer));
 	muxer.proc = copy_to_sideband;
@@ -911,12 +999,101 @@ static const char *unpack_with_sideband(void)
 	if (start_async(&muxer))
 		return NULL;
 
-	ret = unpack(muxer.in);
+	ret = unpack(muxer.in, si);
 
 	finish_async(&muxer);
 	return ret;
 }
 
+static void prepare_shallow_update(struct command *commands,
+				   struct shallow_info *si)
+{
+	int i, j, k, bitmap_size = (si->ref->nr + 31) / 32;
+
+	si->used_shallow = xmalloc(sizeof(*si->used_shallow) *
+				   si->shallow->nr);
+	assign_shallow_commits_to_refs(si, si->used_shallow, NULL);
+
+	si->need_reachability_test =
+		xcalloc(si->shallow->nr, sizeof(*si->need_reachability_test));
+	si->reachable =
+		xcalloc(si->shallow->nr, sizeof(*si->reachable));
+	si->shallow_ref = xcalloc(si->ref->nr, sizeof(*si->shallow_ref));
+
+	for (i = 0; i < si->nr_ours; i++)
+		si->need_reachability_test[si->ours[i]] = 1;
+
+	for (i = 0; i < si->shallow->nr; i++) {
+		if (!si->used_shallow[i])
+			continue;
+		for (j = 0; j < bitmap_size; j++) {
+			if (!si->used_shallow[i][j])
+				continue;
+			si->need_reachability_test[i]++;
+			for (k = 0; k < 32; k++)
+				if (si->used_shallow[i][j] & (1 << k))
+					si->shallow_ref[j * 32 + k]++;
+		}
+
+		/*
+		 * true for those associated with some refs and belong
+		 * in "ours" list aka "step 7 not done yet"
+		 */
+		si->need_reachability_test[i] =
+			si->need_reachability_test[i] > 1;
+	}
+
+	/*
+	 * keep hooks happy by forcing a temporary shallow file via
+	 * env variable because we can't add --shallow-file to every
+	 * command. check_everything_connected() will be done with
+	 * true .git/shallow though.
+	 */
+	setenv(GIT_SHALLOW_FILE_ENVIRONMENT, alt_shallow_file, 1);
+}
+
+static void update_shallow_info(struct command *commands,
+				struct shallow_info *si,
+				struct sha1_array *ref)
+{
+	struct command *cmd;
+	int *ref_status;
+	remove_nonexistent_theirs_shallow(si);
+	if (!si->nr_ours && !si->nr_theirs) {
+		shallow_update = 0;
+		return;
+	}
+
+	for (cmd = commands; cmd; cmd = cmd->next) {
+		if (is_null_sha1(cmd->new_sha1))
+			continue;
+		sha1_array_append(ref, cmd->new_sha1);
+		cmd->index = ref->nr - 1;
+	}
+	si->ref = ref;
+
+	if (shallow_update) {
+		prepare_shallow_update(commands, si);
+		return;
+	}
+
+	ref_status = xmalloc(sizeof(*ref_status) * ref->nr);
+	assign_shallow_commits_to_refs(si, NULL, ref_status);
+	for (cmd = commands; cmd; cmd = cmd->next) {
+		if (is_null_sha1(cmd->new_sha1))
+			continue;
+		if (ref_status[cmd->index]) {
+			cmd->error_string = "shallow update not allowed";
+			cmd->skip_update = 1;
+		}
+	}
+	if (alt_shallow_file && *alt_shallow_file) {
+		unlink(alt_shallow_file);
+		alt_shallow_file = NULL;
+	}
+	free(ref_status);
+}
+
 static void report(struct command *commands, const char *unpack_status)
 {
 	struct command *cmd;
@@ -958,6 +1135,9 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 	int i;
 	char *dir = NULL;
 	struct command *commands;
+	struct sha1_array shallow = SHA1_ARRAY_INIT;
+	struct sha1_array ref = SHA1_ARRAY_INIT;
+	struct shallow_info si;
 
 	packet_trace_identity("receive-pack");
 
@@ -998,9 +1178,6 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 	if (!enter_repo(dir, 0))
 		die("'%s' does not appear to be a git repository", dir);
 
-	if (is_repository_shallow())
-		die("attempt to push into a shallow repository");
-
 	git_config(receive_pack_config, NULL);
 
 	if (0 <= transfer_unpack_limit)
@@ -1014,12 +1191,17 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 	if (advertise_refs)
 		return 0;
 
-	if ((commands = read_head_info()) != NULL) {
+	if ((commands = read_head_info(&shallow)) != NULL) {
 		const char *unpack_status = NULL;
 
-		if (!delete_only(commands))
-			unpack_status = unpack_with_sideband();
-		execute_commands(commands, unpack_status);
+		prepare_shallow_info(&si, &shallow);
+		if (!si.nr_ours && !si.nr_theirs)
+			shallow_update = 0;
+		if (!delete_only(commands)) {
+			unpack_status = unpack_with_sideband(&si);
+			update_shallow_info(commands, &si, &ref);
+		}
+		execute_commands(commands, unpack_status, &si);
 		if (pack_lockfile)
 			unlink_or_warn(pack_lockfile);
 		if (report_status)
@@ -1035,8 +1217,11 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 		}
 		if (auto_update_server_info)
 			update_server_info(0);
+		clear_shallow_info(&si);
 	}
 	if (use_sideband)
 		packet_flush(1);
+	sha1_array_clear(&shallow);
+	sha1_array_clear(&ref);
 	return 0;
 }
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index e7f0b97d31..f420b74665 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -10,6 +10,7 @@
 #include "quote.h"
 #include "transport.h"
 #include "version.h"
+#include "sha1-array.h"
 
 static const char send_pack_usage[] =
 "git send-pack [--all | --mirror] [--dry-run] [--force] [--receive-pack=<git-receive-pack>] [--verbose] [--thin] [<host>:]<directory> [<ref>...]\n"
@@ -99,7 +100,8 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	const char *dest = NULL;
 	int fd[2];
 	struct child_process *conn;
-	struct extra_have_objects extra_have;
+	struct sha1_array extra_have = SHA1_ARRAY_INIT;
+	struct sha1_array shallow = SHA1_ARRAY_INIT;
 	struct ref *remote_refs, *local_refs;
 	int ret;
 	int helper_status = 0;
@@ -228,9 +230,8 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 			args.verbose ? CONNECT_VERBOSE : 0);
 	}
 
-	memset(&extra_have, 0, sizeof(extra_have));
-
-	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL, &extra_have);
+	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL,
+			 &extra_have, &shallow);
 
 	transport_verify_remote_names(nr_refspecs, refspecs);
 
diff --git a/cache.h b/cache.h
index 83a27269b8..c9efe88e9e 100644
--- a/cache.h
+++ b/cache.h
@@ -354,6 +354,7 @@ static inline enum object_type object_type(unsigned int mode)
 #define DB_ENVIRONMENT "GIT_OBJECT_DIRECTORY"
 #define INDEX_ENVIRONMENT "GIT_INDEX_FILE"
 #define GRAFT_ENVIRONMENT "GIT_GRAFT_FILE"
+#define GIT_SHALLOW_FILE_ENVIRONMENT "GIT_SHALLOW_FILE"
 #define TEMPLATE_DIR_ENVIRONMENT "GIT_TEMPLATE_DIR"
 #define CONFIG_ENVIRONMENT "GIT_CONFIG"
 #define CONFIG_DATA_ENVIRONMENT "GIT_CONFIG_PARAMETERS"
@@ -1243,6 +1244,8 @@ __attribute__((format (printf, 2, 3)))
 extern void trace_argv_printf(const char **argv, const char *format, ...);
 extern void trace_repo_setup(const char *prefix);
 extern int trace_want(const char *key);
+__attribute__((format (printf, 2, 3)))
+extern void trace_printf_key(const char *key, const char *fmt, ...);
 extern void trace_strbuf(const char *key, const struct strbuf *buf);
 
 void packet_trace_identity(const char *prog);
diff --git a/commit.h b/commit.h
index f8a451d864..16d9c43513 100644
--- a/commit.h
+++ b/commit.h
@@ -194,6 +194,8 @@ extern struct commit_list *get_octopus_merge_bases(struct commit_list *in);
 /* largest positive number a signed 32-bit integer can contain */
 #define INFINITE_DEPTH 0x7fffffff
 
