remote: allow specifying refs to prefetch

When using 'git fetch --prefetch', all fetchable refs are prefetched
by default. In large repositories with many refs, this can lead to
unnecessary network traffic and increased disk space use.

Introduce a new configuration option 'remote.<name>.prefetchref' that
allows users to specify specific ref patterns to be prefetched during
a 'git fetch --prefetch' operation.

The 'prefetchref' option accepts a space-separated list of ref
patterns (e.g., 'refs/heads/main !refs/heads/feature/*'). When the
'--prefetch' option is used with 'git fetch', only the refs matching
these patterns will be prefetched.

Signed-off-by: Shubham Kanodia <shubham.kanodia10@gmail.com>

Documentation/config/remote.txt

@@ -33,6 +33,13 @@ remote.<name>.fetch::
 	The default set of "refspec" for linkgit:git-fetch[1]. See
 	linkgit:git-fetch[1].
 
+remote.<name>.prefetchref::
+	Specify the refs to be prefetched when fetching from this
+	remote. The value is a space-separated list of ref patterns
+	(e.g., "refs/heads/main !refs/heads/develop*"). This can be
+	used to optimize fetch operations by specifying exactly which
+	refs should be prefetched.
+
 remote.<name>.push::
 	The default set of "refspec" for linkgit:git-push[1]. See
 	linkgit:git-push[1].

builtin/fetch.c

@@ -485,6 +485,32 @@ static void filter_prefetch_refspec(struct refspec *rs)
 	}
 }
 
+static int pattern_matches_ref(const char *pattern, const char *refname)
+{
+	if (strchr(pattern, '*'))
+		return match_refspec_name_with_pattern(pattern, refname, NULL, NULL) != 0;
+	return strcmp(pattern, refname) == 0;
+}
+
+static int matches_prefetch_refs(const char *refname, const struct string_list *prefetch_refs)
+{
+	int has_positive = 0, matched_positive = 0, matched_negative = 0;
+
+	for (int i = 0; i < prefetch_refs->nr; i++) {
+		const char *pattern = prefetch_refs->items[i].string;
+		int is_negative = (*pattern == '!');
+		if (is_negative) pattern++;
+		else has_positive = 1;
+
+		if (pattern_matches_ref(pattern, refname)) {
+			if (is_negative) matched_negative = 1;
+			else matched_positive = 1;
+		}
+	}
+
+	return has_positive ? (matched_positive && !matched_negative) : !matched_negative;
+}
+
 static struct ref *get_ref_map(struct remote *remote,
 			       const struct ref *remote_refs,
 			       struct refspec *rs,
@@ -501,7 +527,11 @@ static struct ref *get_ref_map(struct remote *remote,
 	struct hashmap existing_refs;
 	int existing_refs_populated = 0;
 
+	struct ref *prefetch_filtered_ref_map = NULL, **ref_map_tail = &prefetch_filtered_ref_map;
+	struct ref *next;
+
 	filter_prefetch_refspec(rs);
+
 	if (remote)
 		filter_prefetch_refspec(&remote->fetch);
 
@@ -610,6 +640,29 @@ static struct ref *get_ref_map(struct remote *remote,
 	else
 		ref_map = apply_negative_refspecs(ref_map, &remote->fetch);
 
+	/**
+	 * Filter out advertised refs that we don't want to fetch during
+	 * prefetch if a prefetchref config is set
+	 */
+
+	if (prefetch && remote->prefetch_refs.nr) {
+		prefetch_filtered_ref_map = NULL;
+		ref_map_tail = &prefetch_filtered_ref_map;
+
+		for (rm = ref_map; rm; rm = next) {
+			next = rm->next;
+			rm->next = NULL;
+
+			if (matches_prefetch_refs(rm->name, &remote->prefetch_refs)) {
+				*ref_map_tail = rm;
+				ref_map_tail = &rm->next;
+			} else {
+				free_one_ref(rm);
+			}
+		}
+		ref_map = prefetch_filtered_ref_map;
+	}
+
 	ref_map = ref_remove_duplicates(ref_map);
 
 	for (rm = ref_map; rm; rm = rm->next) {

remote.c

@@ -141,6 +141,7 @@ static struct remote *make_remote(struct remote_state *remote_state,
 	ret->prune = -1;  /* unspecified */
 	ret->prune_tags = -1;  /* unspecified */
 	ret->name = xstrndup(name, len);
+	string_list_init_dup(&ret->prefetch_refs);
 	refspec_init(&ret->push, REFSPEC_PUSH);
 	refspec_init(&ret->fetch, REFSPEC_FETCH);
 
