fetch: support hideRefs to speed up connectivity checks

With roughly 800 remotes all fetching into their own
refs/remotes/$REMOTE/* island, the connectivity check[1] gets
expensive for each fetch on systems which lack sufficient RAM to
cache objects.

To do a no-op fetch on one $REMOTE out of hundreds, hideRefs now
allows the no-op fetch to take ~30 seconds instead of ~20 minutes
on a noisy, RAM-constrained machine (localhost, so no network latency):

   git -c fetch.hideRefs=refs \
	-c fetch.hideRefs='!refs/remotes/$REMOTE/' \
	fetch $REMOTE

[1] `git rev-list --objects --stdin --not --all --quiet --alternate-refs'

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -197,10 +197,11 @@ respectively, and they must begin with `refs/` when applied to `--glob`
 or `--all`. If a trailing '/{asterisk}' is intended, it must be given
 explicitly.
 
---exclude-hidden=[receive|uploadpack]::
+--exclude-hidden=[fetch|receive|uploadpack]::
-	Do not include refs that would be hidden by `git-receive-pack` or
+	Do not include refs that would be hidden by `git-fetch`,
-	`git-upload-pack` by consulting the appropriate `receive.hideRefs` or
+	`git-receive-pack` or `git-upload-pack` by consulting the appropriate
-	`uploadpack.hideRefs` configuration along with `transfer.hideRefs` (see
+	`fetch.hideRefs`, `receive.hideRefs` or `uploadpack.hideRefs`
+	configuration along with `transfer.hideRefs` (see
 	linkgit:git-config[1]). This option affects the next pseudo-ref option
 	`--all` or `--glob` and is cleared after processing them.
 

Documentation/rev-list-options.txt

@@ -195,10 +195,11 @@ respectively, and they must begin with `refs/` when applied to `--glob`
 or `--all`. If a trailing '/{asterisk}' is intended, it must be given
 explicitly.
 
---exclude-hidden=[receive|uploadpack]::
+--exclude-hidden=[fetch|receive|uploadpack]::
-	Do not include refs that would be hidden by `git-receive-pack` or
+	Do not include refs that would be hidden by `git-fetch`,
-	`git-upload-pack` by consulting the appropriate `receive.hideRefs` or
+	`git-receive-pack` or `git-upload-pack` by consulting the appropriate
-	`uploadpack.hideRefs` configuration along with `transfer.hideRefs` (see
+	`fetch.hideRefs`, `receive.hideRefs` or `uploadpack.hideRefs`
+	configuration along with `transfer.hideRefs` (see
 	linkgit:git-config[1]). This option affects the next pseudo-ref option
 	`--all` or `--glob` and is cleared after processing them.
 

builtin/fetch.c

@@ -1132,6 +1132,7 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 	if (!connectivity_checked) {
 		struct check_connected_options opt = CHECK_CONNECTED_INIT;
 
+		opt.exclude_hidden_refs_section = "fetch";
 		rm = ref_map;
 		if (check_connected(iterate_ref_map, &rm, &opt)) {
 			rc = error(_("%s did not send all necessary objects\n"), url);
@@ -1325,6 +1326,7 @@ static int check_exist_and_connected(struct ref *ref_map)
 	}
 
 	opt.quiet = 1;
+	opt.exclude_hidden_refs_section = "fetch";
 	return check_connected(iterate_ref_map, &rm, &opt);
 }
 

builtin/rev-list.c

@@ -38,7 +38,7 @@ static const char rev_list_usage[] =
 "    --tags\n"
 "    --remotes\n"
 "    --stdin\n"
-"    --exclude-hidden=[receive|uploadpack]\n"
+"    --exclude-hidden=[fetch|receive|uploadpack]\n"
 "    --quiet\n"
 "  ordering output:\n"
 "    --topo-order\n"

revision.c

@@ -1574,7 +1574,8 @@ void exclude_hidden_refs(struct ref_exclusions *exclusions, const char *section)
 {
 	struct exclude_hidden_refs_cb cb;
 
-	if (strcmp(section, "receive") && strcmp(section, "uploadpack"))
+	if (strcmp(section, "fetch") && strcmp(section, "receive") &&
+			strcmp(section, "uploadpack"))
 		die(_("unsupported section for hidden refs: %s"), section);
 
 	if (exclusions->hidden_refs_configured)

t/t5510-fetch.sh

@@ -1171,6 +1171,15 @@ test_expect_success '--no-show-forced-updates' '
 	)
 '
 
+for section in fetch transfer
+do
+	test_expect_success "$section.hideRefs affects connectivity check" '
+		GIT_TRACE="$PWD"/trace git -c $section.hideRefs=refs -c \
+			$section.hideRefs="!refs/tags/" fetch &&
+		grep "git rev-list .*--exclude-hidden=fetch" trace
+	'
+done
+
 setup_negotiation_tip () {
 	SERVER="$1"
 	URL="$2"

t/t6018-rev-list-glob.sh

@@ -187,7 +187,7 @@ test_expect_success 'rev-parse --exclude=ref with --remotes=glob' '
 	compare rev-parse "--exclude=upstream/x --remotes=upstream/*" "upstream/one upstream/two"
 '
 
-for section in receive uploadpack
+for section in fetch receive uploadpack
 do
 	test_expect_success "rev-parse --exclude-hidden=$section with --all" '
 		compare "-c transfer.hideRefs=refs/remotes/ rev-parse" "--branches --tags" "--exclude-hidden=$section --all"

t/t6021-rev-list-exclude-hidden.sh

@@ -22,7 +22,7 @@ test_expect_success 'invalid section' '
 	test_cmp expected err
 '
 
-for section in receive uploadpack
+for section in fetch receive uploadpack
 do
 	test_expect_success "$section: passed multiple times" '
 		echo "fatal: --exclude-hidden= passed more than once" >expected &&