remote: default to --limited-fetch in a shallow repository

Adding a second remote to a shallow, single-branch clone used to
still fetch every branch that remote has, since "git remote add"
always set up a wildcard remote.<name>.fetch refspec regardless of
how shallow the repository already was. That defeats the purpose of
having cloned shallow and single-branch in the first place, and can
make a plain "git fetch" on that remote hang or take a very long
time on a repository with many branches.

Turn --limited-fetch on by default when the repository is already
shallow and neither -t/--track nor --mirror was given, so that
adding a remote there does not by itself commit to following every
branch it has. --no-limited-fetch keeps the previous behavior for
whoever wants it.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote.adoc

@@ -52,6 +52,11 @@ Add a remote named _<name>_ for the repository at
 _<URL>_.  The command `git fetch <name>` can then be used to create and
 update remote-tracking branches `<name>/<branch>`.
 +
+If the repository is already a shallow repository (see linkgit:git-clone[1]
+`--depth`) and neither `-t`, `--mirror` nor `--no-limited-fetch` is given,
+`--limited-fetch` is turned on by default, so that `git fetch <name>` does
+not need to negotiate history for every branch the remote has.
++
 With `-f` option, `git fetch <name>` is run immediately after
 the remote information is set up.
 +
@@ -74,7 +79,8 @@ With `--limited-fetch` option, instead of a `remote.<name>.fetch` refspec
 that tracks all branches, `remote.<name>.refmap` is set up so that a
 refspec-less `git fetch <name>` only fetches branches our local branches
 are built on. See the `--refmap` entry in linkgit:git-fetch[1] for
-details.
+details. `--no-limited-fetch` explicitly disables this, overriding the
+shallow-repository default described above.
 +
 With `-m <master>` option, a symbolic-ref `refs/remotes/<name>/HEAD` is set
 up to point at remote's _<master>_ branch. See also the set-head command.

builtin/remote.c

@@ -16,6 +16,7 @@
 #include "rebase.h"
 #include "refs.h"
 #include "refspec.h"
+#include "shallow.h"
 #include "odb.h"
 #include "strvec.h"
 #include "commit-reach.h"
@@ -238,7 +239,9 @@ static int add(int argc, const char **argv, const char *prefix,
 
 	if (!mirror || mirror & MIRROR_FETCH) {
 		int use_limited_fetch = mirror == MIRROR_NONE && track.nr == 0 &&
-			limited_fetch == 1;
+			(limited_fetch == 1 ||
+			 (limited_fetch == -1 &&
+			  is_repository_shallow(the_repository)));
 
 		strbuf_reset(&buf);
 		if (use_limited_fetch) {

t/t5505-remote.sh

@@ -137,6 +137,66 @@ test_expect_success 'filters are listed by git remote -v only' '
 	test_grep ! "\[blob:none\]" out
 '
 
+test_expect_success 'add remote -t keeps an explicit refspec in a shallow repository' '
+	test_when_finished "rm -rf shallow-add" &&
+	git clone --no-local --depth=1 --branch main --single-branch \
+		one shallow-add &&
+	(
+		cd shallow-add &&
+		git remote add -t main upstream ../two &&
+		test_cmp_config "+refs/heads/main:refs/remotes/upstream/main" \
+			remote.upstream.fetch
+	)
+'
+
+test_expect_success 'add remote keeps the wildcard refspec in a full repository' '
+	test_when_finished "rm -rf full-add" &&
+	git clone --no-local one full-add &&
+	(
+		cd full-add &&
+		git remote add upstream ../two &&
+		test_cmp_config "+refs/heads/*:refs/remotes/upstream/*" \
+			remote.upstream.fetch
+	)
+'
+
+test_expect_success 'a remote added in a shallow repository defaults to --limited-fetch' '
+	test_when_finished "rm -rf shallow-add" &&
+	git clone --no-local --depth=1 --branch main --single-branch \
+		one shallow-add &&
+	(
+		cd shallow-add &&
+		git remote add upstream ../two &&
+		test_cmp_config "+refs/heads/*:refs/remotes/upstream/*" \
+			remote.upstream.refmap &&
+		test_must_fail git config get remote.upstream.fetch &&
+		git fetch upstream main &&
+		git branch --set-upstream-to=upstream/main &&
+		test_cmp_config upstream branch.main.remote &&
+		test_cmp_config refs/heads/main branch.main.merge &&
+		git fetch upstream &&
+		git for-each-ref --format="%(refname)" refs/remotes/upstream >actual &&
+		cat >expect <<-\EOF &&
+		refs/remotes/upstream/HEAD
+		refs/remotes/upstream/main
+		EOF
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--no-limited-fetch overrides the shallow-repository default' '
+	test_when_finished "rm -rf shallow-add" &&
+	git clone --no-local --depth=1 --branch main --single-branch \
+		one shallow-add &&
+	(
+		cd shallow-add &&
+		git remote add --no-limited-fetch upstream ../two &&
+		test_cmp_config "+refs/heads/*:refs/remotes/upstream/*" \
+			remote.upstream.fetch &&
+		test_must_fail git config get remote.upstream.refmap
+	)
+'
+
 test_expect_success '--limited-fetch works in a full repository too' '
 	test_when_finished "rm -rf full-add" &&
 	git clone --no-local one full-add &&