@@ -166,6 +167,7 @@ static void remote_clear(struct remote *remote)
 	free((char *)remote->uploadpack);
 	FREE_AND_NULL(remote->http_proxy);
 	FREE_AND_NULL(remote->http_proxy_authmethod);
+	string_list_clear(&remote->prefetch_refs, 0);
 }
 
 static void add_merge(struct branch *branch, const char *name)
@@ -456,6 +458,12 @@ static int handle_config(const char *key, const char *value,
 		remote->prune = git_config_bool(key, value);
 	else if (!strcmp(subkey, "prunetags"))
 		remote->prune_tags = git_config_bool(key, value);
+	else if (!strcmp(subkey, "prefetchref")) {
+		if (!value)
+			return config_error_nonbool(key);
+		string_list_split(&remote->prefetch_refs, value, ' ', -1);
+		return 0;
+	}
 	else if (!strcmp(subkey, "url")) {
 		if (!value)
 			return config_error_nonbool(key);
@@ -868,7 +876,7 @@ struct strvec *push_url_of_remote(struct remote *remote)
 	return remote->pushurl.nr ? &remote->pushurl : &remote->url;
 }
 
-static int match_name_with_pattern(const char *key, const char *name,
+int match_refspec_name_with_pattern(const char *key, const char *name,
 				   const char *value, char **result)
 {
 	const char *kstar = strchr(key, '*');
@@ -900,7 +908,7 @@ static int refspec_match(const struct refspec_item *refspec,
 			 const char *name)
 {
 	if (refspec->pattern)
-		return match_name_with_pattern(refspec->src, name, NULL, NULL);
+		return match_refspec_name_with_pattern(refspec->src, name, NULL, NULL);
 
 	return !strcmp(refspec->src, name);
 }
@@ -969,7 +977,7 @@ static int query_matches_negative_refspec(struct refspec *rs, struct refspec_ite
 			const char *key = refspec->dst ? refspec->dst : refspec->src;
 			const char *value = refspec->src;
 
-			if (match_name_with_pattern(key, needle, value, &expn_name))
+			if (match_refspec_name_with_pattern(key, needle, value, &expn_name))
 				string_list_append_nodup(&reversed, expn_name);
 		} else if (refspec->matching) {
 			/* For the special matching refspec, any query should match */
@@ -1014,7 +1022,7 @@ static void query_refspecs_multiple(struct refspec *rs,
 		if (!refspec->dst || refspec->negative)
 			continue;
 		if (refspec->pattern) {
-			if (match_name_with_pattern(key, needle, value, result))
+			if (match_refspec_name_with_pattern(key, needle, value, result))
 				string_list_append_nodup(results, *result);
 		} else if (!strcmp(needle, key)) {
 			string_list_append(results, value);
@@ -1043,7 +1051,7 @@ int query_refspecs(struct refspec *rs, struct refspec_item *query)
 		if (!refspec->dst || refspec->negative)
 			continue;
 		if (refspec->pattern) {
-			if (match_name_with_pattern(key, needle, value, result)) {
+			if (match_refspec_name_with_pattern(key, needle, value, result)) {
 				query->force = refspec->force;
 				return 0;
 			}
@@ -1456,9 +1464,9 @@ static char *get_ref_match(const struct refspec *rs, const struct ref *ref,
 			const char *dst_side = item->dst ? item->dst : item->src;
 			int match;
 			if (direction == FROM_SRC)
-				match = match_name_with_pattern(item->src, ref->name, dst_side, &name);
+				match = match_refspec_name_with_pattern(item->src, ref->name, dst_side, &name);
 			else
-				match = match_name_with_pattern(dst_side, ref->name, item->src, &name);
+				match = match_refspec_name_with_pattern(dst_side, ref->name, item->src, &name);
 			if (match) {
 				matching_refs = i;
 				break;
@@ -2076,7 +2084,7 @@ static struct ref *get_expanded_map(const struct ref *remote_refs,
 
 		if (strchr(ref->name, '^'))
 			continue; /* a dereference item */
-		if (match_name_with_pattern(refspec->src, ref->name,
+		if (match_refspec_name_with_pattern(refspec->src, ref->name,
 					    refspec->dst, &expn_name) &&
 		    !ignore_symref_update(expn_name, &scratch)) {
 			struct ref *cpy = copy_ref(ref);

remote.h

@@ -5,6 +5,7 @@
 #include "hashmap.h"
 #include "refspec.h"
 #include "strvec.h"
+#include "string-list.h"
 
 struct option;
 struct transport_ls_refs_options;
@@ -77,6 +78,8 @@ struct remote {
 
 	struct refspec fetch;
 
+	struct string_list prefetch_refs;
+
 	/*
 	 * The setting for whether to fetch tags (as a separate rule from the
 	 * configured refspecs);
@@ -207,6 +210,9 @@ int count_refspec_match(const char *, struct ref *refs, struct ref **matched_ref
 
 int check_ref_type(const struct ref *ref, int flags);
 
