symbolic-ref: teach "--[no-]recurse" option

Suppose you are managing many maintenance tracks in your project,
and some of the more recent ones are maint-2.36 and maint-2.37.
Further imagine that your project recently tagged the official 2.38
release, which means you would need to start maint-2.38 track soon,
by doing:

  $ git checkout -b maint-2.38 v2.38.0^0
  $ git branch --list 'maint-2.3[6-9]'
  * maint-2.38
    maint-2.36
    maint-2.37

So far, so good.  But it also is reasonable to want not to have to
worry about which maintenance track is the latest, by pointing a
more generic-sounding 'maint' branch at it, by doing:

  $ git symbolic-ref refs/heads/maint refs/heads/maint-2.38

which would allow you to say "whichever it is, check out the latest
maintenance track", by doing:

  $ git checkout maint
  $ git branch --show-current
  maint-2.38

It is arguably better to say that we are on 'maint-2.38' rather than
on 'maint', and "git merge/pull" would record "into maint-2.38" and
not "into maint", so I think what we have is a good behaviour.

One thing that is slightly irritating, however, is that I do not
think there is a good way (other than "cat .git/HEAD") to learn that
you checked out 'maint' to get into that state.  Just like the output
of "git branch --show-current" shows above, "git symbolic-ref HEAD"
would report 'refs/heads/maint-2.38', bypassing the intermediate
symbolic ref at 'refs/heads/maint' that is pointed at by HEAD.

The internal resolve_ref() API already has the necessary support for
stopping after resolving a single level of a symbolic-ref, and we
can expose it by adding a "--[no-]recurse" option to the command.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 35 insertions, 8 deletions

diff --git a/Documentation/git-symbolic-ref.txt b/Documentation/git-symbolic-ref.txt
index ef68ad2b71..102c83eb19 100644
--- a/Documentation/git-symbolic-ref.txt
+++ b/Documentation/git-symbolic-ref.txt
@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git symbolic-ref' [-m <reason>] <name> <ref>
-'git symbolic-ref' [-q] [--short] <name>
+'git symbolic-ref' [-q] [--short] [--no-recurse] <name>
 'git symbolic-ref' --delete [-q] <name>
 
 DESCRIPTION
@@ -46,6 +46,15 @@ OPTIONS
 	When showing the value of <name> as a symbolic ref, try to shorten the
 	value, e.g. from `refs/heads/master` to `master`.
 
+--recurse::
+--no-recurse::
+	When showing the value of <name> as a symbolic ref, if
+	<name> refers to another symbolic ref, follow such a chain
+	of symbolic refs until the result no longer points at a
+	symbolic ref (`--recurse`, which is the default).
+	`--no-recurse` stops after dereferencing only a single level
+	of symbolic ref.
+
 -m::
 	Update the reflog for <name> with <reason>.  This is valid only
 	when creating or updating a symbolic ref.
diff --git a/builtin/symbolic-ref.c b/builtin/symbolic-ref.c
index e547a08d6c..d5a70e7bd8 100644
--- a/builtin/symbolic-ref.c
+++ b/builtin/symbolic-ref.c
@@ -6,14 +6,17 @@
 
 static const char * const git_symbolic_ref_usage[] = {
 	N_("git symbolic-ref [<options>] <name> [<ref>]"),
-	N_("git symbolic-ref -d [-q] <name>"),
+	N_("git symbolic-ref -d [-q] [--no-recurse] <name>"),
 	NULL
 };
 
-static int check_symref(const char *HEAD, int quiet, int shorten, int print)
+static int check_symref(const char *HEAD, int quiet, int shorten, int recurse, int print)
 {
-	int flag;
-	const char *refname = resolve_ref_unsafe(HEAD, 0, NULL, &flag);
+	int resolve_flags, flag;
+	const char *refname;
+
+	resolve_flags = (recurse ? 0 : RESOLVE_REF_NO_RECURSE);
+	refname = resolve_ref_unsafe(HEAD, resolve_flags, NULL, &flag);
 
 	if (!refname)
 		die("No such ref: %s", HEAD);
@@ -35,13 +38,14 @@ static int check_symref(const char *HEAD, int quiet, int shorten, int print)
 
 int cmd_symbolic_ref(int argc, const char **argv, const char *prefix)
 {
-	int quiet = 0, delete = 0, shorten = 0, ret = 0;
+	int quiet = 0, delete = 0, shorten = 0, recurse = 1, ret = 0;
 	const char *msg = NULL;
 	struct option options[] = {
 		OPT__QUIET(&quiet,
 			N_("suppress error message for non-symbolic (detached) refs")),
 		OPT_BOOL('d', "delete", &delete, N_("delete symbolic ref")),
 		OPT_BOOL(0, "short", &shorten, N_("shorten ref output")),
+		OPT_BOOL(0, "recurse", &recurse, N_("recursively dereference (default)")),
 		OPT_STRING('m', NULL, &msg, N_("reason"), N_("reason of the update")),
 		OPT_END(),
 	};
@@ -55,7 +59,7 @@ int cmd_symbolic_ref(int argc, const char **argv, const char *prefix)
 	if (delete) {
 		if (argc != 1)
 			usage_with_options(git_symbolic_ref_usage, options);
-		ret = check_symref(argv[0], 1, 0, 0);
+		ret = check_symref(argv[0], 1, 0, 0, 0);
 		if (ret)
 			die("Cannot delete %s, not a symbolic ref", argv[0]);
 		if (!strcmp(argv[0], "HEAD"))
@@ -65,7 +69,7 @@ int cmd_symbolic_ref(int argc, const char **argv, const char *prefix)
 
 	switch (argc) {
 	case 1:
-		ret = check_symref(argv[0], quiet, shorten, 1);
+		ret = check_symref(argv[0], quiet, shorten, recurse, 1);
 		break;
 	case 2:
 		if (!strcmp(argv[0], "HEAD") &&
diff --git a/t/t1401-symbolic-ref.sh b/t/t1401-symbolic-ref.sh
index 132a1b885a..773a6e8e01 100755
--- a/t/t1401-symbolic-ref.sh
+++ b/t/t1401-symbolic-ref.sh
@@ -163,4 +163,18 @@ test_expect_success 'symbolic-ref can resolve d/f name (ENOTDIR)' '
 	test_cmp expect actual
 '
 
+test_expect_success 'symbolic-ref pointing at another' '
+	git update-ref refs/heads/maint-2.37 HEAD &&
+	git symbolic-ref refs/heads/maint refs/heads/maint-2.37 &&
+	git checkout maint &&
+
+	git symbolic-ref HEAD >actual &&
+	echo refs/heads/maint-2.37 >expect &&
+	test_cmp expect actual &&
+
+	git symbolic-ref --no-recurse HEAD >actual &&
+	echo refs/heads/maint >expect &&
+	test_cmp expect actual
+'
+
 test_done