[PATCH] tag: allow idempotent "git tag" without "--force"

[Junio C Hamano <gitster@pobox.com>]

When "git tag T O" is told to create a tag pointing at an object O
without the "--force" option, it refuses with "tag T already exists",
even when T points at O (which makes it a no-op).

Let's allow this "idempotent" case by special casing and making it
truly a no-op.

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

 * In thread https://lore.kernel.org/git/xmqqzfefodje.fsf@gitster.g/
   I had a slightly different version but I think the logic flow
   reads better in this version, even though they essentially do the
   same thing.

 builtin/tag.c  |  2 ++
 t/t7004-tag.sh | 12 +++++++++---
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/builtin/tag.c b/builtin/tag.c
index c4bd145831..1dd69b3447 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -647,6 +647,8 @@ int cmd_tag(int argc,
 
 	if (refs_read_ref(get_main_ref_store(the_repository), ref.buf, &prev))
 		oidclr(&prev, the_repository->hash_algo);
+	else if (!create_tag_object && oideq(&object, &prev))
+		exit(0);
 	else if (!force)
 		die(_("tag '%s' already exists"), tag);
 
diff --git a/t/t7004-tag.sh b/t/t7004-tag.sh
index 10835631ca..9a253a44a8 100755
--- a/t/t7004-tag.sh
+++ b/t/t7004-tag.sh
@@ -126,7 +126,7 @@ test_expect_success 'annotated tag with --create-reflog has correct message' '
 '
 
 test_expect_success '--create-reflog does not create reflog on failure' '
-	test_must_fail git tag --create-reflog mytag &&
+	test_must_fail git tag --create-reflog mytag no-such-object &&
 	test_must_fail git reflog exists refs/tags/mytag
 '
 
@@ -183,8 +183,14 @@ test_expect_success 'listing tags using a non-matching pattern should output not
 
 # special cases for creating tags:
 
-test_expect_success 'trying to create a tag with the name of one existing should fail' '
-	test_must_fail git tag mytag
+test_expect_success 'recreating a tag without --force' '
+	# light-weight tag pointing at the same thing
+	# now succeeds
+	git tag mytag HEAD &&
+	# light-weight tag pointing at a different thing
+	test_must_fail git tag mytag HEAD: &&
+	# creating annotated tag, pointing at the same object.
+	test_must_fail git tag -a -m anno mytag $taggedobject
 '
 
 test_expect_success 'trying to create a tag with a non-valid name should fail' '
-- 
2.50.1-396-g5c2aadec13