Users can optionally hide refs from remote users in git-upload-pack(1),
git-receive-pack(1) and others via the `transfer.hideRefs`, but there is
not an easy way to obtain the list of all visible or hidden refs right
now. We'll require just that though for a performance improvement in our
connectivity check.

Add a new pseudo-ref `--visible-refs=` that pretends as if all refs have
been added to the command line that are not hidden. The pseudo-ref
requiers either one of "transfer", "uploadpack" or "receive" as argument
to pay attention to `transfer.hideRefs`, `uploadpack.hideRefs` or
`receive.hideRefs`, respectively.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 Documentation/rev-list-options.txt |  15 +++--
 builtin/rev-list.c                 |   1 +
 builtin/rev-parse.c                |   1 +
 revision.c                         |  34 +++++++++-
 t/t6021-rev-list-visible-refs.sh   | 102 +++++++++++++++++++++++++++++
 5 files changed, 145 insertions(+), 8 deletions(-)
 create mode 100755 t/t6021-rev-list-visible-refs.sh

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 1837509566..a0e34b0e2b 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -180,14 +180,19 @@ endif::git-log[]
 	is automatically prepended if missing. If pattern lacks '?', '{asterisk}',
 	or '[', '/{asterisk}' at the end is implied.
 
+--visible-refs=[transfer|receive|uploadpack]::
+	Pretend as if all the refs that have not been hidden via either one of
+	`transfer.hideRefs`, `receive.hideRefs` or `uploadpack.hideRefs` are
+	listed on the command line.
+
 --exclude=<glob-pattern>::
 
 	Do not include refs matching '<glob-pattern>' that the next `--all`,
-	`--branches`, `--tags`, `--remotes`, or `--glob` would otherwise
-	consider. Repetitions of this option accumulate exclusion patterns
-	up to the next `--all`, `--branches`, `--tags`, `--remotes`, or
-	`--glob` option (other options or arguments do not clear
-	accumulated patterns).
+	`--branches`, `--tags`, `--remotes`, `--glob` or `--visible-refs` would
+	otherwise consider. Repetitions of this option accumulate exclusion
+	patterns up to the next `--all`, `--branches`, `--tags`, `--remotes`,
+	`--glob` or `--visible-refs` option (other options or arguments do not
+	clear accumulated patterns).
 +
 The patterns given should not begin with `refs/heads`, `refs/tags`, or
 `refs/remotes` when applied to `--branches`, `--tags`, or `--remotes`,
diff --git a/builtin/rev-list.c b/builtin/rev-list.c
index 3acd93f71e..f719286cf8 100644
--- a/builtin/rev-list.c
+++ b/builtin/rev-list.c
@@ -38,6 +38,7 @@ static const char rev_list_usage[] =
 "    --tags\n"
 "    --remotes\n"
 "    --stdin\n"
+"    --visible-refs=[transfer|receive|uploadpack]\n"
 "    --quiet\n"
 "  ordering output:\n"
 "    --topo-order\n"
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 8f61050bde..31617bf3d5 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -77,6 +77,7 @@ static int is_rev_argument(const char *arg)
 		"--topo-order",
 		"--date-order",
 		"--unpacked",
+		"--visible-refs=",
 		NULL
 	};
 	const char **p = rev_args;
diff --git a/revision.c b/revision.c
index 0760e78936..ef9e2947af 100644
--- a/revision.c
+++ b/revision.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "config.h"
 #include "object-store.h"
 #include "tag.h"
 #include "blob.h"
@@ -1523,6 +1524,8 @@ struct all_refs_cb {
 	struct rev_info *all_revs;
 	const char *name_for_errormsg;
 	struct worktree *wt;
+	struct string_list hidden_refs;
+	const char *hidden_refs_section;
 };
 
 int ref_excluded(struct string_list *ref_excludes, const char *path)
@@ -1542,11 +1545,13 @@ static int handle_one_ref(const char *path, const struct object_id *oid,
 			  int flag UNUSED,
 			  void *cb_data)
 {
+	const char *stripped_path = strip_namespace(path);
 	struct all_refs_cb *cb = cb_data;
 	struct object *object;
 
-	if (ref_excluded(cb->all_revs->ref_excludes, path))
-	    return 0;
+	if (ref_excluded(cb->all_revs->ref_excludes, path) ||
+	    ref_is_hidden(stripped_path, path, &cb->hidden_refs))
+		return 0;
 
 	object = get_reference(cb->all_revs, path, oid, cb->all_flags);
 	add_rev_cmdline(cb->all_revs, object, path, REV_CMD_REF, cb->all_flags);
@@ -1561,6 +1566,7 @@ static void init_all_refs_cb(struct all_refs_cb *cb, struct rev_info *revs,
 	cb->all_flags = flags;
 	revs->rev_input_given = 1;
 	cb->wt = NULL;
+	string_list_init_dup(&cb->hidden_refs);
 }
 
 void clear_ref_exclusion(struct string_list **ref_excludes_p)
@@ -1596,6 +1602,13 @@ static void handle_refs(struct ref_store *refs,
 	for_each(refs, handle_one_ref, &cb);
 }
 
