pack-objects: finishing touches.

This introduces --no-reuse-delta option to disable reusing of
existing delta, which is a large part of the optimization
introduced by this series.  This may become necessary if
repeated repacking makes delta chain too long.  With this, the
output of the command becomes identical to that of the older
implementation.  But the performance suffers greatly.

It still allows reusing non-deltified representations; there is
no point uncompressing and recompressing the whole text.

It also adds a couple more statistics output, while squelching
it under -q flag, which the last round forgot to do.

  $ time old-git-pack-objects --stdout >/dev/null <RL
  Generating pack...
  Done counting 184141 objects.
  Packing 184141 objects....................
  real    12m8.530s       user    11m1.450s       sys     0m57.920s
  $ time git-pack-objects --stdout >/dev/null <RL
  Generating pack...
  Done counting 184141 objects.
  Packing 184141 objects.....................
  Total 184141, written 184141 (delta 138297), reused 178833 (delta 134081)
  real    0m59.549s       user    0m56.670s       sys     0m2.400s
  $ time git-pack-objects --stdout --no-reuse-delta >/dev/null <RL
  Generating pack...
  Done counting 184141 objects.
  Packing 184141 objects.....................
  Total 184141, written 184141 (delta 134833), reused 47904 (delta 0)
  real    11m13.830s      user    9m45.240s       sys     0m44.330s

There is one remaining issue when --no-reuse-delta option is not
used.  It can create delta chains that are deeper than specified.

    A<--B<--C<--D   E   F   G

Suppose we have a delta chain A to D (A is stored in full either
in a pack or as a loose object. B is depth1 delta relative to A,
C is depth2 delta relative to B...) with loose objects E, F, G.
And we are going to pack all of them.

B, C and D are left as delta against A, B and C respectively.
So A, E, F, and G are examined for deltification, and let's say
we decided to keep E expanded, and store the rest as deltas like
this:

    E<--F<--G<--A

Oops.  We ended up making D a bit too deep, didn't we?  B, C and
D form a chain on top of A!

This is because we did not know what the final depth of A would
be, when we checked objects and decided to keep the existing
delta.  Unfortunately, deferring the decision until just before
the deltification is not an option.  To be able to make B, C,
and D candidates for deltification with the rest, we need to
know the type and final unexpanded size of them, but the major
part of the optimization comes from the fact that we do not read
the delta data to do so -- getting the final size is quite an
expensive operation.

To prevent this from happening, we should keep A from being
deltified.  But how would we tell that, cheaply?

To do this most precisely, after check_object() runs, each
object that is used as the base object of some existing delta
needs to be marked with the maximum depth of the objects we
decided to keep deltified (in this case, D is depth 3 relative
to A, so if no other delta chain that is longer than 3 based on
A exists, mark A with 3).  Then when attempting to deltify A, we
would take that number into account to see if the final delta
chain that leads to D becomes too deep.

However, this is a bit cumbersome to compute, so we would cheat
and reduce the maximum depth for A arbitrarily to depth/4 in
this implementation.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-pack-objects.txt

@@ -8,7 +8,10 @@ git-pack-objects - Create a packed archive of objects.
 
 SYNOPSIS
 --------
-'git-pack-objects' [--non-empty] [--local] [--incremental] [--window=N] [--depth=N] {--stdout | base-name} < object-list
+[verse]
+'git-pack-objects' [-q] [--no-reuse-delta] [--non-empty]
+	[--local] [--incremental] [--window=N] [--depth=N]
+	{--stdout | base-name} < object-list
 
 
 DESCRIPTION
@@ -32,6 +35,10 @@ Placing both in the pack/ subdirectory of $GIT_OBJECT_DIRECTORY (or
 any of the directories on $GIT_ALTERNATE_OBJECT_DIRECTORIES)
 enables git to read from such an archive.
 
+In a packed archive, an object is either stored as a compressed
+whole, or as a difference from some other object.  The latter is
+often called a delta.
+
 
 OPTIONS
 -------
@@ -74,6 +81,18 @@ base-name::
         Only create a packed archive if it would contain at
         least one object.
 
