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[PATCH v4 2/3] reftable/stack: allow locking of outdated stacks

From
Patrick Steinhardt <ps@pks.im>
Date
Sep 24, 2024, 05:33 UTC
Message-ID
<81a836062e95135b4700ed2de93b6f047592624e.1727155858.git.ps@pks.im>
In-Reply-To
<cover.1727155858.git.ps@pks.im>

In `reftable_stack_new_addition()` we first lock the stack and then check whether it is still up-to-date. If it is not we return an error to the caller indicating that the stack is outdated.

This is overly restrictive in our ref transaction interface though: we lock the stack right before we start to verify the transaction, so we do not really care whether it is outdated or not. What we really want is that the stack is up-to-date after it has been locked so that we can verify queued updates against its current state while we know that it is locked for concurrent modification.

Introduce a new flag `REFTABLE_STACK_NEW_ADDITION_RELOAD` that alters the behaviour of `reftable_stack_init_addition()` in this case: when we notice that it is out-of-date we reload it instead of returning an error to the caller.

This logic will be wired up in the reftable backend in the next commit.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/reftable-backend.c         |  4 +-
 reftable/reftable-stack.h       | 13 ++++++-
 reftable/stack.c                | 20 ++++++----
 t/unit-tests/t-reftable-stack.c | 67 ++++++++++++++++++++++++++++++++-
 4 files changed, 91 insertions(+), 13 deletions(-)
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index ca281e39a29..6ca00627dd7 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -770,7 +770,7 @@ static int prepare_transaction_update(struct write_transaction_table_arg **out,
 		if (ret)
 			return ret;
 
-		ret = reftable_stack_new_addition(&addition, stack);
+		ret = reftable_stack_new_addition(&addition, stack, 0);
 		if (ret) {
 			if (ret == REFTABLE_LOCK_ERROR)
 				strbuf_addstr(err, "cannot lock references");
@@ -2207,7 +2207,7 @@ static int reftable_be_reflog_expire(struct ref_store *ref_store,
 	if (ret < 0)
 		goto done;
 
-	ret = reftable_stack_new_addition(&add, stack);
+	ret = reftable_stack_new_addition(&add, stack, 0);
 	if (ret < 0)
 		goto done;
 
diff --git a/reftable/reftable-stack.h b/reftable/reftable-stack.h
index f4f8cabc7fb..6370fe45ddf 100644
--- a/reftable/reftable-stack.h
+++ b/reftable/reftable-stack.h
@@ -37,12 +37,21 @@ uint64_t reftable_stack_next_update_index(struct reftable_stack *st);
 /* holds a transaction to add tables at the top of a stack. */
 struct reftable_addition;
 
+enum {
+	/*
+	 * Reload the stack when the stack is out-of-date after locking it.
+	 */
+	REFTABLE_STACK_NEW_ADDITION_RELOAD = (1 << 0),
+};
+
 /*
  * returns a new transaction to add reftables to the given stack. As a side
- * effect, the ref database is locked.
+ * effect, the ref database is locked. Accepts REFTABLE_STACK_NEW_ADDITION_*
+ * flags.
  */
 int reftable_stack_new_addition(struct reftable_addition **dest,
-				struct reftable_stack *st);
+				struct reftable_stack *st,
+				unsigned int flags);
 
 /* Adds a reftable to transaction. */
 int reftable_addition_add(struct reftable_addition *add,
diff --git a/reftable/stack.c b/reftable/stack.c
index 5ccad2cff31..84cf37a2ad0 100644
--- a/reftable/stack.c
+++ b/reftable/stack.c
@@ -596,7 +596,8 @@ struct reftable_addition {
 #define REFTABLE_ADDITION_INIT {0}
 
 static int reftable_stack_init_addition(struct reftable_addition *add,
-					struct reftable_stack *st)
+					struct reftable_stack *st,
+					unsigned int flags)
 {
 	struct strbuf lock_file_name = STRBUF_INIT;
 	int err;
@@ -626,6 +627,11 @@ static int reftable_stack_init_addition(struct reftable_addition *add,
 	err = stack_uptodate(st);
 	if (err < 0)
 		goto done;
+	if (err > 0 && flags & REFTABLE_STACK_NEW_ADDITION_RELOAD) {
+		err = reftable_stack_reload_maybe_reuse(add->stack, 1);
+		if (err)
+			goto done;
+	}
 	if (err > 0) {
 		err = REFTABLE_OUTDATED_ERROR;
 		goto done;
@@ -633,9 +639,8 @@ static int reftable_stack_init_addition(struct reftable_addition *add,
 
