[PATCH] builtin/fetch: skip unnecessary tasks when using --negotiate-only

[Glen Choo <chooglen@google.com>]

`git fetch --negotiate-only` does not fetch objects and thus, it should
not perform certain auxiliary tasks like updating submodules, updating
the commit graph, or running gc. Although send_pack() invokes `git fetch
--negotiate-only` correctly, cmd_fetch() also reads config variables,
leading to undesirable behavior, like updating submodules if
`submodule.recurse=true`.

Make cmd_fetch() return early if --negotiate-only was specified so that
these auxiliary tasks are skipped.

Signed-off-by: Glen Choo <chooglen@google.com>
---
`git fetch --negotiate-only` is used during push negotiation to
determine the reachability of commits. As its name implies, only
negotiation is performed, not the actual fetching of objects. However,
cmd_fetch() performs certain tasks with the assumption that objects are
fetched:

* Submodules are updated if enabled by recurse.submodules=true, but
  negotiation fetch doesn't actually update the repo, so this doesn't
  make sense (introduced in [1]).
* Commit graphs will be written if enabled by
  fetch.writeCommitGraph=true. But according to
  Documentation/config/fetch.txt [2], this should only be done if a
  pack-file is downloaded
* gc is run, but according to [3], we only do this because we expect
  `git fetch` to introduce objects

Instead of disabling these tasks piecemeal, let's just make cmd_fetch()
return early if --negotiate-only was given. To accommodate possible
future options that don't fetch objects, I opted to introduce another
`if` statement instead of putting the early return in the existing
`if (negotiate_only)` block.

[1] 7dce19d374 (fetch/pull: Add the --recurse-submodules option, 2010-11-12)
[2] 50f26bd035 (fetch: add fetch.writeCommitGraph config setting, 2019-09-02)
[3] 131b8fcbfb (fetch: run gc --auto after fetching, 2013-01-26)

 builtin/fetch.c       | 22 +++++++++++++++++-----
 t/t5516-fetch-push.sh | 12 ++++++++++++
 2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/builtin/fetch.c b/builtin/fetch.c
index f7abbc31ff..01865b5c09 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1996,6 +1996,17 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 
 	argc = parse_options(argc, argv, prefix,
 			     builtin_fetch_options, builtin_fetch_usage, 0);
+
+	if (negotiate_only) {
+		/*
+		 * --negotiate-only should never recurse into
+		 * submodules, so there is no need to read .gitmodules.
+		 */
+		recurse_submodules = RECURSE_SUBMODULES_OFF;
+		if (!negotiation_tip.nr)
+			die(_("--negotiate-only needs one or more --negotiate-tip=*"));
+	}
+
 	if (recurse_submodules != RECURSE_SUBMODULES_OFF) {
 		int *sfjc = submodule_fetch_jobs_config == -1
 			    ? &submodule_fetch_jobs_config : NULL;
@@ -2005,9 +2016,6 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		fetch_config_from_gitmodules(sfjc, rs);
 	}
 
-	if (negotiate_only && !negotiation_tip.nr)
-		die(_("--negotiate-only needs one or more --negotiate-tip=*"));
-
 	if (deepen_relative) {
 		if (deepen_relative < 0)
 			die(_("Negative depth in --deepen is not supported"));
@@ -2112,6 +2120,12 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		result = fetch_multiple(&list, max_children);
 	}
 
+	string_list_clear(&list, 0);
+
+	/* skip irrelevant tasks if objects were not fetched  */
+	if (negotiate_only)
+		return result;
+
 	if (!result && (recurse_submodules != RECURSE_SUBMODULES_OFF)) {
 		struct strvec options = STRVEC_INIT;
 		int max_children = max_jobs;
@@ -2132,8 +2146,6 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		strvec_clear(&options);
 	}
 
-	string_list_clear(&list, 0);
-
 	prepare_repo_settings(the_repository);
 	if (fetch_write_commit_graph > 0 ||
 	    (fetch_write_commit_graph < 0 &&
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index 8212ca56dc..732031085e 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -229,6 +229,18 @@ test_expect_success 'push with negotiation proceeds anyway even if negotiation f
 	test_i18ngrep "push negotiation failed" err
 '
 
+test_expect_success 'push with negotiation does not attempt to fetch submodules' '
+	mk_empty submodule_upstream &&
+	test_commit -C submodule_upstream submodule_commit &&
+	git submodule add ./submodule_upstream submodule &&
+	mk_empty testrepo &&
+	git push testrepo $the_first_commit:refs/remotes/origin/first_commit &&
+	test_commit -C testrepo unrelated_commit &&
+	git -C testrepo config receive.hideRefs refs/remotes/origin/first_commit &&
+	git -c submodule.recurse=true -c protocol.version=2 -c push.negotiate=1 push testrepo refs/heads/main:refs/remotes/origin/main 2>err &&
+	! grep "Fetching submodule" err
+'
+
 test_expect_success 'push without wildcard' '
 	mk_empty testrepo &&
 
-- 
2.33.GIT