[PATCH v10 8/8] fetch set_head: handle mirrored bare repositories

[Bence Ferdinandy <bence@ferdinandy.com>]

When adding a remote to bare repository with "git remote add --mirror",
running fetch will fail to update HEAD to the remote's HEAD, since it
does not know how to handle bare repositories. On the other hand HEAD
already has content, since "git init --bare" has already set HEAD to
whatever is the default branch set for the user. Unless this - by chance
- is the same as the remote's HEAD, HEAD will be pointing to a bad
symref. Teach set_head to handle bare repositories, by overwriting HEAD
so it mirrors the remote's HEAD.

Note, that in this case overriding the local HEAD reference is
necessary, since HEAD will exist before fetch can be run, but this
should not be an issue, since the whole purpose of --mirror is to be an
exact mirror of the remote, so following any changes to HEAD makes
sense.

Also note, that although "git remote set-head" also fails when trying to
update the remote's locally tracked HEAD in a mirrored bare repository,
the usage of the command does not make much sense after this patch:
fetch will update the remote HEAD correctly, and setting it manually to
something else is antithetical to the concept of mirroring.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
---

Notes:
    v10: - new patch
         - handles the issue discovered in
           https://lore.kernel.org/git/D4ZAELFWJMKN.S88LJ6YK31LZ@ferdinandy.com/T/

 builtin/fetch.c   | 16 ++++++++++++----
 t/t5505-remote.sh | 10 ++++++++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/builtin/fetch.c b/builtin/fetch.c
index 31edc67447..249556c9c6 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1632,14 +1632,22 @@ static int set_head(const struct ref *remote_refs)
 	else
 		head_name = xstrdup(heads.items[0].string);
 	if (head_name) {
-		strbuf_addf(&b_head, "refs/remotes/%s/HEAD", remote);
-		strbuf_addf(&b_remote_head, "refs/remotes/%s/%s", remote, head_name);
+		int is_bare = is_bare_repository();
+		if (is_bare) {
+			strbuf_addstr(&b_head, "HEAD");
+			strbuf_addf(&b_remote_head, "refs/heads/%s", head_name);
+		} else {
+			strbuf_addf(&b_head, "refs/remotes/%s/HEAD", remote);
+			strbuf_addf(&b_remote_head, "refs/remotes/%s/%s", remote, head_name);
+		}
 		/* make sure it's valid */
-		if (!refs_ref_exists(refs, b_remote_head.buf))
+		if (!is_bare && !refs_ref_exists(refs, b_remote_head.buf)) {
 			result = 1;
+		}
 		else if (refs_update_symref_extended(refs, b_head.buf, b_remote_head.buf,
-					"fetch", &b_local_head, 1))
+					"fetch", &b_local_head, !is_bare)) {
 			result = 1;
+		}
 		else
 			report_set_head(remote, head_name, &b_local_head);
 
diff --git a/t/t5505-remote.sh b/t/t5505-remote.sh
index 4990d00209..dfa78f3e8d 100755
--- a/t/t5505-remote.sh
+++ b/t/t5505-remote.sh
@@ -545,6 +545,16 @@ test_expect_success 'add --mirror && prune' '
 	)
 '
 
+test_expect_success 'add --mirror setting HEAD' '
+	mkdir headmirror &&
+	(
+		cd headmirror &&
+		git init --bare -b notmain &&
+		git remote add --mirror -f origin ../one &&
+		test "$(git symbolic-ref HEAD)" = "refs/heads/main"
+	)
+'
+
 test_expect_success 'add --mirror=fetch' '
 	mkdir mirror-fetch &&
 	git init -b main mirror-fetch/parent &&
-- 
2.47.0.94.g8861098b6d