[release-branch.go1.15] 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.

Includes a test fix from CL 266097.
Fixes #42884
Updates #40985
Updates #42259

Change-Id: Id52737660ada3cebdfff6efc23366cdc3224b8e8
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>
(cherry picked from commit d4c1ad8829)
Reviewed-on: https://go-review.googlesource.com/c/go/+/284513
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/database/sql/sql.go

@@ -2080,10 +2080,10 @@ 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
@@ -2157,7 +2157,7 @@ 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() {
@@ -2166,7 +2166,7 @@ func (tx *Tx) Commit() error {
 	if err != driver.ErrBadConn {
 		tx.closePrepared()
 	}
-	tx.closeLocked(err)
+	tx.close(err)
 	return err
 }
 
@@ -2189,7 +2189,7 @@ 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() {
@@ -2201,7 +2201,7 @@ func (tx *Tx) rollback(discardConn bool) error {
 	if discardConn {
 		err = driver.ErrBadConn
 	}
-	tx.closeLocked(err)
+	tx.close(err)
 	return err
 }
 

src/database/sql/sql_test.go

@@ -2810,6 +2810,34 @@ func TestTxCannotCommitAfterRollback(t *testing.T) {
 	}
 }
 
+// Issue 40985 transaction statement deadlock while context cancel.
+func TestTxStmtDeadlock(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Millisecond)
+	defer cancel()
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	stmt, err := tx.Prepare("SELECT|people|name,age|age=?")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Run number of stmt queries to reproduce deadlock from context cancel
+	for i := 0; i < 1e3; i++ {
+		// Encounter any close related errors (e.g.. ErrTxDone, stmt is closed)
+		// is expected due to context cancel.
+		_, err = stmt.Query(1)
+		if err != nil {
+			break
+		}
+	}
+	_ = tx.Rollback()
+}
+
 // Issue32530 encounters an issue where a connection may
 // expire right after it comes out of a used connection pool
 // even when a new connection is requested.