box/txn: move journal allocation into separate routine

This makes code easier to read and allows to reuse
txn allocation in sync\async writes.

Acked-by: Konstantin Osipov <kostja.osipov@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

src/box/txn.c

@@ -478,41 +478,49 @@ txn_complete_async(struct journal_entry *entry, void *complete_data)
 	fiber_set_txn(fiber(), NULL);
 }
 
-static int64_t
-txn_write_to_wal(struct txn *txn)
+static struct journal_entry *
+txn_journal_entry_new(struct txn *txn)
 {
+	struct journal_entry *req;
+	struct txn_stmt *stmt;
+
 	assert(txn->n_new_rows + txn->n_applier_rows > 0);
 
-	/* Prepare a journal entry. */
-	struct journal_entry *req = journal_entry_new(txn->n_new_rows +
-						      txn->n_applier_rows,
-						      &txn->region,
-						      txn_complete_async,
-						      txn);
-	if (req == NULL) {
-		txn_rollback(txn);
-		return -1;
-	}
+	req = journal_entry_new(txn->n_new_rows + txn->n_applier_rows,
+				&txn->region, txn_complete_async, txn);
+	if (req == NULL)
+		return NULL;
 
-	struct txn_stmt *stmt;
 	struct xrow_header **remote_row = req->rows;
 	struct xrow_header **local_row = req->rows + txn->n_applier_rows;
+
 	stailq_foreach_entry(stmt, &txn->stmts, next) {
 		if (stmt->has_triggers) {
 			txn_init_triggers(txn);
 			rlist_splice(&txn->on_commit, &stmt->on_commit);
 		}
+
+		/* A read (e.g. select) request */
 		if (stmt->row == NULL)
-			continue; /* A read (e.g. select) request */
+			continue;
+
 		if (stmt->row->replica_id == 0)
 			*local_row++ = stmt->row;
 		else
 			*remote_row++ = stmt->row;
+
 		req->approx_len += xrow_approx_len(stmt->row);
 	}
+
 	assert(remote_row == req->rows + txn->n_applier_rows);
 	assert(local_row == remote_row + txn->n_new_rows);
 
+	return req;
+}
+
+static int64_t
+txn_write_to_wal(struct journal_entry *req)
+{
 	/*
 	 * Send the entry to the journal.
 	 *
@@ -584,6 +592,8 @@ txn_commit_nop(struct txn *txn)
 int
 txn_commit_async(struct txn *txn)
 {
+	struct journal_entry *req;
+
 	if (txn_prepare(txn) != 0) {
 		txn_rollback(txn);
 		return -1;
@@ -592,12 +602,20 @@ txn_commit_async(struct txn *txn)
 	if (txn_commit_nop(txn))
 		return 0;
 
-	return txn_write_to_wal(txn);
+	req = txn_journal_entry_new(txn);
+	if (req == NULL) {
+		txn_rollback(txn);
+		return -1;
+	}
+
+	return txn_write_to_wal(req);
 }
 
 int
 txn_commit(struct txn *txn)
 {
+	struct journal_entry *req;
+
 	txn->fiber = fiber();
 
 	if (txn_prepare(txn) != 0) {
@@ -611,7 +629,14 @@ txn_commit(struct txn *txn)
 		return 0;
 	}
 
-	if (txn_write_to_wal(txn) != 0)
+	req = txn_journal_entry_new(txn);
+	if (req == NULL) {
+		txn_rollback(txn);
+		txn_free(txn);
+		return -1;
+	}
+
+	if (txn_write_to_wal(req) != 0)
 		return -1;
 
 	/*