+struct sha1_array;
+struct ref;
 extern int register_shallow(const unsigned char *sha1);
 extern int unregister_shallow(const unsigned char *sha1);
 extern int for_each_commit_graft(each_commit_graft_fn, void *);
@@ -201,11 +203,38 @@ extern int is_repository_shallow(void);
 extern struct commit_list *get_shallow_commits(struct object_array *heads,
 		int depth, int shallow_flag, int not_shallow_flag);
 extern void check_shallow_file_for_update(void);
-extern void set_alternate_shallow_file(const char *path);
-extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol);
+extern void set_alternate_shallow_file(const char *path, int override);
+extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol,
+				 const struct sha1_array *extra);
 extern void setup_alternate_shallow(struct lock_file *shallow_lock,
-				    const char **alternate_shallow_file);
-extern char *setup_temporary_shallow(void);
+				    const char **alternate_shallow_file,
+				    const struct sha1_array *extra);
+extern char *setup_temporary_shallow(const struct sha1_array *extra);
+extern void advertise_shallow_grafts(int);
+
+struct shallow_info {
+	struct sha1_array *shallow;
+	int *ours, nr_ours;
+	int *theirs, nr_theirs;
+	struct sha1_array *ref;
+
+	/* for receive-pack */
+	uint32_t **used_shallow;
+	int *need_reachability_test;
+	int *reachable;
+	int *shallow_ref;
+	struct commit **commits;
+	int nr_commits;
+};
+
+extern void prepare_shallow_info(struct shallow_info *, struct sha1_array *);
+extern void clear_shallow_info(struct shallow_info *);
+extern void remove_nonexistent_theirs_shallow(struct shallow_info *);
+extern void assign_shallow_commits_to_refs(struct shallow_info *info,
+					   uint32_t **used,
+					   int *ref_status);
+extern int delayed_reachability_test(struct shallow_info *si, int c);
+extern void prune_shallow(int show_only);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/connect.c b/connect.c
index c763eeda86..4150412e2c 100644
--- a/connect.c
+++ b/connect.c
@@ -8,6 +8,7 @@
 #include "connect.h"
 #include "url.h"
 #include "string-list.h"
+#include "sha1-array.h"
 
 static char *server_capabilities;
 static const char *parse_feature_value(const char *, const char *, int *);
@@ -45,13 +46,6 @@ int check_ref_type(const struct ref *ref, int flags)
 	return check_ref(ref->name, strlen(ref->name), flags);
 }
 
-static void add_extra_have(struct extra_have_objects *extra, unsigned char *sha1)
-{
-	ALLOC_GROW(extra->array, extra->nr + 1, extra->alloc);
-	hashcpy(&(extra->array[extra->nr][0]), sha1);
-	extra->nr++;
-}
-
 static void die_initial_contact(int got_at_least_one_head)
 {
 	if (got_at_least_one_head)
@@ -122,7 +116,8 @@ static void annotate_refs_with_symref_info(struct ref *ref)
  */
 struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 			      struct ref **list, unsigned int flags,
-			      struct extra_have_objects *extra_have)
+			      struct sha1_array *extra_have,
+			      struct sha1_array *shallow_points)
 {
 	struct ref **orig_list = list;
 	int got_at_least_one_head = 0;
@@ -148,6 +143,15 @@ struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 		if (len > 4 && starts_with(buffer, "ERR "))
 			die("remote error: %s", buffer + 4);
 
+		if (len == 48 && starts_with(buffer, "shallow ")) {
+			if (get_sha1_hex(buffer + 8, old_sha1))
+				die("protocol error: expected shallow sha-1, got '%s'", buffer + 8);
+			if (!shallow_points)
+				die("repository on the other end cannot be shallow");
+			sha1_array_append(shallow_points, old_sha1);
+			continue;
+		}
+
 		if (len < 42 || get_sha1_hex(buffer, old_sha1) || buffer[40] != ' ')
 			die("protocol error: expected sha/ref, got '%s'", buffer);
 		name = buffer + 41;
@@ -160,7 +164,7 @@ struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 
 		if (extra_have &&
 		    name_len == 5 && !memcmp(".have", name, 5)) {
-			add_extra_have(extra_have, old_sha1);
+			sha1_array_append(extra_have, old_sha1);
 			continue;
 		}
 
diff --git a/connected.c b/connected.c
index 51d8ba4bb7..be0253e21b 100644
--- a/connected.c
+++ b/connected.c
@@ -19,17 +19,17 @@ int check_everything_connected(sha1_iterate_fn fn, int quiet, void *cb_data)
  *
  * Returns 0 if everything is connected, non-zero otherwise.
  */
-int check_everything_connected_with_transport(sha1_iterate_fn fn,
-					      int quiet,
-					      void *cb_data,
-					      struct transport *transport)
+static int check_everything_connected_real(sha1_iterate_fn fn,
+					   int quiet,
+					   void *cb_data,
+					   struct transport *transport,
+					   const char *shallow_file)
 {
 	struct child_process rev_list;
-	const char *argv[] = {"rev-list", "--objects",
-			      "--stdin", "--not", "--all", NULL, NULL};
+	const char *argv[9];
 	char commit[41];
 	unsigned char sha1[20];
-	int err = 0;
+	int err = 0, ac = 0;
 	struct packed_git *new_pack = NULL;
 
 	if (fn(cb_data, sha1))
@@ -47,8 +47,18 @@ int check_everything_connected_with_transport(sha1_iterate_fn fn,
 		strbuf_release(&idx_file);
 	}
 
+	if (shallow_file) {
+		argv[ac++] = "--shallow-file";
+		argv[ac++] = shallow_file;
+	}
+	argv[ac++] = "rev-list";
+	argv[ac++] = "--objects";
+	argv[ac++] = "--stdin";
+	argv[ac++] = "--not";
+	argv[ac++] = "--all";
 	if (quiet)
-		argv[5] = "--quiet";
+		argv[ac++] = "--quiet";
+	argv[ac] = NULL;
 
 	memset(&rev_list, 0, sizeof(rev_list));
 	rev_list.argv = argv;
@@ -92,3 +102,19 @@ int check_everything_connected_with_transport(sha1_iterate_fn fn,
 	sigchain_pop(SIGPIPE);
 	return finish_command(&rev_list) || err;
 }
+
+int check_everything_connected_with_transport(sha1_iterate_fn fn,
+					      int quiet,
+					      void *cb_data,
+					      struct transport *transport)
+{
+	return check_everything_connected_real(fn, quiet, cb_data,
+					       transport, NULL);
+}
+
+int check_shallow_connected(sha1_iterate_fn fn, int quiet, void *cb_data,
+			    const char *shallow_file)
+{
+	return check_everything_connected_real(fn, quiet, cb_data,
+					       NULL, shallow_file);
+}
diff --git a/connected.h b/connected.h
index 0b060b7429..071d408f38 100644
--- a/connected.h
+++ b/connected.h
@@ -18,6 +18,8 @@ typedef int (*sha1_iterate_fn)(void *, unsigned char [20]);
  * Return 0 if Ok, non zero otherwise (i.e. some missing objects)
  */
 extern int check_everything_connected(sha1_iterate_fn, int quiet, void *cb_data);
+extern int check_shallow_connected(sha1_iterate_fn, int quiet, void *cb_data,
+				   const char *shallow_file);
 extern int check_everything_connected_with_transport(sha1_iterate_fn, int quiet,
 						     void *cb_data,
 						     struct transport *transport);
diff --git a/environment.c b/environment.c
index 3c76905b9f..4a3437d8a6 100644
--- a/environment.c
+++ b/environment.c
@@ -10,6 +10,7 @@
 #include "cache.h"
 #include "refs.h"
 #include "fmt-merge-msg.h"
+#include "commit.h"
 
 int trust_executable_bit = 1;
 int trust_ctime = 1;
@@ -97,6 +98,7 @@ const char * const local_repo_env[] = {
 	INDEX_ENVIRONMENT,
 	NO_REPLACE_OBJECTS_ENVIRONMENT,
 	GIT_PREFIX_ENVIRONMENT,
+	GIT_SHALLOW_FILE_ENVIRONMENT,
 	NULL
 };
 
@@ -124,6 +126,7 @@ static char *expand_namespace(const char *raw_namespace)
 static void setup_git_env(void)
 {
 	const char *gitfile;
+	const char *shallow_file;
 
 	git_dir = getenv(GIT_DIR_ENVIRONMENT);
 	if (!git_dir)
@@ -147,6 +150,9 @@ static void setup_git_env(void)
 		read_replace_refs = 0;
 	namespace = expand_namespace(getenv(GIT_NAMESPACE_ENVIRONMENT));
 	namespace_len = strlen(namespace);
+	shallow_file = getenv(GIT_SHALLOW_FILE_ENVIRONMENT);
+	if (shallow_file)
+		set_alternate_shallow_file(shallow_file, 0);
 }
 
 int is_bare_repository(void)
diff --git a/fetch-pack.c b/fetch-pack.c
index 760ed16e79..d52de74c4b 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -13,6 +13,7 @@
 #include "transport.h"
 #include "version.h"
 #include "prio-queue.h"
+#include "sha1-array.h"
 
 static int transfer_unpack_limit = -1;
 static int fetch_unpack_limit = -1;
@@ -309,7 +310,7 @@ static int find_common(struct fetch_pack_args *args,
 	}
 
 	if (is_repository_shallow())
-		write_shallow_commits(&req_buf, 1);
+		write_shallow_commits(&req_buf, 1, NULL);
 	if (args->depth > 0)
 		packet_buf_write(&req_buf, "deepen %d", args->depth);
 	packet_buf_flush(&req_buf);
@@ -772,6 +773,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				 int fd[2],
 				 const struct ref *orig_ref,
 				 struct ref **sought, int nr_sought,
+				 struct shallow_info *si,
 				 char **pack_lockfile)
 {
 	struct ref *ref = copy_ref_list(orig_ref);
@@ -848,7 +850,10 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 	if (args->stateless_rpc)
 		packet_flush(fd[1]);
 	if (args->depth > 0)
-		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file);
+		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file,
+					NULL);
+	else if (si->nr_ours || si->nr_theirs)
+		alternate_shallow_file = setup_temporary_shallow(si->shallow);
 	else
 		alternate_shallow_file = NULL;
 	if (get_pack(args, fd, pack_lockfile))
@@ -922,14 +927,121 @@ static int remove_duplicates_in_refs(struct ref **ref, int nr)
 	return dst;
 }
 