 	add->next_update_index = reftable_stack_next_update_index(st);
 done:
-	if (err) {
+	if (err)
 		reftable_addition_close(add);
-	}
 	strbuf_release(&lock_file_name);
 	return err;
 }
@@ -739,13 +744,14 @@ int reftable_addition_commit(struct reftable_addition *add)
 }
 
 int reftable_stack_new_addition(struct reftable_addition **dest,
-				struct reftable_stack *st)
+				struct reftable_stack *st,
+				unsigned int flags)
 {
 	int err = 0;
 	struct reftable_addition empty = REFTABLE_ADDITION_INIT;
 	REFTABLE_CALLOC_ARRAY(*dest, 1);
 	**dest = empty;
-	err = reftable_stack_init_addition(*dest, st);
+	err = reftable_stack_init_addition(*dest, st, flags);
 	if (err) {
 		reftable_free(*dest);
 		*dest = NULL;
@@ -759,7 +765,7 @@ static int stack_try_add(struct reftable_stack *st,
 			 void *arg)
 {
 	struct reftable_addition add = REFTABLE_ADDITION_INIT;
-	int err = reftable_stack_init_addition(&add, st);
+	int err = reftable_stack_init_addition(&add, st, 0);
 	if (err < 0)
 		goto done;
 
@@ -1608,7 +1614,7 @@ static int reftable_stack_clean_locked(struct reftable_stack *st)
 int reftable_stack_clean(struct reftable_stack *st)
 {
 	struct reftable_addition *add = NULL;
-	int err = reftable_stack_new_addition(&add, st);
+	int err = reftable_stack_new_addition(&add, st, 0);
 	if (err < 0) {
 		goto done;
 	}
diff --git a/t/unit-tests/t-reftable-stack.c b/t/unit-tests/t-reftable-stack.c
index d62a9c1bed5..a37cc698d87 100644
--- a/t/unit-tests/t-reftable-stack.c
+++ b/t/unit-tests/t-reftable-stack.c
@@ -271,7 +271,7 @@ static void t_reftable_stack_transaction_api(void)
 
 	reftable_addition_destroy(add);
 
-	err = reftable_stack_new_addition(&add, st);
+	err = reftable_stack_new_addition(&add, st, 0);
 	check(!err);
 
 	err = reftable_addition_add(add, write_test_ref, &ref);
@@ -292,6 +292,68 @@ static void t_reftable_stack_transaction_api(void)
 	clear_dir(dir);
 }
 
+static void t_reftable_stack_transaction_with_reload(void)
+{
+	char *dir = get_tmp_dir(__LINE__);
+	struct reftable_stack *st1 = NULL, *st2 = NULL;
+	int err;
+	struct reftable_addition *add = NULL;
+	struct reftable_ref_record refs[2] = {
+		{
+			.refname = (char *) "refs/heads/a",
+			.update_index = 1,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { '1' },
+		},
+		{
+			.refname = (char *) "refs/heads/b",
+			.update_index = 2,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { '1' },
+		},
+	};
+	struct reftable_ref_record ref = { 0 };
+
+	err = reftable_new_stack(&st1, dir, NULL);
+	check(!err);
+	err = reftable_new_stack(&st2, dir, NULL);
+	check(!err);
+
+	err = reftable_stack_new_addition(&add, st1, 0);
+	check(!err);
+	err = reftable_addition_add(add, write_test_ref, &refs[0]);
+	check(!err);
+	err = reftable_addition_commit(add);
+	check(!err);
+	reftable_addition_destroy(add);
+
+	/*
+	 * The second stack is now outdated, which we should notice. We do not
+	 * create the addition and lock the stack by default, but allow the
+	 * reload to happen when REFTABLE_STACK_NEW_ADDITION_RELOAD is set.
+	 */
+	err = reftable_stack_new_addition(&add, st2, 0);
+	check_int(err, ==, REFTABLE_OUTDATED_ERROR);
+	err = reftable_stack_new_addition(&add, st2, REFTABLE_STACK_NEW_ADDITION_RELOAD);
+	check(!err);
+	err = reftable_addition_add(add, write_test_ref, &refs[1]);
+	check(!err);
+	err = reftable_addition_commit(add);
+	check(!err);
+	reftable_addition_destroy(add);
+
+	for (size_t i = 0; i < ARRAY_SIZE(refs); i++) {
+		err = reftable_stack_read_ref(st2, refs[i].refname, &ref);
+		check(!err);
+		check(reftable_ref_record_equal(&refs[i], &ref, GIT_SHA1_RAWSZ));
+	}
+
+	reftable_ref_record_release(&ref);
+	reftable_stack_destroy(st1);
+	reftable_stack_destroy(st2);
+	clear_dir(dir);
+}
+
 static void t_reftable_stack_transaction_api_performs_auto_compaction(void)
 {
 	char *dir = get_tmp_dir(__LINE__);
@@ -322,7 +384,7 @@ static void t_reftable_stack_transaction_api_performs_auto_compaction(void)
 		 */
 		st->opts.disable_auto_compact = i != n;
 
