fetch: do not ask for HEAD unnecessarily

In 3f763dd (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 <[email protected]>
[jc: tests are also borrowed from Josh's patch]
Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

builtin/fetch.c

@@ -1643,6 +1643,21 @@ static int set_head(const struct ref *remote_refs)
 	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);

t/t5510-fetch.sh

@@ -189,6 +189,23 @@ test_expect_success 'fetch --prune --tags with refspec prunes based on refspec'
 	git rev-parse sometag
 '
 
+test_expect_success 'fetch --tags gets tags even without a configured remote' '
+	REMOTE="$(pwd)/test_tag_1" &&
+	git init test_tag_1 &&
+	(
+		cd test_tag_1 &&
+		test_commit foo
+	) &&
+	git init test_tag_2 &&
+	(
+		cd test_tag_2 &&
+		git fetch --tags "file://$REMOTE" &&
+		echo "foo" >expect &&
+		git tag >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success REFFILES 'fetch --prune fails to delete branches' '
 	cd "$D" &&
 	git clone . prune-fail &&