clone --single: limit the fetch refspec to fetched branch

After running "git clone --single", the resulting repository has the
usual default "+refs/heads/*:refs/remotes/origin/*" wildcard fetch
refspec installed, which means that a subsequent "git fetch" will
end up grabbing all the other branches.

Update the fetch refspec to cover only the singly cloned ref instead
to correct this.

That means:
If "--single" is used without "--branch" or "--mirror", the
fetch refspec covers the branch on which remote's HEAD points to.
If "--single" is used with "--branch", it'll cover only the branch
specified in the "--branch" option.
If "--single" is combined with "--mirror", then it'll cover all
refs of the cloned repository.
If "--single" is used with "--branch" that specifies a tag, then
it'll cover only the ref for this tag.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -29,7 +29,8 @@ currently active branch.
 After the clone, a plain `git fetch` without arguments will update
 all the remote-tracking branches, and a `git pull` without
 arguments will in addition merge the remote master branch into the
-current master branch, if any.
+current master branch, if any (this is untrue when "--single-branch"
+is given; see below).
 
 This default configuration is achieved by creating references to
 the remote branch heads under `refs/remotes/origin` and
@@ -147,9 +148,10 @@ objects from the source repository into a pack in the cloned repository.
 -b <name>::
 	Instead of pointing the newly created HEAD to the branch pointed
 	to by the cloned repository's HEAD, point to `<name>` branch
-	instead. `--branch` can also take tags and treat them like
+	instead. In a non-bare repository, this is the branch that will
-	detached HEAD. In a non-bare repository, this is the branch
+	be checked out.
-	that will be checked out.
+	`--branch` can also take tags and detaches the HEAD at that commit
+	in the resulting repository.
 
 --upload-pack <upload-pack>::
 -u <upload-pack>::
@@ -188,6 +190,11 @@ objects from the source repository into a pack in the cloned repository.
 	clone with the `--depth` option, this is the default, unless
 	`--no-single-branch` is given to fetch the histories near the
 	tips of all branches.
+	Further fetches into the resulting repository will only update the
+	remote tracking branch for the branch this option was used for the
+	initial cloning.  If the HEAD at the remote did not point at any
+	branch when `--single-branch` clone was made, no remote tracking
+	branch is created.
 
 --recursive::
 --recurse-submodules::

builtin/clone.c

@@ -610,6 +610,54 @@ static void write_config(struct string_list *config)
 	}
 }
 
+static void write_refspec_config(const char* src_ref_prefix,
+		const struct ref* our_head_points_at,
+		const struct ref* remote_head_points_at, struct strbuf* branch_top)
+{
+	struct strbuf key = STRBUF_INIT;
+	struct strbuf value = STRBUF_INIT;
+
+	if (option_mirror || !option_bare) {
+		if (option_single_branch && !option_mirror) {
+			if (option_branch) {
+				if (strstr(our_head_points_at->name, "refs/tags/"))
+					strbuf_addf(&value, "+%s:%s", our_head_points_at->name,
+						our_head_points_at->name);
+				else
+					strbuf_addf(&value, "+%s:%s%s", our_head_points_at->name,
+						branch_top->buf, option_branch);
+			} else if (remote_head_points_at) {
+				strbuf_addf(&value, "+%s:%s%s", remote_head_points_at->name,
+						branch_top->buf,
+						skip_prefix(remote_head_points_at->name, "refs/heads/"));
+			}
+			/*
+			 * otherwise, the next "git fetch" will
+			 * simply fetch from HEAD without updating
+			 * any remote tracking branch, which is what
+			 * we want.
+			 */
+		} else {
+			strbuf_addf(&value, "+%s*:%s*", src_ref_prefix, branch_top->buf);
+		}
+		/* Configure the remote */
+		if (value.len) {
+			strbuf_addf(&key, "remote.%s.fetch", option_origin);
+			git_config_set_multivar(key.buf, value.buf, "^$", 0);
+			strbuf_reset(&key);
+
+			if (option_mirror) {
+				strbuf_addf(&key, "remote.%s.mirror", option_origin);
+				git_config_set(key.buf, "true");
+				strbuf_reset(&key);
+			}
+		}
+	}
+
+	strbuf_release(&key);
+	strbuf_release(&value);
+}
+
 int cmd_clone(int argc, const char **argv, const char *prefix)
 {
 	int is_bundle = 0, is_local;
@@ -755,20 +803,6 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	}
 
 	strbuf_addf(&value, "+%s*:%s*", src_ref_prefix, branch_top.buf);
-
-	if (option_mirror || !option_bare) {
-		/* Configure the remote */
-		strbuf_addf(&key, "remote.%s.fetch", option_origin);
-		git_config_set_multivar(key.buf, value.buf, "^$", 0);
-		strbuf_reset(&key);
-
-		if (option_mirror) {
-			strbuf_addf(&key, "remote.%s.mirror", option_origin);
-			git_config_set(key.buf, "true");
-			strbuf_reset(&key);
-		}
-	}
-
 	strbuf_addf(&key, "remote.%s.url", option_origin);
 	git_config_set(key.buf, repo);
 	strbuf_reset(&key);
@@ -853,6 +887,9 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 					      "refs/heads/master");
 	}
 
