database/sql: prevent Tx statement from committing after rollback

It was possible for a Tx that was aborted for rollback
asynchronously to execute a query after the rollback had completed
on the database, which often would auto commit the query outside
of the transaction.

By W-locking the tx.closemu prior to issuing the rollback
connection it ensures any Tx query either fails or finishes
on the Tx, and never after the Tx has rolled back.

Fixes #34775
Fixes #32942

Change-Id: I017b7932082f2f4ead70bae08b61ed9068ac1d01
Reviewed-on: https://go-review.googlesource.com/c/go/+/216240
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/database/sql/fakedb_test.go

@@ -390,6 +390,7 @@ func setStrictFakeConnClose(t *testing.T) {
 
 func (c *fakeConn) ResetSession(ctx context.Context) error {
 	c.dirtySession = false
+	c.currTx = nil
 	if c.isBad() {
 		return driver.ErrBadConn
 	}
@@ -734,6 +735,9 @@ var hookExecBadConn func() bool
 func (s *fakeStmt) Exec(args []driver.Value) (driver.Result, error) {
 	panic("Using ExecContext")
 }
+
+var errFakeConnSessionDirty = errors.New("fakedb: session is dirty")
+
 func (s *fakeStmt) ExecContext(ctx context.Context, args []driver.NamedValue) (driver.Result, error) {
 	if s.panic == "Exec" {
 		panic(s.panic)
@@ -746,7 +750,7 @@ func (s *fakeStmt) ExecContext(ctx context.Context, args []driver.NamedValue) (d
 		return nil, driver.ErrBadConn
 	}
 	if s.c.isDirtyAndMark() {
-		return nil, errors.New("fakedb: session is dirty")
+		return nil, errFakeConnSessionDirty
 	}
 
 	err := checkSubsetTypes(s.c.db.allowAny, args)
@@ -860,7 +864,7 @@ func (s *fakeStmt) QueryContext(ctx context.Context, args []driver.NamedValue) (
 		return nil, driver.ErrBadConn
 	}
 	if s.c.isDirtyAndMark() {
-		return nil, errors.New("fakedb: session is dirty")
+		return nil, errFakeConnSessionDirty
 	}
 
 	err := checkSubsetTypes(s.c.db.allowAny, args)
@@ -893,6 +897,37 @@ func (s *fakeStmt) QueryContext(ctx context.Context, args []driver.NamedValue) (
 				}
 			}
 		}
+		if s.table == "tx_status" && s.colName[0] == "tx_status" {
+			txStatus := "autocommit"
+			if s.c.currTx != nil {
+				txStatus = "transaction"
+			}
+			cursor := &rowsCursor{
+				parentMem: s.c,
+				posRow:    -1,
+				rows: [][]*row{
+					[]*row{
+						{
+							cols: []interface{}{
+								txStatus,
+							},
+						},
+					},
+				},
+				cols: [][]string{
+					[]string{
+						"tx_status",
+					},
+				},
+				colType: [][]string{
+					[]string{
+						"string",
+					},
+				},
+				errPos: -1,
+			}
+			return cursor, nil
+		}
 
 		t.mu.Lock()
 

src/database/sql/sql.go

@@ -2061,14 +2061,10 @@ 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")
 
-// close returns the connection to the pool and
-// must only be called by Tx.rollback or Tx.Commit.
-func (tx *Tx) close(err error) {
-	tx.cancel()
-
-	tx.closemu.Lock()
-	defer tx.closemu.Unlock()
-
+// closeLocked 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.releaseConn(err)
 	tx.dc = nil
 	tx.txi = nil
@@ -2135,6 +2131,15 @@ func (tx *Tx) Commit() error {
 	if !atomic.CompareAndSwapInt32(&tx.done, 0, 1) {
 		return ErrTxDone
 	}
+
+	// Cancel the Tx to release any active R-closemu locks.
+	// This is safe to do because tx.done has already transitioned
+	// from 0 to 1. Hold the W-closemu lock prior to rollback
+	// to ensure no other connection has an active query.
+	tx.cancel()
+	tx.closemu.Lock()
+	defer tx.closemu.Unlock()
+
 	var err error
 	withLock(tx.dc, func() {
 		err = tx.txi.Commit()
@@ -2142,16 +2147,31 @@ func (tx *Tx) Commit() error {
 	if err != driver.ErrBadConn {
 		tx.closePrepared()
 	}
-	tx.close(err)
+	tx.closeLocked(err)
 	return err
 }
 
