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>

1 files changed, 31 insertions, 0 deletions

diff --git a/src/database/sql/sql_test.go b/src/database/sql/sql_test.go
index 02746a2e30..1ec6217858 100644
--- a/src/database/sql/sql_test.go
+++ b/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)