This commit introduces a new configuration option,

remote.<name>.prefetchref, which allows users to specify specific
ref patterns to be prefetched during a git fetch --prefetch
operation.

The new option accepts a space-separated list of ref patterns.
When the --prefetch option is used with git fetch, only the refs
matching these patterns will be prefetched, instead of the
default behavior of prefetching all fetchable refs.

Example usage in .git/config:
[remote "origin"]
    prefetchref = "refs/heads/main refs/heads/feature/*"

This change allows users to optimize their prefetch operations,
potentially reducing network traffic and improving performance
for large repositories with many refs.

This change allows users to optimize their prefetch operations, potentially
reducing network traffic and improving performance for large repositories
with many refs.

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

Documentation/config/remote.txt

@@ -33,6 +33,12 @@ 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/master refs/heads/develop*").
+    These patterns are used as the source part of the refspecs for prefetching.
+    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

@@ -434,6 +434,30 @@ static void find_non_local_tags(const struct ref *refs,
 	oidset_clear(&fetch_oids);
 }
 
+static void apply_prefetch_refspec(struct remote *remote, struct refspec *rs)
+{
+	if (remote->prefetch_refs.nr > 0) {
+		int i;
+		for (i = 0; i < remote->prefetch_refs.nr; i++) {
+			const char *src = remote->prefetch_refs.items[i].string;
+			struct strbuf dst = STRBUF_INIT;
+
+			strbuf_addf(&dst, "refs/prefetch/%s/", remote->name);
+			if (starts_with(src, "refs/heads/")) {
+				strbuf_addstr(&dst, src + 11);
+			} else if (starts_with(src, "refs/")) {
+				strbuf_addstr(&dst, src + 5);
+			} else {
+				strbuf_addstr(&dst, src);
+			}
+
+			refspec_appendf(rs, "%s:%s", src, dst.buf);
+			strbuf_release(&dst);
+		}
+	}
+}
+
+
 static void filter_prefetch_refspec(struct refspec *rs)
 {
 	int i;
@@ -502,8 +526,11 @@ static struct ref *get_ref_map(struct remote *remote,
 	int existing_refs_populated = 0;
 
 	filter_prefetch_refspec(rs);
-	if (remote)
+	if (remote) {
 		filter_prefetch_refspec(&remote->fetch);
+		if (prefetch)
+			apply_prefetch_refspec(remote, rs);
+	}
 
 	if (rs->nr) {
 		struct refspec *fetch_refspec;

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);

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);

t/t7900-maintenance.sh

@@ -245,6 +245,76 @@ test_expect_success 'prefetch multiple remotes' '
 	test_subcommand git fetch remote2 $fetchargs <skip-remote1.txt
 '
 
+test_expect_success 'prefetch only acts on remote.<name>.prefetchref refs if present' '
+	test_create_repo prefetch-test-mixed-patterns &&
+	(
+		cd prefetch-test-mixed-patterns &&
+		test_commit initial &&
+		git clone . clone1 &&
+		git clone . clone2 &&
+
+		git remote add remote1 "file://$(pwd)/clone1" &&
+		git remote add remote2 "file://$(pwd)/clone2" &&
+
+		# Set single prefetchref pattern for remote1 and multiple for remote2
+		git config remote.remote1.prefetchref "refs/heads/foo" &&
+		git config remote.remote2.prefetchref "refs/heads/feature/* refs/heads/topic" &&
+
+		# Create branches in clone1 and push
+		(
+			cd clone1 &&
+			git checkout -b foo &&
+			test_commit foo-commit &&
+			git checkout -b feature/a &&
+			test_commit feature-a-commit &&
+			git checkout -b other &&
+			test_commit other-commit &&
+			git push origin foo feature/a other
+		) &&
+
+		# Create branches in clone2 and push
+		(
+			cd clone2 &&
+			git checkout -b topic &&
+			test_commit master-commit &&
+			git checkout -b feature/x &&
+			test_commit feature-x-commit &&
+			git checkout -b feature/y &&
+			test_commit feature-y-commit &&
+			git checkout -b dev &&
+			test_commit dev-commit &&
+			git push origin topic feature/x feature/y dev
+		) &&
+
+		# Run maintenance prefetch task
+		GIT_TRACE2_EVENT="$(pwd)/prefetch.txt" git maintenance run --task=prefetch 2>/dev/null &&
+
+		# Check that only specified refs were prefetched
+		fetchargs="--prefetch --prune --no-tags --no-write-fetch-head --recurse-submodules=no --quiet" &&
+		test_subcommand git fetch remote1 $fetchargs <prefetch.txt &&
+		test_subcommand git fetch remote2 $fetchargs <prefetch.txt &&
+		ls -R .git/refs/prefetch &&
+
+		# Verify that only specified refs are in the prefetch refs for remote1
+		 git rev-parse refs/prefetch/remotes/remote1/foo &&
+		test_must_fail git rev-parse refs/prefetch/remotes/remote1/feature/a &&
+		test_must_fail git rev-parse refs/prefetch/remotes/remote1/other &&
+
+				# Verify that only specified refs are in the prefetch refs for remote2
+		git rev-parse refs/prefetch/remotes/remote2/feature/x &&
+		git rev-parse refs/prefetch/remotes/remote2/feature/y &&
+		git rev-parse refs/prefetch/remotes/remote2/topic &&
+		test_must_fail git rev-parse refs/prefetch/remotes/remote2/dev &&
+
+		# Fetch all refs and compare
+		git fetch --all &&
+		test_cmp_rev refs/remotes/remote1/foo refs/prefetch/remotes/remote1/foo &&
+		test_cmp_rev refs/remotes/remote2/feature/x refs/prefetch/remotes/remote2/feature/x &&
+		test_cmp_rev refs/remotes/remote2/feature/y refs/prefetch/remotes/remote2/feature/y &&
+		test_cmp_rev refs/remotes/remote2/topic refs/prefetch/remotes/remote2/topic
+	)
+'
+
 test_expect_success 'loose-objects task' '
 	# Repack everything so we know the state of the object dir
 	git repack -adk &&