+int match_refspec_name_with_pattern(const char *key, const char *name,
+						    const char *value, char **result);
+
 /*
  * Free a single ref and its peer, or an entire list of refs and their peers,
  * respectively.

t/t7900-maintenance.sh

@@ -245,6 +245,91 @@ test_expect_success 'prefetch multiple remotes' '
 	test_subcommand git fetch remote2 $fetchargs <skip-remote1.txt
 '
 
+test_expect_success 'prefetch with positive prefetch ref patterns' '
+	test_create_repo filter-prefetch-positive &&
+	(
+		cd filter-prefetch-positive &&
+		test_commit initial &&
+		git clone . clone2 &&
+		git remote add remote2 "file://$(pwd)/clone2" &&
+
+		cd clone2 &&
+		git checkout -b feature && test_commit feature-commit-2 &&
+		git checkout -b wip/test && test_commit wip-test-commit-2 &&
+		git checkout -b topic/x && test_commit topic-x-commit-2 &&
+		git push -f origin feature wip/test topic/x &&
+		cd .. &&
+
+		git config remote.remote2.prefetchref "refs/heads/feature" &&
+		fetchargs="--prefetch --prune --no-tags --no-write-fetch-head \
+			--recurse-submodules=no --quiet" &&
+		GIT_TRACE2_EVENT="$(pwd)/prefetch-positive.txt" \
+			git maintenance run --task=prefetch 2>/dev/null &&
+		test_subcommand git fetch remote2 $fetchargs <prefetch-positive.txt &&
+
+		git rev-parse refs/prefetch/remotes/remote2/feature &&
+		test_must_fail git rev-parse refs/prefetch/remotes/remote2/wip/test &&
+		test_must_fail git rev-parse refs/prefetch/remotes/remote2/topic/x
+	)
+'
+
+test_expect_success 'prefetch with negative prefetch ref patterns' '
+	test_create_repo filter-prefetch-negative &&
+	(
+		cd filter-prefetch-negative &&
+		test_commit initial &&
+		git clone . clone3 &&
+		git remote add remote3 "file://$(pwd)/clone3" &&
+		cat .git/config &&
+
+		cd clone3 &&
+		git checkout -b feature && test_commit feature-commit-3 &&
+		git checkout -b wip/test && test_commit wip-test-commit-3 &&
+		git checkout -b topic/x && test_commit topic-x-commit-3 &&
+		git push -f origin feature wip/test topic/x &&
+		cd .. &&
+
+		git config remote.remote3.prefetchref "!refs/heads/wip/*" &&
+		fetchargs="--prefetch --prune --no-tags --no-write-fetch-head \
+			--recurse-submodules=no --quiet" &&
+		GIT_TRACE2_EVENT="$(pwd)/prefetch-negative.txt" \
+			git maintenance run --task=prefetch 2>/dev/null &&
+		test_subcommand git fetch remote3 $fetchargs <prefetch-negative.txt &&
+		git rev-parse refs/prefetch/remotes/remote3/feature &&
+		git rev-parse refs/prefetch/remotes/remote3/topic/x &&
+		test_must_fail git rev-parse refs/prefetch/remotes/remote3/wip/test
+	)
+'
+
+test_expect_success 'prefetch with positive & negative prefetch ref patterns' '
+	test_create_repo filter-prefetch-mixed &&
+	(
+		cd filter-prefetch-mixed &&
+		test_commit initial &&
+		git clone . clone4 &&
+		git remote add remote4 "file://$(pwd)/clone4" &&
+
+		cd clone4 &&
+		git checkout -b feature && test_commit feature-commit-4 &&
+		git checkout -b topic/x && test_commit topic-x-commit-4 &&
+		git checkout -b topic/y && test_commit topic-y-commit-4 &&
+		git push -f origin feature topic/x topic/y &&
+		cd .. &&
+
+		git config remote.remote4.prefetchref \
+			"refs/heads/topic/* !refs/heads/topic/y" &&
+		fetchargs="--prefetch --prune --no-tags --no-write-fetch-head \
+			--recurse-submodules=no --quiet" &&
+		GIT_TRACE2_EVENT="$(pwd)/prefetch-mixed.txt" \
+			git maintenance run --task=prefetch 2>/dev/null &&
+		test_subcommand git fetch remote4 $fetchargs <prefetch-mixed.txt &&
+
+		test_must_fail git rev-parse refs/prefetch/remotes/remote4/feature &&
+		test_must_fail git rev-parse refs/prefetch/remotes/remote4/topic/y &&
+		git rev-parse refs/prefetch/remotes/remote4/topic/x
+	)
+'
+
 test_expect_success 'loose-objects task' '
 	# Repack everything so we know the state of the object dir
 	git repack -adk &&