tag: "git tag" refuses to use HEAD as a tagname

Even though the plumbing level allows you to create refs/tags/HEAD
and refs/heads/HEAD, doing so makes it confusing within the context
of the UI Git Porcelain commands provides.  Just like we prevent a
branch from getting called "HEAD" at the Porcelain layer (i.e. "git
branch" command), teach "git tag" to refuse to create a tag "HEAD".

With a few new tests, we make sure that

 - "git tag HEAD" and "git tag -a HEAD" are rejected

 - "git update-ref refs/tags/HEAD" is still allowed (this is a
   deliberate design decision to allow others to create their own UI
   on top of Git infrastructure that may be different from our UI).

 - "git tag -d HEAD" can remove refs/tags/HEAD to recover from an
   mistake.

Helped-by: Jeff King <[email protected]>
Helped-by: Rubén Justo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

refs.c

@@ -735,7 +735,7 @@ int check_branch_ref(struct strbuf *sb, const char *name)
 
 int check_tag_ref(struct strbuf *sb, const char *name)
 {
-	if (name[0] == '-')
+	if (name[0] == '-' || !strcmp(name, "HEAD"))
 		return -1;
 
 	strbuf_reset(sb);

t/t7004-tag.sh

@@ -91,6 +91,18 @@ test_expect_success 'creating a tag using default HEAD should succeed' '
 	test_must_fail git reflog exists refs/tags/mytag
 '
 
+test_expect_success 'HEAD is forbidden as a tagname' '
+	test_when_finished "git update-ref --no-deref -d refs/tags/HEAD || :" &&
+	test_must_fail git tag HEAD &&
+	test_must_fail git tag -a -m "useless" HEAD
+'
+
+test_expect_success '"git tag" can remove a tag named HEAD' '
+	test_when_finished "git update-ref --no-deref -d refs/tags/HEAD || :" &&
+	git update-ref refs/tags/HEAD HEAD &&
+	git tag -d HEAD
+'
+
 test_expect_success 'creating a tag with --create-reflog should create reflog' '
 	git log -1 \
 		--format="format:tag: tagging %h (%s, %cd)%n" \