sql: update ptr to VDBE after its creation in sql_txn

VDBE object is used in struct sql_txn to add new autoincrement ids in
sequence_next(). List of these ids is returned later as a query
execution result. sql_txn is created once SQL statement is executed
inside transaction and exists till commit or rollback. After its
creation it contains pointer to current VDBE. Each VDBE is freed after
statement is executed. Hence, after first SQL statement within
transaction is executed, sql_txn will point to freed memory (dangling
pointer). This leads to crash in the next processed statement. Fix to
this bug is simple: we must re-assign pointer to VDBE in sql_txn before
VDBE execution.

Closes #4157

src/box/sql/vdbeaux.c

@@ -110,6 +110,7 @@ sql_vdbe_prepare(struct Vdbe *vdbe)
 			if (txn->psql_txn == NULL)
 				return -1;
 		}
+		txn->psql_txn->vdbe = vdbe;
 	}
 	return 0;
 }

test/sql/transitive-transactions.result

@@ -134,3 +134,31 @@ box.execute('DROP TABLE parent;');
 ---
 - row_count: 1
 ...
+-- gh-4157: autoincrement within transaction started in SQL
+-- leads to seagfault.
+--
+box.execute('CREATE TABLE t (id INT PRIMARY KEY AUTOINCREMENT);');
+---
+- row_count: 1
+...
+box.execute('START TRANSACTION')
+box.execute('INSERT INTO t VALUES (null), (null);')
+box.execute('INSERT INTO t VALUES (null), (null);')
+box.execute('SAVEPOINT sp;')
+box.execute('INSERT INTO t VALUES (null);')
+box.execute('ROLLBACK TO sp;')
+box.execute('INSERT INTO t VALUES (null);')
+box.commit();
+---
+...
+box.space.T:select();
+---
+- - [1]
+  - [2]
+  - [3]
+  - [4]
+  - [6]
+...
+box.space.T:drop();
+---
+...

test/sql/transitive-transactions.test.lua

@@ -67,3 +67,18 @@ box.execute('PRAGMA defer_foreign_keys = 0;')
 -- Cleanup
 box.execute('DROP TABLE child;');
 box.execute('DROP TABLE parent;');
+
+-- gh-4157: autoincrement within transaction started in SQL
+-- leads to seagfault.
+--
+box.execute('CREATE TABLE t (id INT PRIMARY KEY AUTOINCREMENT);');
+box.execute('START TRANSACTION')
+box.execute('INSERT INTO t VALUES (null), (null);')
+box.execute('INSERT INTO t VALUES (null), (null);')
+box.execute('SAVEPOINT sp;')
+box.execute('INSERT INTO t VALUES (null);')
+box.execute('ROLLBACK TO sp;')
+box.execute('INSERT INTO t VALUES (null);')
+box.commit();
+box.space.T:select();
+box.space.T:drop();