+static void update_shallow(struct fetch_pack_args *args,
+			   struct ref **sought, int nr_sought,
+			   struct shallow_info *si)
+{
+	struct sha1_array ref = SHA1_ARRAY_INIT;
+	int *status;
+	int i;
+
+	if (args->depth > 0 && alternate_shallow_file) {
+		if (*alternate_shallow_file == '\0') { /* --unshallow */
+			unlink_or_warn(git_path("shallow"));
+			rollback_lock_file(&shallow_lock);
+		} else
+			commit_lock_file(&shallow_lock);
+		return;
+	}
+
+	if (!si->shallow || !si->shallow->nr)
+		return;
+
+	if (alternate_shallow_file) {
+		/*
+		 * The temporary shallow file is only useful for
+		 * index-pack and unpack-objects because it may
+		 * contain more roots than we want. Delete it.
+		 */
+		if (*alternate_shallow_file)
+			unlink(alternate_shallow_file);
+		free((char *)alternate_shallow_file);
+	}
+
+	if (args->cloning) {
+		/*
+		 * remote is shallow, but this is a clone, there are
+		 * no objects in repo to worry about. Accept any
+		 * shallow points that exist in the pack (iow in repo
+		 * after get_pack() and reprepare_packed_git())
+		 */
+		struct sha1_array extra = SHA1_ARRAY_INIT;
+		unsigned char (*sha1)[20] = si->shallow->sha1;
+		for (i = 0; i < si->shallow->nr; i++)
+			if (has_sha1_file(sha1[i]))
+				sha1_array_append(&extra, sha1[i]);
+		if (extra.nr) {
+			setup_alternate_shallow(&shallow_lock,
+						&alternate_shallow_file,
+						&extra);
+			commit_lock_file(&shallow_lock);
+		}
+		sha1_array_clear(&extra);
+		return;
+	}
+
+	if (!si->nr_ours && !si->nr_theirs)
+		return;
+
+	remove_nonexistent_theirs_shallow(si);
+	if (!si->nr_ours && !si->nr_theirs)
+		return;
+	for (i = 0; i < nr_sought; i++)
+		sha1_array_append(&ref, sought[i]->old_sha1);
+	si->ref = &ref;
+
+	if (args->update_shallow) {
+		/*
+		 * remote is also shallow, .git/shallow may be updated
+		 * so all refs can be accepted. Make sure we only add
+		 * shallow roots that are actually reachable from new
+		 * refs.
+		 */
+		struct sha1_array extra = SHA1_ARRAY_INIT;
+		unsigned char (*sha1)[20] = si->shallow->sha1;
+		assign_shallow_commits_to_refs(si, NULL, NULL);
+		if (!si->nr_ours && !si->nr_theirs) {
+			sha1_array_clear(&ref);
+			return;
+		}
+		for (i = 0; i < si->nr_ours; i++)
+			sha1_array_append(&extra, sha1[si->ours[i]]);
+		for (i = 0; i < si->nr_theirs; i++)
+			sha1_array_append(&extra, sha1[si->theirs[i]]);
+		setup_alternate_shallow(&shallow_lock,
+					&alternate_shallow_file,
+					&extra);
+		commit_lock_file(&shallow_lock);
+		sha1_array_clear(&extra);
+		sha1_array_clear(&ref);
+		return;
+	}
+
+	/*
+	 * remote is also shallow, check what ref is safe to update
+	 * without updating .git/shallow
+	 */
+	status = xcalloc(nr_sought, sizeof(*status));
+	assign_shallow_commits_to_refs(si, NULL, status);
+	if (si->nr_ours || si->nr_theirs) {
+		for (i = 0; i < nr_sought; i++)
+			if (status[i])
+				sought[i]->status = REF_STATUS_REJECT_SHALLOW;
+	}
+	free(status);
+	sha1_array_clear(&ref);
+}
+
 struct ref *fetch_pack(struct fetch_pack_args *args,
 		       int fd[], struct child_process *conn,
 		       const struct ref *ref,
 		       const char *dest,
 		       struct ref **sought, int nr_sought,
+		       struct sha1_array *shallow,
 		       char **pack_lockfile)
 {
 	struct ref *ref_cpy;
+	struct shallow_info si;
 
 	fetch_pack_setup();
 	if (nr_sought)
@@ -939,16 +1051,11 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		packet_flush(fd[1]);
 		die("no matching remote head");
 	}
-	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought, pack_lockfile);
-
-	if (args->depth > 0 && alternate_shallow_file) {
-		if (*alternate_shallow_file == '\0') { /* --unshallow */
-			unlink_or_warn(git_path("shallow"));
-			rollback_lock_file(&shallow_lock);
-		} else
-			commit_lock_file(&shallow_lock);
-	}
-
+	prepare_shallow_info(&si, shallow);
+	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought,
+				&si, pack_lockfile);
 	reprepare_packed_git();
+	update_shallow(args, sought, nr_sought, &si);
+	clear_shallow_info(&si);
 	return ref_cpy;
 }
diff --git a/fetch-pack.h b/fetch-pack.h
index 20ccc12e57..bb7fd76e59 100644
--- a/fetch-pack.h
+++ b/fetch-pack.h
@@ -4,23 +4,27 @@
 #include "string-list.h"
 #include "run-command.h"
 
+struct sha1_array;
+
 struct fetch_pack_args {
 	const char *uploadpack;
 	int unpacklimit;
 	int depth;
-	unsigned quiet:1,
-		keep_pack:1,
-		lock_pack:1,
-		use_thin_pack:1,
-		fetch_all:1,
-		stdin_refs:1,
-		diag_url:1,
-		verbose:1,
-		no_progress:1,
-		include_tag:1,
-		stateless_rpc:1,
-		check_self_contained_and_connected:1,
-		self_contained_and_connected:1;
+	unsigned quiet:1;
+	unsigned keep_pack:1;
+	unsigned lock_pack:1;
+	unsigned use_thin_pack:1;
+	unsigned fetch_all:1;
+	unsigned stdin_refs:1;
+	unsigned diag_url:1;
+	unsigned verbose:1;
+	unsigned no_progress:1;
+	unsigned include_tag:1;
+	unsigned stateless_rpc:1;
+	unsigned check_self_contained_and_connected:1;
+	unsigned self_contained_and_connected:1;
+	unsigned cloning:1;
+	unsigned update_shallow:1;
 };
 
 /*
@@ -34,6 +38,7 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		       const char *dest,
 		       struct ref **sought,
 		       int nr_sought,
+		       struct sha1_array *shallow,
 		       char **pack_lockfile);
 