+-q::
+	This flag makes the command not to report its progress
+	on the standard error stream.
+
+--no-reuse-delta::
+	When creating a packed archive in a repository that
+	has existing packs, the command reuses existing deltas.
+	This sometimes results in a slightly suboptimal pack.
+	This flag tells the command not to reuse existing deltas
+	but compute them from scratch.
+
+
 Author
 ------
 Written by Linus Torvalds <torvalds@osdl.org>

pack-objects.c

@@ -5,7 +5,7 @@
 #include "csum-file.h"
 #include <sys/time.h>
 
-static const char pack_usage[] = "git-pack-objects [-q] [--non-empty] [--local] [--incremental] [--window=N] [--depth=N] {--stdout | base-name} < object-list";
+static const char pack_usage[] = "git-pack-objects [-q] [--no-reuse-delta] [--non-empty] [--local] [--incremental] [--window=N] [--depth=N] {--stdout | base-name} < object-list";
 
 struct object_entry {
 	unsigned char sha1[20];
@@ -14,10 +14,11 @@ struct object_entry {
 	unsigned int depth;	/* delta depth */
 	unsigned int hash;	/* name hint hash */
 	enum object_type type;
+	unsigned char edge;	/* reused delta chain points at this entry. */
+	enum object_type in_pack_type;	/* could be delta */
 	unsigned long delta_size;	/* delta data size (uncompressed) */
 	struct object_entry *delta;	/* delta base object */
 	struct packed_git *in_pack; 	/* already in pack */
-	enum object_type in_pack_type;	/* could be delta */
 	unsigned int in_pack_offset;
 };
 
@@ -36,6 +37,7 @@ struct object_entry {
 
 static unsigned char object_list_sha1[20];
 static int non_empty = 0;
+static int no_reuse_delta = 0;
 static int local = 0;
 static int incremental = 0;
 static struct object_entry **sorted_by_sha, **sorted_by_type;
@@ -75,7 +77,9 @@ static int pack_revindex_hashsz = 0;
  * stats
  */
 static int written = 0;
+static int written_delta = 0;
 static int reused = 0;
+static int reused_delta = 0;
 
 static int pack_revindex_ix(struct packed_git *p)
 {
@@ -227,10 +231,23 @@ static unsigned long write_object(struct sha1file *f, struct object_entry *entry
 	unsigned char header[10];
 	unsigned hdrlen, datalen;
 	enum object_type obj_type;
+	int to_reuse = 0;
 
 	obj_type = entry->type;
-	if (!entry->in_pack ||
-	    (obj_type != entry->in_pack_type)) {
+	if (! entry->in_pack)
+		to_reuse = 0;	/* can't reuse what we don't have */
+	else if (obj_type == OBJ_DELTA)
+		to_reuse = 1;	/* check_object() decided it for us */
+	else if (obj_type != entry->in_pack_type)
+		to_reuse = 0;	/* pack has delta which is unusable */
+	else if (entry->delta)
+		to_reuse = 0;	/* we want to pack afresh */
+	else
+		to_reuse = 1;	/* we have it in-pack undeltified,
+				 * and we do not need to deltify it.
+				 */
+
+	if (! to_reuse) {
 		buf = read_sha1_file(entry->sha1, type, &size);
 		if (!buf)
 			die("unable to read %s", sha1_to_hex(entry->sha1));
@@ -266,8 +283,12 @@ static unsigned long write_object(struct sha1file *f, struct object_entry *entry
 		sha1write(f, buf, datalen);
 		unuse_packed_git(p);
 		hdrlen = 0; /* not really */
+		if (obj_type == OBJ_DELTA)
+			reused_delta++;
 		reused++;
 	}
+	if (obj_type == OBJ_DELTA)
+		written_delta++;
 	written++;
 	return hdrlen + datalen;
 }
@@ -294,7 +315,6 @@ static void write_pack_file(void)
 	int i;
 	struct sha1file *f;
 	unsigned long offset;
-	unsigned long mb;
 	struct pack_header hdr;
 
 	if (!base_name)
@@ -357,10 +377,9 @@ static int add_object_entry(unsigned char *sha1, unsigned int hash)
 	unsigned int idx = nr_objects;
 	struct object_entry *entry;
 	struct packed_git *p;
-	unsigned int found_offset;
-	struct packed_git *found_pack;
+	unsigned int found_offset = 0;
+	struct packed_git *found_pack = NULL;
 
