[PATCH v8 1/2] rerere: wait for MERGE_RR.lock before giving up

["Thomas Bachem via GitGitGadget" <gitgitgadget@gmail.com>]

From: Thomas Bachem <mail@thomasbachem.com>

setup_rerere() takes MERGE_RR.lock with LOCK_DIE_ON_ERROR, so when two
processes want it at the same time, the second one dies. That
used to be rare. Since 452b12c2e0 (builtin/maintenance: use
"geometric" strategy by default, 2026-02-24) the auto maintenance
after a commit runs "git rerere gc" whenever rr-cache has enough
stale entries, and the gc holds the lock while it prunes.

If a rebase's next pick conflicts while the gc holds the lock, the
rebase dies inside repo_rerere(), before the sequencer has written what
"git rebase --continue" needs. Every later "git rebase --continue"
then fails with "you have staged changes in your working tree".

Wait for the lock for up to rerere.lockTimeout milliseconds, 1000 by
default, and only then fail as before. Pruning a few thousand entries
takes well under a second, so the default covers a rebase that runs
into the gc.

Assisted-by: Claude Fable 5.1
Signed-off-by: Thomas Bachem <mail@thomasbachem.com>
---
 Documentation/config/rerere.adoc |  7 +++++
 rerere.c                         | 22 ++++++++++---
 t/t4200-rerere.sh                | 53 ++++++++++++++++++++++++++++++++
 3 files changed, 77 insertions(+), 5 deletions(-)

diff --git a/Documentation/config/rerere.adoc b/Documentation/config/rerere.adoc
index 3a78b5ebb1..ab035e2d7d 100644
--- a/Documentation/config/rerere.adoc
+++ b/Documentation/config/rerere.adoc
@@ -10,3 +10,10 @@ rerere.enabled::
 	enabled if there is an `rr-cache` directory under the
 	`$GIT_DIR`, e.g. if "rerere" was previously used in the
 	repository.
+
+rerere.lockTimeout::
+	The length of time, in milliseconds, to wait for the rerere
+	lock when another process holds it, typically a background
+	`git rerere gc`.  Value 0 means not to wait at all; -1 means
+	to wait indefinitely.  Default is 1000 (i.e., wait for 1
+	second).
diff --git a/rerere.c b/rerere.c
index 1c3745d9e3..64fac07c71 100644
--- a/rerere.c
+++ b/rerere.c
@@ -33,6 +33,9 @@ static int rerere_enabled = -1;
 /* automatically update cleanly resolved paths to the index */
 static int rerere_autoupdate;
 
+/* how long to wait for MERGE_RR.lock, in milliseconds */
+static int rerere_lock_timeout_ms = 1000;
+
 #define RR_HAS_POSTIMAGE 1
 #define RR_HAS_PREIMAGE 2
 struct rerere_dir {
@@ -850,6 +853,8 @@ static void git_rerere_config(void)
 {
 	repo_config_get_bool(the_repository, "rerere.enabled", &rerere_enabled);
 	repo_config_get_bool(the_repository, "rerere.autoupdate", &rerere_autoupdate);
+	repo_config_get_int(the_repository, "rerere.locktimeout",
+			    &rerere_lock_timeout_ms);
 	repo_config(the_repository, git_default_config, NULL);
 }
 
@@ -882,12 +887,19 @@ int setup_rerere(struct repository *r, struct string_list *merge_rr, int flags)
 
 	if (flags & (RERERE_AUTOUPDATE|RERERE_NOAUTOUPDATE))
 		rerere_autoupdate = !!(flags & RERERE_AUTOUPDATE);
-	if (flags & RERERE_READONLY)
+	if (flags & RERERE_READONLY) {
 		fd = 0;
-	else
-		fd = repo_hold_lock_file_for_update(r, &write_lock,
-						    git_path_merge_rr(r),
-						    LOCK_DIE_ON_ERROR);
+	} else {
+		/*
+		 * Another process may hold the lock for a while, e.g.
+		 * "git rerere gc" while it prunes rr-cache, so wait for
+		 * it instead of dying right away.
+		 */
+		fd = repo_hold_lock_file_for_update_timeout(r, &write_lock,
+							    git_path_merge_rr(r),
+							    LOCK_DIE_ON_ERROR,
+							    rerere_lock_timeout_ms);
+	}
 	read_rr(r, merge_rr);
 	return fd;
 }
diff --git a/t/t4200-rerere.sh b/t/t4200-rerere.sh
index 7bb601e117..94165c8417 100755
--- a/t/t4200-rerere.sh
+++ b/t/t4200-rerere.sh
@@ -242,6 +242,59 @@ test_expect_success 'old records rest in peace' '
 	test_path_is_missing $rr2/preimage
 '
 
+test_expect_success 'a held lock is waited out within rerere.lockTimeout' '
+	git reset --hard &&
+	rm -rf $rr &&
+	test_when_finished "wait && rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	{
+		( sleep 1 && rm -f .git/MERGE_RR.lock ) &
+	} &&
+	test_must_fail git -c rerere.lockTimeout=5000 merge first 2>err &&
+	test_grep ! "Unable to create.*MERGE_RR\.lock" err &&
+	test_grep "^=======\$" $rr/preimage
+'
+
+test_expect_success 'merge fails on a lock it cannot take' '
+	git reset --hard &&
+	rm -rf $rr &&
+	test_when_finished "rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	test_must_fail git -c rerere.lockTimeout=0 merge first 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_grep "^=======\$" a1 &&
+	test_path_is_missing $rr/preimage
+'
+
+test_expect_success 'rerere, forget, clear and gc fail on a lock they cannot take' '
+	test_when_finished "rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere forget a1 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere clear 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere gc 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err
+'
+
+test_expect_success 'rebase --abort fails on a lock it cannot take' '
+	git reset --hard &&
+	git checkout -b lock-held-abort third &&
+	test_when_finished "git checkout third && git branch -D lock-held-abort" &&
+	test_must_fail git rebase first &&
+	test_when_finished "test_might_fail git rebase --abort" &&
+	test_when_finished "rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	test_must_fail git -c rerere.lockTimeout=0 rebase --abort 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_path_is_dir .git/rebase-merge &&
+	rm .git/MERGE_RR.lock &&
+	git rebase --abort &&
+	test_path_is_missing .git/rebase-merge
+'
+
 rerere_gc_custom_expiry_test () {
 	five_days="$1" right_now="$2"
 	test_expect_success "rerere gc with custom expiry ($five_days, $right_now)" '
-- 
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