 #endif
diff --git a/git.c b/git.c
index bba4378458..7cf2953eff 100644
--- a/git.c
+++ b/git.c
@@ -162,7 +162,7 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 		} else if (!strcmp(cmd, "--shallow-file")) {
 			(*argv)++;
 			(*argc)--;
-			set_alternate_shallow_file((*argv)[0]);
+			set_alternate_shallow_file((*argv)[0], 1);
 			if (envchanged)
 				*envchanged = 1;
 		} else if (!strcmp(cmd, "-C")) {
diff --git a/remote-curl.c b/remote-curl.c
index e38c4b026f..10cb0114ea 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -10,6 +10,7 @@
 #include "sideband.h"
 #include "argv-array.h"
 #include "credential.h"
+#include "sha1-array.h"
 
 static struct remote *remote;
 /* always ends with a trailing slash */
@@ -20,6 +21,8 @@ struct options {
 	unsigned long depth;
 	unsigned progress : 1,
 		check_self_contained_and_connected : 1,
+		cloning : 1,
+		update_shallow : 1,
 		followtags : 1,
 		dry_run : 1,
 		thin : 1;
@@ -87,8 +90,23 @@ static int set_option(const char *name, const char *value)
 		string_list_append(&cas_options, val.buf);
 		strbuf_release(&val);
 		return 0;
-	}
-	else {
+	} else if (!strcmp(name, "cloning")) {
+		if (!strcmp(value, "true"))
+			options.cloning = 1;
+		else if (!strcmp(value, "false"))
+			options.cloning = 0;
+		else
+			return -1;
+		return 0;
+	} else if (!strcmp(name, "update-shallow")) {
+		if (!strcmp(value, "true"))
+			options.update_shallow = 1;
+		else if (!strcmp(value, "false"))
+			options.update_shallow = 0;
+		else
+			return -1;
+		return 0;
+	} else {
 		return 1 /* unsupported */;
 	}
 }
@@ -99,6 +117,7 @@ struct discovery {
 	char *buf;
 	size_t len;
 	struct ref *refs;
+	struct sha1_array shallow;
 	unsigned proto_git : 1;
 };
 static struct discovery *last_discovery;
@@ -107,7 +126,7 @@ static struct ref *parse_git_refs(struct discovery *heads, int for_push)
 {
 	struct ref *list = NULL;
 	get_remote_heads(-1, heads->buf, heads->len, &list,
-			 for_push ? REF_NORMAL : 0, NULL);
+			 for_push ? REF_NORMAL : 0, NULL, &heads->shallow);
 	return list;
 }
 
@@ -168,6 +187,7 @@ static void free_discovery(struct discovery *d)
 	if (d) {
 		if (d == last_discovery)
 			last_discovery = NULL;
+		free(d->shallow.sha1);
 		free(d->buf_alloc);
 		free_refs(d->refs);
 		free(d);
@@ -699,7 +719,7 @@ static int fetch_git(struct discovery *heads,
 	struct strbuf preamble = STRBUF_INIT;
 	char *depth_arg = NULL;
 	int argc = 0, i, err;
-	const char *argv[16];
+	const char *argv[17];
 
 	argv[argc++] = "fetch-pack";
 	argv[argc++] = "--stateless-rpc";
@@ -715,6 +735,10 @@ static int fetch_git(struct discovery *heads,
 	}
 	if (options.check_self_contained_and_connected)
 		argv[argc++] = "--check-self-contained-and-connected";
+	if (options.cloning)
+		argv[argc++] = "--cloning";
+	if (options.update_shallow)
+		argv[argc++] = "--update-shallow";
 	if (!options.progress)
 		argv[argc++] = "--no-progress";
 	if (options.depth) {
@@ -730,7 +754,8 @@ static int fetch_git(struct discovery *heads,
 		struct ref *ref = to_fetch[i];
 		if (!ref->name || !*ref->name)
 			die("cannot fetch by sha1 over smart http");
-		packet_buf_write(&preamble, "%s\n", ref->name);
+		packet_buf_write(&preamble, "%s %s\n",
+				 sha1_to_hex(ref->old_sha1), ref->name);
 	}
 	packet_buf_flush(&preamble);
 
diff --git a/remote.h b/remote.h
index 00c6a76ef8..fb7647fab9 100644
--- a/remote.h
+++ b/remote.h
@@ -109,6 +109,7 @@ struct ref {
 		REF_STATUS_REJECT_FETCH_FIRST,
 		REF_STATUS_REJECT_NEEDS_FORCE,
 		REF_STATUS_REJECT_STALE,
+		REF_STATUS_REJECT_SHALLOW,
 		REF_STATUS_UPTODATE,
 		REF_STATUS_REMOTE_REJECT,
 		REF_STATUS_EXPECTING_REPORT
@@ -138,13 +139,11 @@ int check_ref_type(const struct ref *ref, int flags);
  */
 void free_refs(struct ref *ref);
 
-struct extra_have_objects {
-	int nr, alloc;
-	unsigned char (*array)[20];
-};
+struct sha1_array;
 extern struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 				     struct ref **list, unsigned int flags,
-				     struct extra_have_objects *);
+				     struct sha1_array *extra_have,
+				     struct sha1_array *shallow);
 
 int resolve_remote_symref(struct ref *ref, struct ref *list);
 int ref_newer(const unsigned char *new_sha1, const unsigned char *old_sha1);
diff --git a/send-pack.c b/send-pack.c
index ac14a4d090..6129b0fd8e 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -10,6 +10,7 @@
 #include "quote.h"
 #include "transport.h"
 #include "version.h"
+#include "sha1-array.h"
 
 static int feed_object(const unsigned char *sha1, int fd, int negative)
 {
@@ -28,7 +29,7 @@ static int feed_object(const unsigned char *sha1, int fd, int negative)
 /*
  * Make a pack stream and spit it out into file descriptor fd
  */
-static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *extra, struct send_pack_args *args)
+static int pack_objects(int fd, struct ref *refs, struct sha1_array *extra, struct send_pack_args *args)
 {
 	/*
 	 * The child becomes pack-objects --revs; we feed
@@ -71,7 +72,7 @@ static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *ext
 	 * parameters by writing to the pipe.
 	 */
 	for (i = 0; i < extra->nr; i++)
-		if (!feed_object(extra->array[i], po.in, 1))
+		if (!feed_object(extra->sha1[i], po.in, 1))
 			break;
 
 	while (refs) {
@@ -174,10 +175,25 @@ static int sideband_demux(int in, int out, void *data)
 	return ret;
 }
 
+static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *cb)
+{
+	struct strbuf *sb = cb;
+	if (graft->nr_parent == -1)
+		packet_buf_write(sb, "shallow %s\n", sha1_to_hex(graft->sha1));
+	return 0;
+}
+
+static void advertise_shallow_grafts_buf(struct strbuf *sb)
+{
+	if (!is_repository_shallow())
+		return;
+	for_each_commit_graft(advertise_shallow_grafts_cb, sb);
+}
+
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
-	      struct extra_have_objects *extra_have)
+	      struct sha1_array *extra_have)
 {
 	int in = fd[0];
 	int out = fd[1];
@@ -215,6 +231,9 @@ int send_pack(struct send_pack_args *args,
 		return 0;
 	}
 
+	if (!args->dry_run)
+		advertise_shallow_grafts_buf(&req_buf);
+
 	/*
 	 * Finally, tell the other end!
 	 */
@@ -274,7 +293,7 @@ int send_pack(struct send_pack_args *args,
 	}
 
 	if (args->stateless_rpc) {
-		if (!args->dry_run && cmds_sent) {
+		if (!args->dry_run && (cmds_sent || is_repository_shallow())) {
 			packet_buf_flush(&req_buf);
 			send_sideband(out, -1, req_buf.buf, req_buf.len, LARGE_PACKET_MAX);
 		}
diff --git a/send-pack.h b/send-pack.h
index 05d7ab118b..8e843924cf 100644
--- a/send-pack.h
+++ b/send-pack.h
@@ -16,6 +16,6 @@ struct send_pack_args {
 
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
-	      struct ref *remote_refs, struct extra_have_objects *extra_have);
+	      struct ref *remote_refs, struct sha1_array *extra_have);
 
 #endif
diff --git a/shallow.c b/shallow.c
index 961cf6f024..bbc98b55c0 100644
--- a/shallow.c
+++ b/shallow.c
@@ -2,15 +2,23 @@
 #include "commit.h"
 #include "tag.h"
 #include "pkt-line.h"
+#include "remote.h"
+#include "refs.h"
+#include "sha1-array.h"
+#include "diff.h"
+#include "revision.h"
+#include "commit-slab.h"
 
 static int is_shallow = -1;
 static struct stat shallow_stat;
 static char *alternate_shallow_file;
 
-void set_alternate_shallow_file(const char *path)
+void set_alternate_shallow_file(const char *path, int override)
 {
 	if (is_shallow != -1)
 		die("BUG: is_repository_shallow must not be called before set_alternate_shallow_file");
+	if (alternate_shallow_file && !override)
+		return;
 	free(alternate_shallow_file);
 	alternate_shallow_file = path ? xstrdup(path) : NULL;
 }
@@ -69,6 +77,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 	struct commit_list *result = NULL;
 	struct object_array stack = OBJECT_ARRAY_INIT;
 	struct commit *commit = NULL;
+	struct commit_graft *graft;
 
 	while (commit || i < heads->nr || stack.nr) {
 		struct commit_list *p;
@@ -92,7 +101,10 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 		}
 		parse_commit_or_die(commit);
 		cur_depth++;
-		if (cur_depth >= depth) {
+		if ((depth != INFINITE_DEPTH && cur_depth >= depth) ||
+		    (is_repository_shallow() && !commit->parents &&
+		     (graft = lookup_commit_graft(commit->object.sha1)) != NULL &&
+		     graft->nr_parent < 0)) {
 			commit_list_insert(commit, &result);
 			commit->object.flags |= shallow_flag;
 			commit = NULL;
@@ -142,10 +154,14 @@ void check_shallow_file_for_update(void)
 		die("shallow file was changed during fetch");
 }
 