+	write_refspec_config(src_ref_prefix, our_head_points_at,
+			remote_head_points_at, &branch_top);
+
 	if (is_local)
 		clone_local(path, git_dir);
 	else if (refs && complete_refs_before_fetch)

t/t5709-clone-refspec.sh

@@ -0,0 +1,156 @@
+#!/bin/sh
+
+test_description='test refspec written by clone-command'
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	# Make two branches, "master" and "side"
+	echo one >file &&
+	git add file &&
+	git commit -m one &&
+	echo two >file &&
+	git commit -a -m two &&
+	git tag two &&
+	echo three >file &&
+	git commit -a -m three &&
+	git checkout -b side &&
+	echo four >file &&
+	git commit -a -m four &&
+	git checkout master &&
+
+	# default clone
+	git clone . dir_all &&
+
+	# default --single that follows HEAD=master
+	git clone --single-branch . dir_master &&
+
+	# default --single that follows HEAD=side
+	git checkout side &&
+	git clone --single-branch . dir_side &&
+
+	# explicit --single that follows side
+	git checkout master &&
+	git clone --single-branch --branch side . dir_side2 &&
+
+	# default --single with --mirror
+	git clone --single-branch --mirror . dir_mirror &&
+
+	# default --single with --branch and --mirror
+	git clone --single-branch --mirror --branch side . dir_mirror_side &&
+
+	# --single that does not know what branch to follow
+	git checkout two^ &&
+	git clone --single-branch . dir_detached &&
+
+	# explicit --single with tag
+	git clone --single-branch --branch two . dir_tag &&
+
+	# advance both "master" and "side" branches
+	git checkout side &&
+	echo five >file &&
+	git commit -a -m five &&
+	git checkout master &&
+	echo six >file &&
+	git commit -a -m six &&
+
+	# update tag
+	git tag -d two && git tag two
+'
+
+test_expect_success 'by default all branches will be kept updated' '
+	(
+		cd dir_all && git fetch &&
+		git for-each-ref refs/remotes/origin |
+		sed -e "/HEAD$/d" \
+		    -e "s|/remotes/origin/|/heads/|" >../actual
+	) &&
+	# follow both master and side
+	git for-each-ref refs/heads >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'by default no tags will be kept updated' '
+	(
+		cd dir_all && git fetch &&
+		git for-each-ref refs/tags >../actual
+	) &&
+	git for-each-ref refs/tags >expect &&
+	test_must_fail test_cmp expect actual
+'
+
+test_expect_success '--single-branch while HEAD pointing at master' '
+	(
+		cd dir_master && git fetch &&
+		git for-each-ref refs/remotes/origin |
+		sed -e "/HEAD$/d" \
+		    -e "s|/remotes/origin/|/heads/|" >../actual
+	) &&
+	# only follow master
+	git for-each-ref refs/heads/master >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--single-branch while HEAD pointing at side' '
+	(
+		cd dir_side && git fetch &&
+		git for-each-ref refs/remotes/origin |
+		sed -e "/HEAD$/d" \
+		    -e "s|/remotes/origin/|/heads/|" >../actual
+	) &&
+	# only follow side
+	git for-each-ref refs/heads/side >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--single-branch with explicit --branch side' '
+	(
+		cd dir_side2 && git fetch &&
+		git for-each-ref refs/remotes/origin |
+		sed -e "/HEAD$/d" \
+		    -e "s|/remotes/origin/|/heads/|" >../actual
+	) &&
+	# only follow side
+	git for-each-ref refs/heads/side >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--single-branch with explicit --branch with tag fetches updated tag' '
+	(
+		cd dir_tag && git fetch &&
+		git for-each-ref refs/tags >../actual
+	) &&
+	git for-each-ref refs/tags >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--single-branch with --mirror' '
+	(
+		cd dir_mirror && git fetch &&
+		git for-each-ref refs > ../actual
+	) &&
+	git for-each-ref refs >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--single-branch with explicit --branch and --mirror' '
+	(
+		cd dir_mirror_side && git fetch &&
+		git for-each-ref refs > ../actual
+	) &&
+	git for-each-ref refs >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--single-branch with detached' '
+	(
+		cd dir_detached && git fetch &&
+		git for-each-ref refs/remotes/origin |
+		sed -e "/HEAD$/d" \
+		    -e "s|/remotes/origin/|/heads/|" >../actual
+	)
+	# nothing
+	>expect &&
+	test_cmp expect actual
+'
+
+test_done