Cheaper receive pack connectivity check

Documentation/git-rev-parse.adoc

@@ -199,6 +199,10 @@ If a `pattern` is given, only refs matching the given shell glob are
 shown.  If the pattern does not contain a globbing character (`?`,
 `*`, or `[`), it is turned into a prefix match by appending `/*`.
 
+--toplevel-branches::
+	Show every ref directly under `refs/heads/` (that is, a branch
+	whose short name does not contain a `/`).
+
 --glob=<pattern>::
 	Show all refs matching the shell glob pattern `pattern`. If
 	the pattern does not start with `refs/`, this is automatically

Documentation/rev-list-options.adoc

@@ -170,6 +170,12 @@ endif::git-log[]
 	branches to ones matching given shell glob. If _<pattern>_ lacks '?',
 	'{asterisk}', or '[', '/{asterisk}' at the end is implied.
 
+`--toplevel-branches`::
+	Pretend as if every ref directly under `refs/heads/` is listed on
+	the command line as _<commit>_.  This is a subset of --branches
+	which excludes branches in deeper hierarchies, i.e. excluding
+	branches whose short name contains a `/`.
+
 `--tags[=<pattern>]`::
 	Pretend as if all the refs in `refs/tags` are listed
 	on the command line as _<commit>_. If _<pattern>_ is given, limit

builtin/receive-pack.c

@@ -2058,7 +2058,7 @@ static void execute_commands(struct command *commands,
 		opt.err_fd = err_fd;
 		opt.progress = err_fd && !quiet;
 		opt.env = tmp_objdir_env(tmp_objdir);
-		opt.exclude_hidden_refs_section = "receive";
+		opt.use_toplevel_branches_for_reachability = 1;
 
 		if (check_connected(iterate_receive_command_list, &data, &opt))
 			set_connectivity_errors(commands, si);

builtin/rev-parse.c

@@ -70,6 +70,7 @@ static int is_rev_argument(const char *arg)
 		"--dense",
 		"--branches=",
 		"--branches",
+		"--toplevel-branches",
 		"--header",
 		"--ignore-missing",
 		"--max-age=",
@@ -960,6 +961,15 @@ int cmd_rev_parse(int argc,
 				free(term_bad);
 				continue;
 			}
+			if (!strcmp(arg, "--toplevel-branches")) {
+				if (ref_excludes.hidden_refs_configured)
+					return error(_("options '%s' and '%s' cannot be used together"),
+						     "--exclude-hidden", "--toplevel-branches");
+				refs_for_each_toplevel_branch_ref(get_main_ref_store(the_repository),
+								  show_reference, NULL);
+				clear_ref_exclusions(&ref_excludes);
+				continue;
+			}
 			if (opt_with_value(arg, "--branches", &arg)) {
 				if (ref_excludes.hidden_refs_configured)
 					return error(_("options '%s' and '%s' cannot be used together"),

connected.c

@@ -8,6 +8,7 @@
 #include "sigchain.h"
 #include "connected.h"
 #include "transport.h"
+#include "object-name.h"
 #include "packfile.h"
 #include "promisor-remote.h"
 
@@ -93,10 +94,17 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 		strvec_push(&rev_list.args, "--exclude-promisor-objects");
 	if (!opt->is_deepening_fetch) {
 		strvec_push(&rev_list.args, "--not");
-		if (opt->exclude_hidden_refs_section)
-			strvec_pushf(&rev_list.args, "--exclude-hidden=%s",
-				     opt->exclude_hidden_refs_section);
-		strvec_push(&rev_list.args, "--all");
+		if (opt->use_toplevel_branches_for_reachability) {
+			struct object_id head_oid;
+			strvec_push(&rev_list.args, "--toplevel-branches");
+			if (!repo_get_oid(the_repository, "HEAD", &head_oid))
+				strvec_push(&rev_list.args, "HEAD");
+		} else {
+			if (opt->exclude_hidden_refs_section)
+				strvec_pushf(&rev_list.args, "--exclude-hidden=%s",
+					     opt->exclude_hidden_refs_section);
+			strvec_push(&rev_list.args, "--all");
+		}
 	}
 	strvec_push(&rev_list.args, "--quiet");
 	strvec_push(&rev_list.args, "--alternate-refs");

connected.h

@@ -50,9 +50,19 @@ struct check_connected_options {
 	/*
 	 * If not NULL, use `--exclude-hidden=$section` to exclude all refs
 	 * hidden via the `$section.hideRefs` config from the set of
-	 * already-reachable refs.
+	 * already-reachable refs; irrelevant if
+	 * use_toplevel_branches_for_reachability is set.
 	 */
 	const char *exclude_hidden_refs_section;
+
+	/*
+	 * If set, use only toplevel branches (and HEAD) for the
+	 * reachability check.  This avoids the linear-in-refcount
+	 * enumeration of every visible ref in repositories with many
+	 * branches/tags, at the cost of walking a little further into
+	 * already-reachable history.
+	 */
+	unsigned use_toplevel_branches_for_reachability : 1;
 };
 