+#define SEEN_ONLY 1
+#define VERBOSE   2
+
 struct write_shallow_data {
 	struct strbuf *out;
 	int use_pack_protocol;
 	int count;
+	unsigned flags;
 };
 
 static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
@@ -154,6 +170,15 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 	const char *hex = sha1_to_hex(graft->sha1);
 	if (graft->nr_parent != -1)
 		return 0;
+	if (data->flags & SEEN_ONLY) {
+		struct commit *c = lookup_commit(graft->sha1);
+		if (!c || !(c->object.flags & SEEN)) {
+			if (data->flags & VERBOSE)
+				printf("Removing %s from .git/shallow\n",
+				       sha1_to_hex(c->object.sha1));
+			return 0;
+		}
+	}
 	data->count++;
 	if (data->use_pack_protocol)
 		packet_buf_write(data->out, "shallow %s", hex);
@@ -164,22 +189,39 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 	return 0;
 }
 
-int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
+static int write_shallow_commits_1(struct strbuf *out, int use_pack_protocol,
+				   const struct sha1_array *extra,
+				   unsigned flags)
 {
 	struct write_shallow_data data;
+	int i;
 	data.out = out;
 	data.use_pack_protocol = use_pack_protocol;
 	data.count = 0;
+	data.flags = flags;
 	for_each_commit_graft(write_one_shallow, &data);
+	if (!extra)
+		return data.count;
+	for (i = 0; i < extra->nr; i++) {
+		strbuf_addstr(out, sha1_to_hex(extra->sha1[i]));
+		strbuf_addch(out, '\n');
+		data.count++;
+	}
 	return data.count;
 }
 
-char *setup_temporary_shallow(void)
+int write_shallow_commits(struct strbuf *out, int use_pack_protocol,
+			  const struct sha1_array *extra)
+{
+	return write_shallow_commits_1(out, use_pack_protocol, extra, 0);
+}
+
+char *setup_temporary_shallow(const struct sha1_array *extra)
 {
 	struct strbuf sb = STRBUF_INIT;
 	int fd;
 
-	if (write_shallow_commits(&sb, 0)) {
+	if (write_shallow_commits(&sb, 0, extra)) {
 		struct strbuf path = STRBUF_INIT;
 		strbuf_addstr(&path, git_path("shallow_XXXXXX"));
 		fd = xmkstemp(path.buf);
@@ -198,7 +240,8 @@ char *setup_temporary_shallow(void)
 }
 
