remote: prefetch config

Large repositories often contain numerous branches and refs, many of
which individual users may not need. This commit introduces a new
configuration option (`remote.<remote>.prefetch`) to allow
users to specify which remotes to prefetch during
the maintenance task.

Key behaviors:
1. If `remote.<remote>.prefetch` is unset or true, running
   `git-maintenance` will prefetch all refs for the remote.
2. If `remote.<remote>.prefetch` is set to false, the remote
   will be ignored for prefetching.

In a future change, we could also allow restricting the refs that are
prefetched per remote using the `prefetchref` config option per remote.

Both of these options in unison would allow users to optimize their
prefetch operations, reducing network traffic and disk usage.

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

Documentation/config/remote.txt

@@ -33,6 +33,11 @@ remote.<name>.fetch::
 	The default set of "refspec" for linkgit:git-fetch[1]. See
 	linkgit:git-fetch[1].
 
+remote.<name>.prefetch::
+	If false, refs from the remote would not be prefetched for
+	the prefetch task in linkgit:git-maintenance[1]. If not set,
+	the value is assumed to be true.
+
 remote.<name>.push::
 	The default set of "refspec" for linkgit:git-push[1]. See
 	linkgit:git-push[1].

Documentation/git-maintenance.txt

@@ -97,9 +97,10 @@ commit-graph::
 
 prefetch::
 	The `prefetch` task updates the object directory with the latest
-	objects from all registered remotes. For each remote, a `git fetch`
-	command is run. The configured refspec is modified to place all
-	requested refs within `refs/prefetch/`. Also, tags are not updated.
+	objects from all registered remotes unless they've disabled prefetch
+	using `remote.<remote>.prefetch` set to `false`. For each such remote,
+	a `git fetch` command is run. The configured refspec is modified to place
+	all requested refs within `refs/prefetch/`. Also, tags are not updated.
 +
 This is done to avoid disrupting the remote-tracking branches. The end users
 expect these refs to stay unmoved unless they initiate a fetch.  However,

builtin/gc.c

@@ -1027,6 +1027,9 @@ static int fetch_remote(struct remote *remote, void *cbdata)
 	if (remote->skip_default_update)
 		return 0;
 
+	if (remote->prefetch == 0)
+		return 0;
+
 	child.git_cmd = 1;
 	strvec_pushl(&child.args, "fetch", remote->name,
 		     "--prefetch", "--prune", "--no-tags",

remote.c

@@ -140,6 +140,7 @@ static struct remote *make_remote(struct remote_state *remote_state,
 	CALLOC_ARRAY(ret, 1);
 	ret->prune = -1;  /* unspecified */
 	ret->prune_tags = -1;  /* unspecified */
+	ret->prefetch = -1;  /* unspecified */
 	ret->name = xstrndup(name, len);
 	refspec_init(&ret->push, REFSPEC_PUSH);
 	refspec_init(&ret->fetch, REFSPEC_FETCH);
@@ -456,6 +457,8 @@ 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, "prefetch"))
+		remote->prefetch = git_config_bool(key, value);
 	else if (!strcmp(subkey, "url")) {
 		if (!value)
 			return config_error_nonbool(key);

remote.h

@@ -77,6 +77,15 @@ struct remote {
 
 	struct refspec fetch;
 
+	/*
+	 * The setting for whether to prefetch from a remote
+	 * when a fetch is invoked with a prefetch flag.
+	 *  -1 = unset
+	 *   0 = don't prefetch from this remote
+	 *   1 = prefetch from this remote
+	 */
+	int prefetch;
+
 	/*
 	 * The setting for whether to fetch tags (as a separate rule from the
 	 * configured refspecs);

t/t7900-maintenance.sh

@@ -245,6 +245,48 @@ test_expect_success 'prefetch multiple remotes' '
 	test_subcommand git fetch remote2 $fetchargs <skip-remote1.txt
 '
 
+test_expect_success 'prefetch respects remote.*.prefetch config' '
+	test_create_repo prefetch-test-config &&
+	(
+		cd prefetch-test-config &&
+		test_commit initial &&
+		test_create_repo clone1 &&
+		test_create_repo clone2 &&
+		test_create_repo clone3 &&
+
+		git remote add remote1 "file://$(pwd)/clone1" &&
+		git remote add remote2 "file://$(pwd)/clone2" &&
+		git remote add remote3 "file://$(pwd)/clone3" &&
+
+		git config remote.remote1.prefetch false &&
+		git config remote.remote2.prefetch true &&
+		# remote3 is left unset
+
+		# Make changes in all clones
+		git -C clone1 switch -c one &&
+		git -C clone2 switch -c two &&
+		git -C clone3 switch -c three &&
+		test_commit -C clone1 one &&
+		test_commit -C clone2 two &&
+		test_commit -C clone3 three &&
+
+		# Run maintenance prefetch task
+		GIT_TRACE2_EVENT="$(pwd)/prefetch.txt" git maintenance run --task=prefetch 2>/dev/null &&
+
+		# Check that if remotes were prefetched properly
+		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 &&
+		test_subcommand git fetch remote3 $fetchargs <prefetch.txt &&
+
+		# Verify that changes are in the prefetch refs for remote2 and remote3, but not remote1
+		test_must_fail git rev-parse refs/prefetch/remotes/remote1/one &&
+		git fetch --all &&
+		test_cmp_rev refs/remotes/remote2/two refs/prefetch/remotes/remote2/two &&
+		test_cmp_rev refs/remotes/remote3/three refs/prefetch/remotes/remote3/three
+	)
+'
+
 test_expect_success 'loose-objects task' '
 	# Repack everything so we know the state of the object dir
 	git repack -adk &&