-	found_pack = NULL;
 	for (p = packed_git; p; p = p->next) {
 		struct pack_entry e;
 		if (find_pack_entry_one(sha1, &e, p)) {
@@ -420,32 +439,39 @@ static void check_object(struct object_entry *entry)
 	char type[20];
 
 	if (entry->in_pack) {
+		unsigned char base[20];
+		unsigned long size;
+		struct object_entry *base_entry;
+
+		/* We want in_pack_type even if we do not reuse delta.
+		 * There is no point not reusing non-delta representations.
+		 */
+		check_reuse_pack_delta(entry->in_pack,
+				       entry->in_pack_offset,
+				       base, &size,
+				       &entry->in_pack_type);
+
 		/* Check if it is delta, and the base is also an object
 		 * we are going to pack.  If so we will reuse the existing
 		 * delta.
 		 */
-		unsigned char base[20];
-		unsigned long size;
-		struct object_entry *base_entry;
-		if (!check_reuse_pack_delta(entry->in_pack,
-					    entry->in_pack_offset,
-					    base, &size,
-					    &entry->in_pack_type) &&
+		if (!no_reuse_delta &&
+		    entry->in_pack_type == OBJ_DELTA &&
 		    (base_entry = locate_object_entry(base))) {
-			/* We do not know depth at this point, but it
-			 * does not matter.  Getting delta_chain_length
-			 * with packed_object_info_detail() is not so
-			 * expensive, so we could do that later if we
-			 * wanted to.  Calling sha1_object_info to get
-			 * the true size (and later an uncompressed
-			 * representation) of deeply deltified object
-			 * is quite expensive.
+
+			/* Depth value does not matter - find_deltas()
+			 * will never consider reused delta as the
+			 * base object to deltify other objects
+			 * against, in order to avoid circular deltas.
 			 */
-			entry->depth = 1;
-			/* uncompressed size */
+
+			/* uncompressed size of the delta data */
 			entry->size = entry->delta_size = size;
 			entry->delta = base_entry;
 			entry->type = OBJ_DELTA;
+
+			base_entry->edge = 1;
+
 			return;
 		}
 		/* Otherwise we would do the usual */
@@ -568,6 +594,13 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
 	if (cur_entry->type != old_entry->type)
 		return -1;
 
+	/* If the current object is at edge, take the depth the objects
+	 * that depend on the current object into account -- otherwise
+	 * they would become too deep.
+	 */
+	if (cur_entry->edge)
+		max_depth /= 4;
+
 	size = cur_entry->size;
 	if (size < 50)
 		return -1;
@@ -627,7 +660,7 @@ static void find_deltas(struct object_entry **list, int window, int depth)
 
 		if (entry->delta)
 			/* This happens if we decided to reuse existing
-			 * delta from a pack.
+			 * delta from a pack.  "!no_reuse_delta &&" is implied.
 			 */
 			continue;
 
@@ -636,6 +669,7 @@ static void find_deltas(struct object_entry **list, int window, int depth)
 		n->data = read_sha1_file(entry->sha1, type, &size);
 		if (size != entry->size)
 			die("object %s inconsistent object length (%lu vs %lu)", sha1_to_hex(entry->sha1), size, entry->size);
+
 		j = window;
 		while (--j > 0) {
 			unsigned int other_idx = idx + j;
@@ -664,7 +698,7 @@ static void prepare_pack(int window, int depth)
 		fprintf(stderr, "Packing %d objects", nr_objects);
 	get_object_details();
 	if (progress)
-		fprintf(stderr, ".");
+		fputc('.', stderr);
 
 	sorted_by_type = create_sorted_list(type_size_sort);
 	if (window && depth)
@@ -694,8 +728,9 @@ static int reuse_cached_pack(unsigned char *sha1, int pack_to_stdout)
 		}
 	}
 
-	fprintf(stderr, "Reusing %d objects pack %s\n", nr_objects,
-		sha1_to_hex(sha1));
+	if (progress)
+		fprintf(stderr, "Reusing %d objects pack %s\n", nr_objects,
+			sha1_to_hex(sha1));
 
 	if (pack_to_stdout) {
 		if (copy_fd(ifd, 1))
@@ -775,6 +810,10 @@ int main(int argc, char **argv)
 				progress = 0;
 				continue;
 			}
+			if (!strcmp("--no-reuse-delta", arg)) {
+				no_reuse_delta = 1;
+				continue;
+			}
 			if (!strcmp("--stdout", arg)) {
 				pack_to_stdout = 1;
 				continue;
@@ -850,7 +889,8 @@ int main(int argc, char **argv)
 			puts(sha1_to_hex(object_list_sha1));
 		}
 	}
-	fprintf(stderr, "Total %d, written %d, reused %d\n",
-		nr_objects, written, reused);
+	if (progress)
+		fprintf(stderr, "Total %d, written %d (delta %d), reused %d (delta %d)\n",
+			nr_objects, written, written_delta, reused, reused_delta);
 	return 0;
 }