 void setup_alternate_shallow(struct lock_file *shallow_lock,
-			     const char **alternate_shallow_file)
+			     const char **alternate_shallow_file,
+			     const struct sha1_array *extra)
 {
 	struct strbuf sb = STRBUF_INIT;
 	int fd;
@@ -206,7 +249,7 @@ void setup_alternate_shallow(struct lock_file *shallow_lock,
 	check_shallow_file_for_update();
 	fd = hold_lock_file_for_update(shallow_lock, git_path("shallow"),
 				       LOCK_DIE_ON_ERROR);
-	if (write_shallow_commits(&sb, 0)) {
+	if (write_shallow_commits(&sb, 0, extra)) {
 		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
 			die_errno("failed to write to %s",
 				  shallow_lock->filename);
@@ -219,3 +262,415 @@ void setup_alternate_shallow(struct lock_file *shallow_lock,
 		*alternate_shallow_file = "";
 	strbuf_release(&sb);
 }
+
+static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *cb)
+{
+	int fd = *(int *)cb;
+	if (graft->nr_parent == -1)
+		packet_write(fd, "shallow %s\n", sha1_to_hex(graft->sha1));
+	return 0;
+}
+
+void advertise_shallow_grafts(int fd)
+{
+	if (!is_repository_shallow())
+		return;
+	for_each_commit_graft(advertise_shallow_grafts_cb, &fd);
+}
+
+/*
+ * mark_reachable_objects() should have been run prior to this and all
+ * reachable commits marked as "SEEN".
+ */
+void prune_shallow(int show_only)
+{
+	static struct lock_file shallow_lock;
+	struct strbuf sb = STRBUF_INIT;
+	int fd;
+
+	if (show_only) {
+		write_shallow_commits_1(&sb, 0, NULL, SEEN_ONLY | VERBOSE);
+		strbuf_release(&sb);
+		return;
+	}
+	check_shallow_file_for_update();
+	fd = hold_lock_file_for_update(&shallow_lock, git_path("shallow"),
+				       LOCK_DIE_ON_ERROR);
+	if (write_shallow_commits_1(&sb, 0, NULL, SEEN_ONLY)) {
+		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
+			die_errno("failed to write to %s",
+				  shallow_lock.filename);
+		commit_lock_file(&shallow_lock);
+	} else {
+		unlink(git_path("shallow"));
+		rollback_lock_file(&shallow_lock);
+	}
+	strbuf_release(&sb);
+}
+
+#define TRACE_KEY "GIT_TRACE_SHALLOW"
+
+/*
+ * Step 1, split sender shallow commits into "ours" and "theirs"
+ * Step 2, clean "ours" based on .git/shallow
+ */
+void prepare_shallow_info(struct shallow_info *info, struct sha1_array *sa)
+{
+	int i;
+	trace_printf_key(TRACE_KEY, "shallow: prepare_shallow_info\n");
+	memset(info, 0, sizeof(*info));
+	info->shallow = sa;
+	if (!sa)
+		return;
+	info->ours = xmalloc(sizeof(*info->ours) * sa->nr);
+	info->theirs = xmalloc(sizeof(*info->theirs) * sa->nr);
+	for (i = 0; i < sa->nr; i++) {
+		if (has_sha1_file(sa->sha1[i])) {
+			struct commit_graft *graft;
+			graft = lookup_commit_graft(sa->sha1[i]);
+			if (graft && graft->nr_parent < 0)
+				continue;
+			info->ours[info->nr_ours++] = i;
+		} else
+			info->theirs[info->nr_theirs++] = i;
+	}
+}
+
+void clear_shallow_info(struct shallow_info *info)
+{
+	free(info->ours);
+	free(info->theirs);
+}
+
+/* Step 4, remove non-existent ones in "theirs" after getting the pack */
+
+void remove_nonexistent_theirs_shallow(struct shallow_info *info)
+{
+	unsigned char (*sha1)[20] = info->shallow->sha1;
+	int i, dst;
+	trace_printf_key(TRACE_KEY, "shallow: remove_nonexistent_theirs_shallow\n");
+	for (i = dst = 0; i < info->nr_theirs; i++) {
+		if (i != dst)
+			info->theirs[dst] = info->theirs[i];
+		if (has_sha1_file(sha1[info->theirs[i]]))
+			dst++;
+	}
+	info->nr_theirs = dst;
+}
+
+define_commit_slab(ref_bitmap, uint32_t *);
+
+struct paint_info {
+	struct ref_bitmap ref_bitmap;
+	unsigned nr_bits;
+	char **slab;
+	char *free, *end;
+	unsigned slab_count;
+};
+
+static uint32_t *paint_alloc(struct paint_info *info)
+{
+	unsigned nr = (info->nr_bits + 31) / 32;
+	unsigned size = nr * sizeof(uint32_t);
+	void *p;
+	if (!info->slab_count || info->free + size > info->end) {
+		info->slab_count++;
+		info->slab = xrealloc(info->slab,
+				      info->slab_count * sizeof(*info->slab));
+		info->free = xmalloc(COMMIT_SLAB_SIZE);
+		info->slab[info->slab_count - 1] = info->free;
+		info->end = info->free + COMMIT_SLAB_SIZE;
+	}
+	p = info->free;
+	info->free += size;
+	return p;
+}
+
+/*
+ * Given a commit SHA-1, walk down to parents until either SEEN,
+ * UNINTERESTING or BOTTOM is hit. Set the id-th bit in ref_bitmap for
+ * all walked commits.
+ */
+static void paint_down(struct paint_info *info, const unsigned char *sha1,
+		       int id)
+{
+	unsigned int i, nr;
+	struct commit_list *head = NULL;
+	int bitmap_nr = (info->nr_bits + 31) / 32;
+	int bitmap_size = bitmap_nr * sizeof(uint32_t);
+	uint32_t *tmp = xmalloc(bitmap_size); /* to be freed before return */
+	uint32_t *bitmap = paint_alloc(info);
+	struct commit *c = lookup_commit_reference_gently(sha1, 1);
+	if (!c)
+		return;
+	memset(bitmap, 0, bitmap_size);
+	bitmap[id / 32] |= (1 << (id % 32));
+	commit_list_insert(c, &head);
+	while (head) {
+		struct commit_list *p;
+		struct commit *c = head->item;
+		uint32_t **refs = ref_bitmap_at(&info->ref_bitmap, c);
+
+		p = head;
+		head = head->next;
+		free(p);
+
+		/* XXX check "UNINTERESTING" from pack bitmaps if available */
+		if (c->object.flags & (SEEN | UNINTERESTING))
+			continue;
+		else
+			c->object.flags |= SEEN;
+
+		if (*refs == NULL)
+			*refs = bitmap;
+		else {
+			memcpy(tmp, *refs, bitmap_size);
+			for (i = 0; i < bitmap_nr; i++)
+				tmp[i] |= bitmap[i];
+			if (memcmp(tmp, *refs, bitmap_size)) {
+				*refs = paint_alloc(info);
+				memcpy(*refs, tmp, bitmap_size);
+			}
+		}
+
+		if (c->object.flags & BOTTOM)
+			continue;
+
+		if (parse_commit(c))
+			die("unable to parse commit %s",
+			    sha1_to_hex(c->object.sha1));
+
+		for (p = c->parents; p; p = p->next) {
+			uint32_t **p_refs = ref_bitmap_at(&info->ref_bitmap,
+							  p->item);
+			if (p->item->object.flags & SEEN)
+				continue;
+			if (*p_refs == NULL || *p_refs == *refs)
+				*p_refs = *refs;
+			commit_list_insert(p->item, &head);
+		}
+	}
+
+	nr = get_max_object_index();
+	for (i = 0; i < nr; i++) {
+		struct object *o = get_indexed_object(i);
+		if (o && o->type == OBJ_COMMIT)
+			o->flags &= ~SEEN;
+	}
+
+	free(tmp);
+}
+
+static int mark_uninteresting(const char *refname,
+			      const unsigned char *sha1,
+			      int flags, void *cb_data)
+{
+	struct commit *commit = lookup_commit_reference_gently(sha1, 1);
+	if (!commit)
+		return 0;
+	commit->object.flags |= UNINTERESTING;
+	mark_parents_uninteresting(commit);
+	return 0;
+}
+
+static void post_assign_shallow(struct shallow_info *info,
+				struct ref_bitmap *ref_bitmap,
+				int *ref_status);
+/*
+ * Step 6(+7), associate shallow commits with new refs
+ *
+ * info->ref must be initialized before calling this function.
+ *
+ * If used is not NULL, it's an array of info->shallow->nr
+ * bitmaps. The n-th bit set in the m-th bitmap if ref[n] needs the
+ * m-th shallow commit from info->shallow.
+ *
+ * If used is NULL, "ours" and "theirs" are updated. And if ref_status
+ * is not NULL it's an array of ref->nr ints. ref_status[i] is true if
+ * the ref needs some shallow commits from either info->ours or
+ * info->theirs.
+ */
+void assign_shallow_commits_to_refs(struct shallow_info *info,
+				    uint32_t **used, int *ref_status)
+{
+	unsigned char (*sha1)[20] = info->shallow->sha1;
+	struct sha1_array *ref = info->ref;
+	unsigned int i, nr;
+	int *shallow, nr_shallow = 0;
+	struct paint_info pi;
+
+	trace_printf_key(TRACE_KEY, "shallow: assign_shallow_commits_to_refs\n");
+	shallow = xmalloc(sizeof(*shallow) * (info->nr_ours + info->nr_theirs));
+	for (i = 0; i < info->nr_ours; i++)
+		shallow[nr_shallow++] = info->ours[i];
+	for (i = 0; i < info->nr_theirs; i++)
+		shallow[nr_shallow++] = info->theirs[i];
+
+	/*
+	 * Prepare the commit graph to track what refs can reach what
+	 * (new) shallow commits.
+	 */
+	nr = get_max_object_index();
+	for (i = 0; i < nr; i++) {
+		struct object *o = get_indexed_object(i);
+		if (!o || o->type != OBJ_COMMIT)
+			continue;
+
+		o->flags &= ~(UNINTERESTING | BOTTOM | SEEN);
+	}
+
+	memset(&pi, 0, sizeof(pi));
+	init_ref_bitmap(&pi.ref_bitmap);
+	pi.nr_bits = ref->nr;
+
+	/*
+	 * "--not --all" to cut short the traversal if new refs
+	 * connect to old refs. If not (e.g. force ref updates) it'll
+	 * have to go down to the current shallow commits.
+	 */
+	head_ref(mark_uninteresting, NULL);
+	for_each_ref(mark_uninteresting, NULL);
+
+	/* Mark potential bottoms so we won't go out of bound */
+	for (i = 0; i < nr_shallow; i++) {
+		struct commit *c = lookup_commit(sha1[shallow[i]]);
+		c->object.flags |= BOTTOM;
+	}
+
+	for (i = 0; i < ref->nr; i++)
+		paint_down(&pi, ref->sha1[i], i);
+
+	if (used) {
+		int bitmap_size = ((pi.nr_bits + 31) / 32) * sizeof(uint32_t);
+		memset(used, 0, sizeof(*used) * info->shallow->nr);
+		for (i = 0; i < nr_shallow; i++) {
+			const struct commit *c = lookup_commit(sha1[shallow[i]]);
+			uint32_t **map = ref_bitmap_at(&pi.ref_bitmap, c);
+			if (*map)
+				used[shallow[i]] = xmemdupz(*map, bitmap_size);
+		}
+		/*
+		 * unreachable shallow commits are not removed from
+		 * "ours" and "theirs". The user is supposed to run
+		 * step 7 on every ref separately and not trust "ours"
+		 * and "theirs" any more.
+		 */
+	} else
+		post_assign_shallow(info, &pi.ref_bitmap, ref_status);
+
+	clear_ref_bitmap(&pi.ref_bitmap);
+	for (i = 0; i < pi.slab_count; i++)
+		free(pi.slab[i]);
+	free(pi.slab);
+	free(shallow);
+}
+
+struct commit_array {
+	struct commit **commits;
+	int nr, alloc;
+};
+
+static int add_ref(const char *refname,
+		   const unsigned char *sha1, int flags, void *cb_data)
+{
+	struct commit_array *ca = cb_data;
+	ALLOC_GROW(ca->commits, ca->nr + 1, ca->alloc);
+	ca->commits[ca->nr] = lookup_commit_reference_gently(sha1, 1);
+	if (ca->commits[ca->nr])
+		ca->nr++;
+	return 0;
+}
+
+static void update_refstatus(int *ref_status, int nr, uint32_t *bitmap)
+{
+	int i;
+	if (!ref_status)
+		return;
+	for (i = 0; i < nr; i++)
+		if (bitmap[i / 32] & (1 << (i % 32)))
+			ref_status[i]++;
+}
+
+/*
+ * Step 7, reachability test on "ours" at commit level
+ */
+static void post_assign_shallow(struct shallow_info *info,
+				struct ref_bitmap *ref_bitmap,
+				int *ref_status)
+{
+	unsigned char (*sha1)[20] = info->shallow->sha1;
+	struct commit *c;
+	uint32_t **bitmap;
+	int dst, i, j;
+	int bitmap_nr = (info->ref->nr + 31) / 32;
+	struct commit_array ca;
+
+	trace_printf_key(TRACE_KEY, "shallow: post_assign_shallow\n");
+	if (ref_status)
+		memset(ref_status, 0, sizeof(*ref_status) * info->ref->nr);
+
+	/* Remove unreachable shallow commits from "theirs" */
+	for (i = dst = 0; i < info->nr_theirs; i++) {
+		if (i != dst)
+			info->theirs[dst] = info->theirs[i];
+		c = lookup_commit(sha1[info->theirs[i]]);
+		bitmap = ref_bitmap_at(ref_bitmap, c);
+		if (!*bitmap)
+			continue;
+		for (j = 0; j < bitmap_nr; j++)
+			if (bitmap[0][j]) {
+				update_refstatus(ref_status, info->ref->nr, *bitmap);
+				dst++;
+				break;
+			}
+	}
+	info->nr_theirs = dst;
+
+	memset(&ca, 0, sizeof(ca));
+	head_ref(add_ref, &ca);
+	for_each_ref(add_ref, &ca);
+
+	/* Remove unreachable shallow commits from "ours" */
+	for (i = dst = 0; i < info->nr_ours; i++) {
+		if (i != dst)
+			info->ours[dst] = info->ours[i];
+		c = lookup_commit(sha1[info->ours[i]]);
+		bitmap = ref_bitmap_at(ref_bitmap, c);
+		if (!*bitmap)
+			continue;
+		for (j = 0; j < bitmap_nr; j++)
+			if (bitmap[0][j] &&
+			    /* Step 7, reachability test at commit level */
+			    !in_merge_bases_many(c, ca.nr, ca.commits)) {
+				update_refstatus(ref_status, info->ref->nr, *bitmap);
+				dst++;
+				break;
+			}
+	}
+	info->nr_ours = dst;
+
+	free(ca.commits);
+}
+
+/* (Delayed) step 7, reachability test at commit level */
+int delayed_reachability_test(struct shallow_info *si, int c)
+{
+	if (si->need_reachability_test[c]) {
+		struct commit *commit = lookup_commit(si->shallow->sha1[c]);
+
+		if (!si->commits) {
+			struct commit_array ca;
+			memset(&ca, 0, sizeof(ca));
+			head_ref(add_ref, &ca);
+			for_each_ref(add_ref, &ca);
+			si->commits = ca.commits;
+			si->nr_commits = ca.nr;
+		}
+
+		si->reachable[c] = in_merge_bases_many(commit,
+						       si->nr_commits,
+						       si->commits);
+		si->need_reachability_test[c] = 0;
+	}
+	return si->reachable[c];
+}
diff --git a/t/t5304-prune.sh b/t/t5304-prune.sh
index e4bb3a1457..66c9a41739 100755
--- a/t/t5304-prune.sh
+++ b/t/t5304-prune.sh
@@ -221,4 +221,14 @@ EOF
 	test_cmp expected actual
 '
 
