[Tarantool-patches] [PATCH v2 6/7] qsync: implement direct write of CONFIRM/ROLLBACK into a journal

[Cyrill Gorcunov <gorcunov@gmail.com>]

When we need to write CONFIRM or ROLLBACK message (which is just
a binary record in msgpack format) into a journal we use txn code
to allocate a new transaction, encode there a message and pass it
to walk the long txn path before it hit the journal. This is not
only resource wasting but also somehow strange from arhitectural
point of view.

Instead lets encode a record on the stack and write it
directly to the journal.

Closes #5129

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
---
 src/box/txn_limbo.c | 92 +++++++++++++++++++++++++--------------------
 1 file changed, 51 insertions(+), 41 deletions(-)

diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
index a74bfe244..3eca244dc 100644
--- a/src/box/txn_limbo.c
+++ b/src/box/txn_limbo.c
@@ -32,6 +32,9 @@
 #include "txn_limbo.h"
 #include "replication.h"
 
+#include "iproto_constants.h"
+#include "journal.h"
+
 struct txn_limbo txn_limbo;
 
 static inline void
@@ -237,62 +240,69 @@ txn_limbo_wait_complete(struct txn_limbo *limbo, struct txn_limbo_entry *entry)
 	return 0;
 }
 
+static void
+txn_limbo_write_cb(struct journal_entry *entry)
+{
+	/*
+	 * It is safe to call fiber_wakeup on
+	 * active fiber but we need to call wakeup
+	 * in case if we're sitting in WAL thread.
+	 */
+	assert(entry->complete_data != NULL);
+	fiber_wakeup(entry->complete_data);
+}
+
+/**
+ * Write CONFIRM or ROLLBACK message to a journal directly
+ * without involving transaction engine because using txn
+ * engine is far from being cheap while we only need to
+ * write a small message.
+ */
 static int
-txn_limbo_write_confirm_rollback(struct txn_limbo *limbo, int64_t lsn,
-				 bool is_confirm)
+txn_limbo_write(uint32_t replica_id, int64_t lsn, int type)
 {
+	assert(replica_id != REPLICA_ID_NIL);
+	assert(type == IPROTO_CONFIRM || type == IPROTO_ROLLBACK);
 	assert(lsn > 0);
 
-	struct xrow_header row;
-	struct request request = {
-		.header = &row,
-	};
+	char buf[sizeof(struct journal_entry) +
+		 sizeof(struct xrow_header *) +
+		 sizeof(struct xrow_header)];
 
-	struct txn *txn = txn_begin();
-	if (txn == NULL)
-		return -1;
+	struct journal_entry *entry = (void *)buf;
+	struct xrow_header *row = (void *)&entry->rows[1];
+	entry->rows[0] = row;
 
-	int res = 0;
-	if (is_confirm) {
-		res = xrow_encode_confirm(&row, &txn->region,
-					  limbo->instance_id, lsn);
-	} else {
-		/*
-		 * This LSN is the first to be rolled back, so
-		 * the last "safe" lsn is lsn - 1.
-		 */
-		res = xrow_encode_rollback(&row, &txn->region,
-					   limbo->instance_id, lsn);
+	struct request_synchro_body body;
+	xrow_encode_confirm_rollback(row, &body, replica_id,
+				     lsn, type);
+
+	journal_entry_create(entry, 1, xrow_approx_len(row),
+			     txn_limbo_write_cb, fiber());
+
+	if (journal_write(entry) != 0) {
+		diag_set(ClientError, ER_WAL_IO);
+		diag_log();
+		return -1;
 	}
-	if (res == -1)
-		goto rollback;
-	/*
-	 * This is not really a transaction. It just uses txn API
-	 * to put the data into WAL. And obviously it should not
-	 * go to the limbo and block on the very same sync
-	 * transaction which it tries to confirm now.
-	 */
-	txn_set_flag(txn, TXN_FORCE_ASYNC);
 
-	if (txn_begin_stmt(txn, NULL) != 0)
-		goto rollback;
-	if (txn_commit_stmt(txn, &request) != 0)
-		goto rollback;
+	if (entry->res < 0) {
+		diag_set(ClientError, ER_WAL_IO);
+		diag_log();
+		return -1;
+	}
 
-	return txn_commit(txn);
-rollback:
-	txn_rollback(txn);
-	return -1;
+	return 0;
 }
 
 /**
  * Write a confirmation entry to WAL. After it's written all the
  * transactions waiting for confirmation may be finished.
  */
-static int
+static inline int
 txn_limbo_write_confirm(struct txn_limbo *limbo, int64_t lsn)
 {
-	return txn_limbo_write_confirm_rollback(limbo, lsn, true);
+	return txn_limbo_write(limbo->instance_id, lsn, IPROTO_CONFIRM);
 }
 
 void
@@ -338,10 +348,10 @@ txn_limbo_read_confirm(struct txn_limbo *limbo, int64_t lsn)
  * transactions following the current one and waiting for
  * confirmation must be rolled back.
  */
-static int
+static inline int
 txn_limbo_write_rollback(struct txn_limbo *limbo, int64_t lsn)
 {
-	return txn_limbo_write_confirm_rollback(limbo, lsn, false);
+	return txn_limbo_write(limbo->instance_id, lsn, IPROTO_ROLLBACK);
 }
 
 void
-- 
2.26.2