database/sql: ensure Commit and Rollback return ErrTxDone

Ensure documented behavior of returning ErrTxDone if the Tx has
already been committed or rolled back.

Fixes #18147

Change-Id: I07dc75bef4dbd4dd88dd252c96dc8ab99f28c00e
Reviewed-on: https://go-review.googlesource.com/33793
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -1449,14 +1449,14 @@ func (tx *Tx) closePrepared() {
 
 // Commit commits the transaction.
 func (tx *Tx) Commit() error {
+	if tx.isDone() {
+		return ErrTxDone
+	}
 	select {
 	default:
 	case <-tx.ctx.Done():
 		return tx.ctx.Err()
 	}
-	if tx.isDone() {
-		return ErrTxDone
-	}
 	var err error
 	withLock(tx.dc, func() {
 		err = tx.txi.Commit()

src/database/sql/sql_test.go

@@ -683,6 +683,37 @@ func TestQueryRow(t *testing.T) {
 	}
 }
 
+func TestTxRollbackCommitErr(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tx.Rollback()
+	if err != nil {
+		t.Errorf("expected nil error from Rollback; got %v", err)
+	}
+	err = tx.Commit()
+	if err != ErrTxDone {
+		t.Errorf("expected %q from Commit; got %q", ErrTxDone, err)
+	}
+
+	tx, err = db.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tx.Commit()
+	if err != nil {
+		t.Errorf("expected nil error from Commit; got %v", err)
+	}
+	err = tx.Rollback()
+	if err != ErrTxDone {
+		t.Errorf("expected %q from Rollback; got %q", ErrTxDone, err)
+	}
+}
+
 func TestStatementErrorAfterClose(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)