+test_expect_success 'prune .git/shallow' '
+	SHA1=`echo hi|git commit-tree HEAD^{tree}` &&
+	echo $SHA1 >.git/shallow &&
+	git prune --dry-run >out &&
+	grep $SHA1 .git/shallow &&
+	grep $SHA1 out &&
+	git prune &&
+	! test -f .git/shallow
+'
+
 test_done
diff --git a/t/t5537-fetch-shallow.sh b/t/t5537-fetch-shallow.sh
new file mode 100755
index 0000000000..b0fa7387cb
--- /dev/null
+++ b/t/t5537-fetch-shallow.sh
@@ -0,0 +1,204 @@
+#!/bin/sh
+
+test_description='fetch/clone from a shallow clone'
+
+. ./test-lib.sh
+
+commit() {
+	echo "$1" >tracked &&
+	git add tracked &&
+	git commit -m "$1"
+}
+
+test_expect_success 'setup' '
+	commit 1 &&
+	commit 2 &&
+	commit 3 &&
+	commit 4 &&
+	git config --global transfer.fsckObjects true
+'
+
+test_expect_success 'setup shallow clone' '
+	git clone --no-local --depth=2 .git shallow &&
+	git --git-dir=shallow/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'clone from shallow clone' '
+	git clone --no-local shallow shallow2 &&
+	(
+	cd shallow2 &&
+	git fsck &&
+	git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch from shallow clone' '
+	(
+	cd shallow &&
+	commit 5
+	) &&
+	(
+	cd shallow2 &&
+	git fetch &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch --depth from shallow clone' '
+	(
+	cd shallow &&
+	commit 6
+	) &&
+	(
+	cd shallow2 &&
+	git fetch --depth=2 &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+6
+5
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch --unshallow from shallow clone' '
+	(
+	cd shallow2 &&
+	git fetch --unshallow &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+6
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch something upstream has but hidden by clients shallow boundaries' '
+	# the blob "1" is available in .git but hidden by the
+	# shallow2/.git/shallow and it should be resent
+	! git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` >/dev/null &&
+	echo 1 >1.t &&
+	git add 1.t &&
+	git commit -m add-1-back &&
+	(
+	cd shallow2 &&
+	git fetch ../.git +refs/heads/master:refs/remotes/top/master &&
+	git fsck &&
+	git log --format=%s top/master >actual &&
+	cat <<EOF >expect &&
+add-1-back
+4
+3
+EOF
+	test_cmp expect actual
+	) &&
+	git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` >/dev/null
+
+'
+
+test_expect_success 'fetch that requires changes in .git/shallow is filtered' '
+	(
+	cd shallow &&
+	git checkout --orphan no-shallow &&
+	commit no-shallow
+	) &&
+	git init notshallow &&
+	(
+	cd notshallow &&
+	git fetch ../shallow/.git refs/heads/*:refs/remotes/shallow/*&&
+	git for-each-ref --format="%(refname)" >actual.refs &&
+	cat <<EOF >expect.refs &&
+refs/remotes/shallow/no-shallow
+EOF
+	test_cmp expect.refs actual.refs &&
+	git log --format=%s shallow/no-shallow >actual &&
+	cat <<EOF >expect &&
+no-shallow
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch --update-shallow' '
+	(
+	cd shallow &&
+	git checkout master &&
+	commit 7 &&
+	git tag -m foo heavy-tag HEAD^ &&
+	git tag light-tag HEAD^:tracked
+	) &&
+	(
+	cd notshallow &&
+	git fetch --update-shallow ../shallow/.git refs/heads/*:refs/remotes/shallow/* &&
+	git fsck &&
+	git for-each-ref --sort=refname --format="%(refname)" >actual.refs &&
+	cat <<EOF >expect.refs &&
+refs/remotes/shallow/master
+refs/remotes/shallow/no-shallow
+refs/tags/heavy-tag
+refs/tags/light-tag
+EOF
+	test_cmp expect.refs actual.refs &&
+	git log --format=%s shallow/master >actual &&
+	cat <<EOF >expect &&
+7
+6
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+if test -n "$NO_CURL" -o -z "$GIT_TEST_HTTPD"; then
+	say 'skipping remaining tests, git built without http support'
+	test_done
+fi
+
+LIB_HTTPD_PORT=${LIB_HTTPD_PORT-'5537'}
+. "$TEST_DIRECTORY"/lib-httpd.sh
+start_httpd
+
+test_expect_success 'clone http repository' '
+	git clone --bare --no-local shallow "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git clone $HTTPD_URL/smart/repo.git clone &&
+	(
+	cd clone &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+7
+6
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+stop_httpd
+test_done
diff --git a/t/t5538-push-shallow.sh b/t/t5538-push-shallow.sh
new file mode 100755
index 0000000000..0a6e40f144
--- /dev/null
+++ b/t/t5538-push-shallow.sh
@@ -0,0 +1,183 @@
+#!/bin/sh
+
+test_description='push from/to a shallow clone'
+
+. ./test-lib.sh
+
+commit() {
+	echo "$1" >tracked &&
+	git add tracked &&
+	git commit -m "$1"
+}
+
+test_expect_success 'setup' '
+	git config --global transfer.fsckObjects true &&
+	commit 1 &&
+	commit 2 &&
+	commit 3 &&
+	commit 4 &&
+	git clone . full &&
+	(
+	git init full-abc &&
+	cd full-abc &&
+	commit a &&
+	commit b &&
+	commit c
+	) &&
+	git clone --no-local --depth=2 .git shallow &&
+	git --git-dir=shallow/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual &&
+	git clone --no-local --depth=2 full-abc/.git shallow2 &&
+	git --git-dir=shallow2/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+c
+b
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'push from shallow clone' '
+	(
+	cd shallow &&
+	commit 5 &&
+	git push ../.git +master:refs/remotes/shallow/master
+	) &&
+	git log --format=%s shallow/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+5
+4
+3
+2
+1
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'push from shallow clone, with grafted roots' '
+	(
+	cd shallow2 &&
+	test_must_fail git push ../.git +master:refs/remotes/shallow2/master 2>err &&
+	grep "shallow2/master.*shallow update not allowed" err
+	) &&
+	test_must_fail git rev-parse shallow2/master &&
+	git fsck
+'
+
+test_expect_success 'add new shallow root with receive.updateshallow on' '
+	test_config receive.shallowupdate true &&
+	(
+	cd shallow2 &&
+	git push ../.git +master:refs/remotes/shallow2/master
+	) &&
+	git log --format=%s shallow2/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+c
+b
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'push from shallow to shallow' '
+	(
+	cd shallow &&
+	git --git-dir=../shallow2/.git config receive.shallowupdate true &&
+	git push ../shallow2/.git +master:refs/remotes/shallow/master &&
+	git --git-dir=../shallow2/.git config receive.shallowupdate false
+	) &&
+	(
+	cd shallow2 &&
+	git log --format=%s shallow/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'push from full to shallow' '
+	! git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` &&
+	commit 1 &&
+	git push shallow2/.git +master:refs/remotes/top/master &&
+	(
+	cd shallow2 &&
+	git log --format=%s top/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+1
+4
+3
+EOF
+	test_cmp expect actual &&
+	git cat-file blob `echo 1|git hash-object --stdin` >/dev/null
+	)
+'
+
+if test -n "$NO_CURL" -o -z "$GIT_TEST_HTTPD"; then
+	say 'skipping remaining tests, git built without http support'
+	test_done
+fi
+
+LIB_HTTPD_PORT=${LIB_HTTPD_PORT-'5537'}
+. "$TEST_DIRECTORY"/lib-httpd.sh
+start_httpd
+
+test_expect_success 'push to shallow repo via http' '
+	git clone --bare --no-local shallow "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git config http.receivepack true
+	) &&
+	(
+	cd full &&
+	commit 9 &&
+	git push $HTTPD_URL/smart/repo.git +master:refs/remotes/top/master
+	) &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git fsck &&
+	git log --format=%s top/master >actual &&
+	cat <<EOF >expect &&
+9
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'push from shallow repo via http' '
+	mv "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" shallow-upstream.git &&
+	git clone --bare --no-local full "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git config http.receivepack true
+	) &&
+	commit 10 &&
+	git push $HTTPD_URL/smart/repo.git +master:refs/remotes/top/master &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git fsck &&
+	git log --format=%s top/master >actual &&
+	cat <<EOF >expect &&
+10
+1
+4
+3
+2
+1
+EOF
+	test_cmp expect actual
+	)
+'
+
+stop_httpd
+test_done
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index 62fbd7e664..5e67035be8 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -430,4 +430,11 @@ test_expect_success 'clone from a repository with two identical branches' '
 