+static int hide_refs_config(const char *var, const char *value, void *cb_data)
+{
+	struct all_refs_cb *cb = cb_data;
+	return parse_hide_refs_config(var, value, cb->hidden_refs_section,
+				      &cb->hidden_refs);
+}
+
 static void handle_one_reflog_commit(struct object_id *oid, void *cb_data)
 {
 	struct all_refs_cb *cb = cb_data;
@@ -2225,7 +2238,7 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	    !strcmp(arg, "--bisect") || starts_with(arg, "--glob=") ||
 	    !strcmp(arg, "--indexed-objects") ||
 	    !strcmp(arg, "--alternate-refs") ||
-	    starts_with(arg, "--exclude=") ||
+	    starts_with(arg, "--exclude=") || starts_with(arg, "--visible-refs=") ||
 	    starts_with(arg, "--branches=") || starts_with(arg, "--tags=") ||
 	    starts_with(arg, "--remotes=") || starts_with(arg, "--no-walk="))
 	{
@@ -2759,6 +2772,21 @@ static int handle_revision_pseudo_opt(struct rev_info *revs,
 		parse_list_objects_filter(&revs->filter, arg);
 	} else if (!strcmp(arg, ("--no-filter"))) {
 		list_objects_filter_set_no_filter(&revs->filter);
+	} else if (skip_prefix(arg, "--visible-refs=", &arg)) {
+		struct all_refs_cb cb;
+
+		if (strcmp(arg, "transfer") && strcmp(arg, "receive") &&
+		    strcmp(arg, "uploadpack"))
+			die(_("unsupported section for --visible-refs: %s"), arg);
+
+		init_all_refs_cb(&cb, revs, *flags);
+		cb.hidden_refs_section = arg;
+		git_config(hide_refs_config, &cb);
+
+		refs_for_each_ref(refs, handle_one_ref, &cb);
+
+		string_list_clear(&cb.hidden_refs, 1);
+		clear_ref_exclusion(&revs->ref_excludes);
 	} else {
 		return 0;
 	}
diff --git a/t/t6021-rev-list-visible-refs.sh b/t/t6021-rev-list-visible-refs.sh
new file mode 100755
index 0000000000..9e12384dcf
--- /dev/null
+++ b/t/t6021-rev-list-visible-refs.sh
@@ -0,0 +1,102 @@
+#!/bin/sh
+
+test_description='git rev-list --visible-refs test'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit_bulk --id=commit --ref=refs/heads/main 1 &&
+	COMMIT=$(git rev-parse refs/heads/main) &&
+	test_commit_bulk --id=tag --ref=refs/tags/lightweight 1 &&
+	TAG=$(git rev-parse refs/tags/lightweight) &&
+	test_commit_bulk --id=hidden --ref=refs/hidden/commit 1 &&
+	HIDDEN=$(git rev-parse refs/hidden/commit)
+'
+
+test_expect_success 'invalid section' '
+	echo "fatal: unsupported section for --visible-refs: unsupported" >expected &&
+	test_must_fail git rev-list --visible-refs=unsupported 2>err &&
+	test_cmp expected err
+'
+
+test_expect_success '--visible-refs without hiddenRefs' '
+	git rev-list --visible-refs=transfer >out &&
+	cat >expected <<-EOF &&
+	$HIDDEN
+	$TAG
+	$COMMIT
+	EOF
+	test_cmp expected out
+'
+
+test_expect_success 'hidden via transfer.hideRefs' '
+	git -c transfer.hideRefs=refs/hidden/ rev-list --visible-refs=transfer >out &&
+	cat >expected <<-EOF &&
+	$TAG
+	$COMMIT
+	EOF
+	test_cmp expected out
+'
+
+test_expect_success '--all --not --visible-refs=transfer without hidden refs' '
+	git rev-list --all --not --visible-refs=transfer >out &&
+	test_must_be_empty out
+'
+
+test_expect_success '--all --not --visible-refs=transfer with hidden ref' '
+	git -c transfer.hideRefs=refs/hidden/ rev-list --all --not --visible-refs=transfer >out &&
+	cat >expected <<-EOF &&
+	$HIDDEN
+	EOF
+	test_cmp expected out
+'
+
+test_expect_success '--visible-refs with --exclude' '
+	git -c transfer.hideRefs=refs/hidden/ rev-list --exclude=refs/tags/* --visible-refs=transfer >out &&
+	cat >expected <<-EOF &&
+	$COMMIT
+	EOF
+	test_cmp expected out
+'
+
+for section in receive uploadpack
+do
+	test_expect_success "hidden via $section.hideRefs" '
+		git -c receive.hideRefs=refs/hidden/ rev-list --visible-refs=receive >out &&
+		cat >expected <<-EOF &&
+		$TAG
+		$COMMIT
+		EOF
+		test_cmp expected out
+	'
+
+	test_expect_success "--visible-refs=$section respects transfer.hideRefs" '
+		git -c transfer.hideRefs=refs/hidden/ rev-list --visible-refs=$section >out &&
+		cat >expected <<-EOF &&
+		$TAG
+		$COMMIT
+		EOF
+		test_cmp expected out
+	'
+
+	test_expect_success "--visible-refs=transfer ignores $section.hideRefs" '
+		git -c $section.hideRefs=refs/hidden/ rev-list --visible-refs=transfer >out &&
+		cat >expected <<-EOF &&
+		$HIDDEN
+		$TAG
+		$COMMIT
+		EOF
+		test_cmp expected out
+	'
+
+	test_expect_success "--visible-refs=$section respects both transfer.hideRefs and $section.hideRefs" '
+		git -c transfer.hideRefs=refs/tags/ -c $section.hideRefs=refs/hidden/ rev-list --visible-refs=$section >out &&
+		cat >expected <<-EOF &&
+		$COMMIT
+		EOF
+		test_cmp expected out
+	'
+done
+
+test_done
-- 
2.38.1