fetch: do not ask for HEAD unnecessarily

In 3f763ddf28 (fetch: set remote/HEAD if it does not exist,
2024-11-22), git-fetch learned to opportunistically set $REMOTE/HEAD
when fetching by always asking for remote HEAD, in the hope that it
will help setting refs/remotes/<name>/HEAD if missing.

But it is not needed to always ask for remote HEAD.  When we are
fetching from a remote, for which we have remote-tracking branches,
we do need to know about HEAD.  But if we are doing one-shot fetch,
e.g.,

  $ git fetch --tags https://github.com/git/git

we do not even know what sub-hierarchy of refs/remotes/<remote>/
we need to adjust the remote HEAD for.  There is no need to ask for
HEAD in such a case.

Incidentally, because the unconditional request to list "HEAD"
affected the number of ref-prefixes requested in the ls-remote
request, this affected how the requests for tags are added to the
same ls-remote request, breaking "git fetch --tags $URL" performed
against a URL that is not configured as a remote.

Reported-by: Josh Steadmon <steadmon@google.com>
[jc: tests are also borrowed from Josh's patch]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 19 insertions, 1 deletions

diff --git a/builtin/fetch.c b/builtin/fetch.c
index a64de4485f..3eb6f3acc9 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1643,6 +1643,21 @@ cleanup:
 	return result;
 }
 
+static int uses_remote_tracking(struct transport *transport, struct refspec *rs)
+{
+	if (!remote_is_configured(transport->remote, 0))
+		return 0;
+
+	if (!rs->nr)
+		rs = &transport->remote->fetch;
+
+	for (int i = 0; i < rs->nr; i++)
+		if (rs->items[i].dst)
+			return 1;
+
+	return 0;
+}
+
 static int do_fetch(struct transport *transport,
 		    struct refspec *rs,
 		    const struct fetch_config *config)
@@ -1712,7 +1727,10 @@ static int do_fetch(struct transport *transport,
 				    "refs/tags/");
 	}
 
-	strvec_push(&transport_ls_refs_options.ref_prefixes, "HEAD");
+	if (uses_remote_tracking(transport, rs)) {
+		must_list_refs = 1;
+		strvec_push(&transport_ls_refs_options.ref_prefixes, "HEAD");
+	}
 
 	if (must_list_refs) {
 		trace2_region_enter("fetch", "remote_refs", the_repository);