 '
 
+test_expect_success 'shallow clone locally' '
+	git clone --depth=1 --no-local src ssrrcc &&
+	git clone ssrrcc ddsstt &&
+	test_cmp ssrrcc/.git/shallow ddsstt/.git/shallow &&
+	( cd ddsstt && git fsck )
+'
+
 test_done
diff --git a/trace.c b/trace.c
index 3d744d1d4d..08180a90bc 100644
--- a/trace.c
+++ b/trace.c
@@ -76,7 +76,7 @@ static void trace_vprintf(const char *key, const char *fmt, va_list ap)
 }
 
 __attribute__((format (printf, 2, 3)))
-static void trace_printf_key(const char *key, const char *fmt, ...)
+void trace_printf_key(const char *key, const char *fmt, ...)
 {
 	va_list ap;
 	va_start(ap, fmt);
diff --git a/transport-helper.c b/transport-helper.c
index 2010674bb4..087f617d39 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -360,6 +360,12 @@ static int fetch_with_fetch(struct transport *transport,
 	    data->transport_options.check_self_contained_and_connected)
 		set_helper_option(transport, "check-connectivity", "true");
 
+	if (transport->cloning)
+		set_helper_option(transport, "cloning", "true");
+
+	if (data->transport_options.update_shallow)
+		set_helper_option(transport, "update-shallow", "true");
+
 	for (i = 0; i < nr_heads; i++) {
 		const struct ref *posn = to_fetch[i];
 		if (posn->status & REF_STATUS_UPTODATE)
diff --git a/transport.c b/transport.c
index 824c5b93f9..ca7bb441bf 100644
--- a/transport.c
+++ b/transport.c
@@ -14,6 +14,7 @@
 #include "url.h"
 #include "submodule.h"
 #include "string-list.h"
+#include "sha1-array.h"
 
 /* rsync support */
 
@@ -454,7 +455,8 @@ struct git_transport_data {
 	struct child_process *conn;
 	int fd[2];
 	unsigned got_remote_heads : 1;
-	struct extra_have_objects extra_have;
+	struct sha1_array extra_have;
+	struct sha1_array shallow;
 };
 
 static int set_git_option(struct git_transport_options *opts,
@@ -475,6 +477,9 @@ static int set_git_option(struct git_transport_options *opts,
 	} else if (!strcmp(name, TRANS_OPT_KEEP)) {
 		opts->keep = !!value;
 		return 0;
+	} else if (!strcmp(name, TRANS_OPT_UPDATE_SHALLOW)) {
+		opts->update_shallow = !!value;
+		return 0;
 	} else if (!strcmp(name, TRANS_OPT_DEPTH)) {
 		if (!value)
 			opts->depth = 0;
@@ -511,7 +516,9 @@ static struct ref *get_refs_via_connect(struct transport *transport, int for_pus
 
 	connect_setup(transport, for_push, 0);
 	get_remote_heads(data->fd[0], NULL, 0, &refs,
-			 for_push ? REF_NORMAL : 0, &data->extra_have);
+			 for_push ? REF_NORMAL : 0,
+			 &data->extra_have,
+			 &data->shallow);
 	data->got_remote_heads = 1;
 
 	return refs;
@@ -538,16 +545,19 @@ static int fetch_refs_via_pack(struct transport *transport,
 	args.depth = data->options.depth;
 	args.check_self_contained_and_connected =
 		data->options.check_self_contained_and_connected;
+	args.cloning = transport->cloning;
+	args.update_shallow = data->options.update_shallow;
 
 	if (!data->got_remote_heads) {
 		connect_setup(transport, 0, 0);
-		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0,
+				 NULL, &data->shallow);
 		data->got_remote_heads = 1;
 	}
 
 	refs = fetch_pack(&args, data->fd, data->conn,
 			  refs_tmp ? refs_tmp : transport->remote_refs,
-			  dest, to_fetch, nr_heads,
+			  dest, to_fetch, nr_heads, &data->shallow,
 			  &transport->pack_lockfile);
 	close(data->fd[0]);
 	close(data->fd[1]);
@@ -713,6 +723,10 @@ static int print_one_push_status(struct ref *ref, const char *dest, int count, i
 		print_ref_status('!', "[rejected]", ref, ref->peer_ref,
 						 "stale info", porcelain);
 		break;
+	case REF_STATUS_REJECT_SHALLOW:
+		print_ref_status('!', "[rejected]", ref, ref->peer_ref,
+						 "new shallow roots not allowed", porcelain);
+		break;
 	case REF_STATUS_REMOTE_REJECT:
 		print_ref_status('!', "[remote rejected]", ref,
 						 ref->deletion ? NULL : ref->peer_ref,
@@ -805,7 +819,8 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 		struct ref *tmp_refs;
 		connect_setup(transport, 1, 0);
 
-		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL,
+				 NULL, &data->shallow);
 		data->got_remote_heads = 1;
 	}
 
diff --git a/transport.h b/transport.h
index 8f96bed775..02ea248db1 100644
--- a/transport.h
+++ b/transport.h
@@ -11,6 +11,7 @@ struct git_transport_options {
 	unsigned followtags : 1;
 	unsigned check_self_contained_and_connected : 1;
 	unsigned self_contained_and_connected : 1;
+	unsigned update_shallow : 1;
 	int depth;
 	const char *uploadpack;
 	const char *receivepack;
@@ -35,6 +36,12 @@ struct transport {
 	 */
 	unsigned cannot_reuse : 1;
 
+	/*
+	 * A hint from caller that it will be performing a clone, not
+	 * normal fetch. IOW the repository is guaranteed empty.
+	 */
+	unsigned cloning : 1;
+
 	/**
 	 * Returns 0 if successful, positive if the option is not
 	 * recognized or is inapplicable, and negative if the option
@@ -146,6 +153,9 @@ struct transport *transport_get(struct remote *, const char *);
 /* Aggressively fetch annotated tags if possible */
 #define TRANS_OPT_FOLLOWTAGS "followtags"
 
+/* Accept refs that may update .git/shallow without --depth */
+#define TRANS_OPT_UPDATE_SHALLOW "updateshallow"
+
 /**
  * Returns 0 if the option was used, non-zero otherwise. Prints a
  * message to stderr if the option is not used.
@@ -193,10 +203,4 @@ void transport_print_push_status(const char *dest, struct ref *refs,
 
 typedef void alternate_ref_fn(const struct ref *, void *);
 extern void for_each_alternate_ref(alternate_ref_fn, void *);
-
-struct send_pack_args;
-extern int send_pack(struct send_pack_args *args,
-		     int fd[], struct child_process *conn,
-		     struct ref *remote_refs,
-		     struct extra_have_objects *extra_have);
 #endif
diff --git a/upload-pack.c b/upload-pack.c
index ec56cdbce9..0c44f6b292 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -84,7 +84,7 @@ static void create_pack_file(void)
 	char *shallow_file = NULL;
 
 	if (shallow_nr) {
-		shallow_file = setup_temporary_shallow();
+		shallow_file = setup_temporary_shallow(NULL);
 		argv[arg++] = "--shallow-file";
 		argv[arg++] = shallow_file;
 	}
@@ -619,7 +619,7 @@ static void receive_needs(void)
 	if (depth > 0) {
 		struct commit_list *result = NULL, *backup = NULL;
 		int i;
-		if (depth == INFINITE_DEPTH)
+		if (depth == INFINITE_DEPTH && !is_repository_shallow())
 			for (i = 0; i < shallows.nr; i++) {
 				struct object *object = shallows.objects[i].item;
 				object->flags |= NOT_SHALLOW;
@@ -757,6 +757,7 @@ static void upload_pack(void)
 		reset_timeout();
 		head_ref_namespaced(send_ref, &symref);
 		for_each_namespaced_ref(send_ref, &symref);
+		advertise_shallow_grafts(1);
 		packet_flush(1);
 	} else {
 		head_ref_namespaced(mark_our_ref, NULL);
@@ -834,8 +835,7 @@ int main(int argc, char **argv)
 
 	if (!enter_repo(dir, strict))
 		die("'%s' does not appear to be a git repository", dir);
-	if (is_repository_shallow())
-		die("attempt to fetch/clone from a shallow repository");
+
 	git_config(upload_pack_config, NULL);
 	upload_pack();
 	return 0;