-		err = reftable_stack_new_addition(&add, st);
+		err = reftable_stack_new_addition(&add, st, 0);
 		check(!err);
 
 		err = reftable_addition_add(add, write_test_ref, &ref);
@@ -1314,6 +1376,7 @@ int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
 	TEST(t_reftable_stack_reload_with_missing_table(), "stack iteration with garbage tables");
 	TEST(t_reftable_stack_tombstone(), "'tombstone' refs in stack");
 	TEST(t_reftable_stack_transaction_api(), "update transaction to stack");
+	TEST(t_reftable_stack_transaction_with_reload(), "transaction with reload");
 	TEST(t_reftable_stack_transaction_api_performs_auto_compaction(), "update transaction triggers auto-compaction");
 	TEST(t_reftable_stack_update_index_check(), "update transactions with equal update indices");
 	TEST(t_reftable_stack_uptodate(), "stack must be reloaded before ref update");

-- 
2.46.0.551.gc5ee8f2d1c.dirty
Previous: Patrick SteinhardtNext: Patrick Steinhardt
Message 31 of 42 in “reftable: graceful concurrent writes”
  1. 0/3 reftable: graceful concurrent writesPatrick Steinhardt, Sep 17, 2024
  2. 1/3 refs/reftable: introduce "reftable.lockTimeout"Patrick Steinhardt, Sep 17, 2024
  3. karthik nayakSep 17, 2024
  4. Eric SunshineSep 17, 2024
  5. Patrick SteinhardtSep 18, 2024
  6. Patrick SteinhardtSep 18, 2024
  7. 2/3 reftable/stack: allow locking of outdated stacksPatrick Steinhardt, Sep 17, 2024
  8. 3/3 refs/reftable: reload locked stack when preparing transactionPatrick Steinhardt, Sep 17, 2024
  9. karthik nayakSep 17, 2024
  10. Patrick SteinhardtSep 18, 2024
  11. 0/3 reftable: graceful concurrent writesPatrick Steinhardt, Sep 18, 2024
  12. 1/3 refs/reftable: introduce "reftable.lockTimeout"Patrick Steinhardt, Sep 18, 2024
  13. James LiuSep 18, 2024
  14. Patrick SteinhardtSep 18, 2024
  15. 2/3 reftable/stack: allow locking of outdated stacksPatrick Steinhardt, Sep 18, 2024
  16. James LiuSep 18, 2024
  17. Patrick SteinhardtSep 18, 2024
  18. 3/3 refs/reftable: reload locked stack when preparing transactionPatrick Steinhardt, Sep 18, 2024
  19. James LiuSep 18, 2024
  20. 0/3 reftable: graceful concurrent writesPatrick Steinhardt, Sep 18, 2024
  21. 1/3 refs/reftable: introduce "reftable.lockTimeout"Patrick Steinhardt, Sep 18, 2024
  22. Junio C HamanoSep 19, 2024
  23. 2/3 reftable/stack: allow locking of outdated stacksPatrick Steinhardt, Sep 18, 2024
  24. Junio C HamanoSep 20, 2024
  25. Patrick SteinhardtSep 24, 2024
  26. 3/3 refs/reftable: reload locked stack when preparing transactionPatrick Steinhardt, Sep 18, 2024
  27. James LiuSep 18, 2024
  28. Patrick SteinhardtSep 24, 2024
  29. 0/3 reftable: graceful concurrent writesPatrick Steinhardt, Sep 24, 2024
  30. 1/3 refs/reftable: introduce "reftable.lockTimeout"Patrick Steinhardt, Sep 24, 2024
  31. 2/3 reftable/stack: allow locking of outdated stacksPatrick Steinhardt, Sep 24, 2024
  32. 3/3 refs/reftable: reload locked stack when preparing transactionPatrick Steinhardt, Sep 24, 2024
  33. Jeff KingSep 27, 2024
  34. Patrick SteinhardtSep 30, 2024
  35. Josh SteadmonSep 30, 2024
  36. Patrick SteinhardtOct 1, 2024
  37. Jeff KingOct 1, 2024
  38. Junio C HamanoOct 1, 2024
  39. Patrick SteinhardtOct 2, 2024
  40. Junio C HamanoOct 1, 2024
  41. Josh SteadmonOct 1, 2024
  42. Jeff KingOct 1, 2024

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