exp/sql: fix statement leak

Also verified in external test suite that this fixes MySQL
resource exhaustion problems, and also exposed a double-free
bug in the gosqlite3 driver (where gosqlite3 either got lucky
before, or was working around this bug)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5544057

src/pkg/exp/sql/fakedb_test.go

@@ -77,6 +77,17 @@ type fakeConn struct {
 	db *fakeDB // where to return ourselves to
 
 	currTx *fakeTx
+
+	// Stats for tests:
+	mu          sync.Mutex
+	stmtsMade   int
+	stmtsClosed int
+}
+
+func (c *fakeConn) incrStat(v *int) {
+	c.mu.Lock()
+	*v++
+	c.mu.Unlock()
 }
 
 type fakeTx struct {
@@ -338,6 +349,7 @@ func (c *fakeConn) Prepare(query string) (driver.Stmt, error) {
 	cmd := parts[0]
 	parts = parts[1:]
 	stmt := &fakeStmt{q: query, c: c, cmd: cmd}
+	c.incrStat(&c.stmtsMade)
 	switch cmd {
 	case "WIPE":
 		// Nothing
@@ -358,7 +370,10 @@ func (s *fakeStmt) ColumnConverter(idx int) driver.ValueConverter {
 }
 
 func (s *fakeStmt) Close() error {
-	s.closed = true
+	if !s.closed {
+		s.c.incrStat(&s.c.stmtsClosed)
+		s.closed = true
+	}
 	return nil
 }
 

src/pkg/exp/sql/sql.go

@@ -243,8 +243,13 @@ func (db *DB) Query(query string, args ...interface{}) (*Rows, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer stmt.Close()
-	return stmt.Query(args...)
+	rows, err := stmt.Query(args...)
+	if err != nil {
+		stmt.Close()
+		return nil, err
+	}
+	rows.closeStmt = stmt
+	return rows, nil
 }
 
 // QueryRow executes a query that is expected to return at most one row.
@@ -706,9 +711,10 @@ type Rows struct {
 	releaseConn func()
 	rowsi       driver.Rows
 
-	closed   bool
-	lastcols []interface{}
-	lasterr  error
+	closed    bool
+	lastcols  []interface{}
+	lasterr   error
+	closeStmt *Stmt // if non-nil, statement to Close on close
 }
 
 // Next prepares the next result row for reading with the Scan method.
@@ -789,6 +795,9 @@ func (rs *Rows) Close() error {
 	rs.closed = true
 	err := rs.rowsi.Close()
 	rs.releaseConn()
+	if rs.closeStmt != nil {
+		rs.closeStmt.Close()
+	}
 	return err
 }
 

src/pkg/exp/sql/sql_test.go

@@ -276,3 +276,21 @@ func TestIssue2542Deadlock(t *testing.T) {
 		}
 	}
 }
+
+func TestQueryRowClosingStmt(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+	var name string
+	var age int
+	err := db.QueryRow("SELECT|people|age,name|age=?", 3).Scan(&age, &name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(db.freeConn) != 1 {
+		t.Fatalf("expected 1 free conn")
+	}
+	fakeConn := db.freeConn[0].(*fakeConn)
+	if made, closed := fakeConn.stmtsMade, fakeConn.stmtsClosed; made != closed {
+		t.Logf("statement close mismatch: made %d, closed %d", made, closed)
+	}
+}