+var rollbackHook func()
+
 // rollback aborts the transaction and optionally forces the pool to discard
 // the connection.
 func (tx *Tx) rollback(discardConn bool) error {
 	if !atomic.CompareAndSwapInt32(&tx.done, 0, 1) {
 		return ErrTxDone
 	}
+
+	if rollbackHook != nil {
+		rollbackHook()
+	}
+
+	// Cancel the Tx to release any active R-closemu locks.
+	// This is safe to do because tx.done has already transitioned
+	// from 0 to 1. Hold the W-closemu lock prior to rollback
+	// to ensure no other connection has an active query.
+	tx.cancel()
+	tx.closemu.Lock()
+	defer tx.closemu.Unlock()
+
 	var err error
 	withLock(tx.dc, func() {
 		err = tx.txi.Rollback()
@@ -2162,7 +2182,7 @@ func (tx *Tx) rollback(discardConn bool) error {
 	if discardConn {
 		err = driver.ErrBadConn
 	}
-	tx.close(err)
+	tx.closeLocked(err)
 	return err
 }
 

src/database/sql/sql_test.go

@@ -80,6 +80,11 @@ func newTestDBConnector(t testing.TB, fc *fakeConnector, name string) *DB {
 		exec(t, db, "CREATE|magicquery|op=string,millis=int32")
 		exec(t, db, "INSERT|magicquery|op=sleep,millis=10")
 	}
+	if name == "tx_status" {
+		// Magic table name and column, known by fakedb_test.go.
+		exec(t, db, "CREATE|tx_status|tx_status=string")
+		exec(t, db, "INSERT|tx_status|tx_status=invalid")
+	}
 	return db
 }
 
@@ -2707,6 +2712,70 @@ func TestManyErrBadConn(t *testing.T) {
 	}
 }
 
+// Issue 34755: Ensure that a Tx cannot commit after a rollback.
+func TestTxCannotCommitAfterRollback(t *testing.T) {
+	db := newTestDB(t, "tx_status")
+	defer closeDB(t, db)
+
+	// First check query reporting is correct.
+	var txStatus string
+	err := db.QueryRow("SELECT|tx_status|tx_status|").Scan(&txStatus)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, w := txStatus, "autocommit"; g != w {
+		t.Fatalf("tx_status=%q, wanted %q", g, w)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Ignore dirty session for this test.
+	// A failing test should trigger the dirty session flag as well,
+	// but that isn't exactly what this should test for.
+	tx.txi.(*fakeTx).c.skipDirtySession = true
+
+	defer tx.Rollback()
+
+	err = tx.QueryRow("SELECT|tx_status|tx_status|").Scan(&txStatus)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, w := txStatus, "transaction"; g != w {
+		t.Fatalf("tx_status=%q, wanted %q", g, w)
+	}
+
+	// 1. Begin a transaction.
+	// 2. (A) Start a query, (B) begin Tx rollback through a ctx cancel.
+	// 3. Check if 2.A has committed in Tx (pass) or outside of Tx (fail).
+	sendQuery := make(chan struct{})
+	hookTxGrabConn = func() {
+		cancel()
+		<-sendQuery
+	}
+	rollbackHook = func() {
+		close(sendQuery)
+	}
+	defer func() {
+		hookTxGrabConn = nil
+		rollbackHook = nil
+	}()
+
+	err = tx.QueryRow("SELECT|tx_status|tx_status|").Scan(&txStatus)
+	if err != nil {
+		// A failure here would be expected if skipDirtySession was not set to true above.
+		t.Fatal(err)
+	}
+	if g, w := txStatus, "transaction"; g != w {
+		t.Fatalf("tx_status=%q, wanted %q", g, w)
+	}
+}
+
 // 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.