remote: introduce config to set prefetch refs

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.

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 &&