database/sql: Close per-tx prepared statements when the associated tx ends

LGTM=bradfitz
R=golang-codereviews, bradfitz, mattn.jp
CC=golang-codereviews
https://golang.org/cl/131650043

src/database/sql/sql.go

@@ -1043,6 +1043,13 @@ type Tx struct {
 	// or Rollback. once done, all operations fail with
 	// ErrTxDone.
 	done bool
+
+	// All Stmts prepared for this transaction.  These will be closed after the
+	// transaction has been committed or rolled back.
+	stmts struct {
+		sync.Mutex
+		v []*Stmt
+	}
 }
 
 var ErrTxDone = errors.New("sql: Transaction has already been committed or rolled back")
@@ -1064,6 +1071,15 @@ func (tx *Tx) grabConn() (*driverConn, error) {
 	return tx.dc, nil
 }
 
+// Closes all Stmts prepared for this transaction.
+func (tx *Tx) closePrepared() {
+	tx.stmts.Lock()
+	for _, stmt := range tx.stmts.v {
+		stmt.Close()
+	}
+	tx.stmts.Unlock()
+}
+
 // Commit commits the transaction.
 func (tx *Tx) Commit() error {
 	if tx.done {
@@ -1071,8 +1087,12 @@ func (tx *Tx) Commit() error {
 	}
 	defer tx.close()
 	tx.dc.Lock()
-	defer tx.dc.Unlock()
-	return tx.txi.Commit()
+	err := tx.txi.Commit()
+	tx.dc.Unlock()
+	if err != driver.ErrBadConn {
+		tx.closePrepared()
+	}
+	return err
 }
 
 // Rollback aborts the transaction.
@@ -1082,8 +1102,12 @@ func (tx *Tx) Rollback() error {
 	}
 	defer tx.close()
 	tx.dc.Lock()
-	defer tx.dc.Unlock()
-	return tx.txi.Rollback()
+	err := tx.txi.Rollback()
+	tx.dc.Unlock()
+	if err != driver.ErrBadConn {
+		tx.closePrepared()
+	}
+	return err
 }
 
 // Prepare creates a prepared statement for use within a transaction.
@@ -1127,6 +1151,9 @@ func (tx *Tx) Prepare(query string) (*Stmt, error) {
 		},
 		query: query,
 	}
+	tx.stmts.Lock()
+	tx.stmts.v = append(tx.stmts.v, stmt)
+	tx.stmts.Unlock()
 	return stmt, nil
 }
 
@@ -1155,7 +1182,7 @@ func (tx *Tx) Stmt(stmt *Stmt) *Stmt {
 	dc.Lock()
 	si, err := dc.ci.Prepare(stmt.query)
 	dc.Unlock()
-	return &Stmt{
+	txs := &Stmt{
 		db: tx.db,
 		tx: tx,
 		txsi: &driverStmt{
@@ -1165,6 +1192,10 @@ func (tx *Tx) Stmt(stmt *Stmt) *Stmt {
 		query:     stmt.query,
 		stickyErr: err,
 	}
+	tx.stmts.Lock()
+	tx.stmts.v = append(tx.stmts.v, txs)
+	tx.stmts.Unlock()
+	return txs
 }
 
 // Exec executes a query that doesn't return rows.