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>

diff --git a/src/database/sql/sql.go b/src/database/sql/sql.go
index fc7e3e44850a058044defc03dc7258bf5f773198..d8f19520c8914d65774a2ef586bea61927f83ac2 100644 (file)
--- 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
 }
 

diff --git a/src/database/sql/sql_test.go b/src/database/sql/sql_test.go
index 762d42f54b4e06d3469f9ddbdf340eb8018671c8..8ae6e1339e18f45f57deabd8e0c1954ba8460c96 100644 (file)
--- a/src/database/sql/sql_test.go
+++ b/src/database/sql/sql_test.go
@@ -2810,6 +2810,36 @@ 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++ {
+               _, err = stmt.Query(1)
+               if err != nil {
+                       // Encounter ErrTxDone here is expected due to context cancel
+                       if err != ErrTxDone {
+                               t.Fatalf("unexpected error while executing stmt, err: %v", err)
+                       }
+                       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.