 #define CHECK_CONNECTED_INIT { 0 }

refs.c

@@ -552,6 +552,41 @@ int refs_for_each_branch_ref(struct ref_store *refs, refs_for_each_cb cb, void *
 	return refs_for_each_ref_ext(refs, cb, cb_data, &opts);
 }
 
+struct toplevel_branch_filter {
+	refs_for_each_cb *fn;
+	void *cb_data;
+};
+
+static int filter_toplevel_branch(const struct reference *ref, void *data)
+{
+	struct toplevel_branch_filter *filter = data;
+
+	/*
+	 * ref->name has had the "refs/heads/" prefix trimmed, so a
+	 * top-level branch like refs/heads/main appears as "main",
+	 * while a sub-directory branch like refs/heads/dscho/wip
+	 * appears as "dscho/wip".
+	 */
+	if (strchr(ref->name, '/'))
+		return 0;
+	return filter->fn(ref, filter->cb_data);
+}
+
+int refs_for_each_toplevel_branch_ref(struct ref_store *refs,
+				      refs_for_each_cb cb, void *cb_data)
+{
+	struct toplevel_branch_filter filter = {
+		.fn = cb,
+		.cb_data = cb_data,
+	};
+	struct refs_for_each_ref_options opts = {
+		.prefix = "refs/heads/",
+		.trim_prefix = strlen("refs/heads/"),
+	};
+	return refs_for_each_ref_ext(refs, filter_toplevel_branch, &filter,
+				     &opts);
+}
+
 int refs_for_each_remote_ref(struct ref_store *refs, refs_for_each_cb cb, void *cb_data)
 {
 	struct refs_for_each_ref_options opts = {

refs.h

@@ -506,6 +506,8 @@ int refs_for_each_tag_ref(struct ref_store *refs,
 			  refs_for_each_cb fn, void *cb_data);
 int refs_for_each_branch_ref(struct ref_store *refs,
 			     refs_for_each_cb fn, void *cb_data);
+int refs_for_each_toplevel_branch_ref(struct ref_store *refs,
+				      refs_for_each_cb fn, void *cb_data);
 int refs_for_each_remote_ref(struct ref_store *refs,
 			     refs_for_each_cb fn, void *cb_data);
 int refs_for_each_replace_ref(struct ref_store *refs,

revision.c

@@ -2328,6 +2328,7 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 
 	/* pseudo revision arguments */
 	if (!strcmp(arg, "--all") || !strcmp(arg, "--branches") ||
+	    !strcmp(arg, "--toplevel-branches") ||
 	    !strcmp(arg, "--tags") || !strcmp(arg, "--remotes") ||
 	    !strcmp(arg, "--reflog") || !strcmp(arg, "--not") ||
 	    !strcmp(arg, "--no-walk") || !strcmp(arg, "--do-walk") ||
@@ -2807,6 +2808,12 @@ static int handle_revision_pseudo_opt(struct rev_info *revs,
 				     "--exclude-hidden", "--branches");
 		handle_refs(refs, revs, *flags, refs_for_each_branch_ref);
 		clear_ref_exclusions(&revs->ref_excludes);
+	} else if (!strcmp(arg, "--toplevel-branches")) {
+		if (revs->ref_excludes.hidden_refs_configured)
+			return error(_("options '%s' and '%s' cannot be used together"),
+				     "--exclude-hidden", "--toplevel-branches");
+		handle_refs(refs, revs, *flags, refs_for_each_toplevel_branch_ref);
+		clear_ref_exclusions(&revs->ref_excludes);
 	} else if (!strcmp(arg, "--bisect")) {
 		read_bisect_terms(&term_bad, &term_good);
 		handle_refs(refs, revs, *flags, for_each_bad_bisect_ref);

t/t6018-rev-list-glob.sh

@@ -423,4 +423,30 @@ test_expect_failure 'shortlog --glob is not confused by option-like argument' '
 
 '
 
+test_expect_success 'rev-parse --toplevel-branches matches only top-level branches' '
+
+	git rev-parse main someref subspace-x | sort >expect &&
+	git rev-parse --toplevel-branches | sort >actual &&
+	test_cmp expect actual
+
+'
+
+test_expect_success 'rev-list --toplevel-branches matches only top-level branches' '
+
+	git rev-list --no-walk main someref subspace-x | sort >expect &&
+	git rev-list --no-walk --toplevel-branches | sort >actual &&
+	test_cmp expect actual
+
+'
+
+test_expect_success 'rev-list --not --toplevel-branches excludes top-level reachable' '
+
+	# "main" itself is a top-level branch, so excluding all
+	# top-level branches from the walk starting at main leaves
+	# nothing to enumerate.
+	git rev-list main --not --toplevel-branches >actual &&
+	test_must_be_empty actual
+
+'
+
 test_done