txn: add helper to detect transaction boundaries

Add txn_is_first_statement() function, which returns true if this is the
first statement of the transaction. The function is supposed to be used
from on_replace trigger to detect transaction boundaries.

Needed for #2129

src/box/txn.c

@@ -398,8 +398,7 @@ txn_abort(struct txn *txn)
 int
 txn_check_singlestatement(struct txn *txn, const char *where)
 {
-	if (!txn->is_autocommit ||
-	    stailq_last(&txn->stmts) != stailq_first(&txn->stmts)) {
+	if (!txn->is_autocommit || !txn_is_first_statement(txn)) {
 		diag_set(ClientError, ER_UNSUPPORTED,
 			 where, "multi-statement transactions");
 		return -1;

src/box/txn.h

@@ -280,6 +280,17 @@ txn_rollback_stmt();
 int
 txn_check_singlestatement(struct txn *txn, const char *where);
 
+/**
+ * Returns true if the transaction has a single statement.
+ * Supposed to be used from a space on_replace trigger to
+ * detect transaction boundaries.
+ */
+static inline bool
+txn_is_first_statement(struct txn *txn)
+{
+	return stailq_last(&txn->stmts) == stailq_first(&txn->stmts);
+}
+
 /** The current statement of the transaction. */
 static inline struct txn_stmt *
 txn_current_stmt(struct txn *txn)