database/sql: fix tx stmt deadlock when rollback

Tx acquires tx.closemu W-lock and then acquires stmt.closemu.W-lock
to fully close the transaction and associated prepared statement.
Stmt query and execution run in reverse ways - acquires
stmt.closemu.R-lock and then acquires tx.closemu.R-lock to grab tx
connection, which may cause deadlock.

Prevent the lock is held around tx.closePrepared to ensure no
deadlock happens.

Fixes #40985

Change-Id: If53909822b87bce11861a6e3035ecb9476d2cd17
Reviewed-on: https://go-review.googlesource.com/c/go/+/250178
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

1 files changed, 7 insertions, 7 deletions

diff --git a/src/database/sql/sql.go b/src/database/sql/sql.go
index fc7e3e4485..d8f19520c8 100644
--- a/src/database/sql/sql.go
+++ b/src/database/sql/sql.go
@@ -2087,10 +2087,10 @@ func (tx *Tx) isDone() bool {
 // that has already been committed or rolled back.
 var ErrTxDone = errors.New("sql: transaction has already been committed or rolled back")
 
-// closeLocked returns the connection to the pool and
+// close returns the connection to the pool and
 // must only be called by Tx.rollback or Tx.Commit while
-// closemu is Locked and tx already canceled.
-func (tx *Tx) closeLocked(err error) {
+// tx is already canceled and won't be executed concurrently.
+func (tx *Tx) close(err error) {
 	tx.releaseConn(err)
 	tx.dc = nil
 	tx.txi = nil
@@ -2164,7 +2164,7 @@ func (tx *Tx) Commit() error {
 	// to ensure no other connection has an active query.
 	tx.cancel()
 	tx.closemu.Lock()
-	defer tx.closemu.Unlock()
+	tx.closemu.Unlock()
 
 	var err error
 	withLock(tx.dc, func() {
@@ -2173,7 +2173,7 @@ func (tx *Tx) Commit() error {
 	if err != driver.ErrBadConn {
 		tx.closePrepared()
 	}
-	tx.closeLocked(err)
+	tx.close(err)
 	return err
 }
 
@@ -2196,7 +2196,7 @@ func (tx *Tx) rollback(discardConn bool) error {
 	// to ensure no other connection has an active query.
 	tx.cancel()
 	tx.closemu.Lock()
-	defer tx.closemu.Unlock()
+	tx.closemu.Unlock()
 
 	var err error
 	withLock(tx.dc, func() {
@@ -2208,7 +2208,7 @@ func (tx *Tx) rollback(discardConn bool) error {
 	if discardConn {
 		err = driver.ErrBadConn
 	}
-	tx.closeLocked(err)
+	tx.close(err)
 	return err
 }