Do not attempt manual transaction rollback in Tx.Copy

The typical use these days is with a managed transaction, via db.View.

The first case (error when re-opening database file) is not tested;
it is harder to instrument, and I have other plans for it.

tx.go

@@ -242,7 +242,6 @@ func (tx *Tx) Copy(w io.Writer) error {
 	// Open reader on the database.
 	f, err := os.OpenFile(tx.db.path, os.O_RDONLY|odirect, 0)
 	if err != nil {
-		_ = tx.Rollback()
 		return err
 	}
 
@@ -251,14 +250,12 @@ func (tx *Tx) Copy(w io.Writer) error {
 	_, err = io.CopyN(w, f, int64(tx.db.pageSize*2))
 	tx.db.metalock.Unlock()
 	if err != nil {
-		_ = tx.Rollback()
 		_ = f.Close()
 		return fmt.Errorf("meta copy: %s", err)
 	}
 
 	// Copy data pages.
 	if _, err := io.CopyN(w, f, tx.Size()-int64(tx.db.pageSize*2)); err != nil {
-		_ = tx.Rollback()
 		_ = f.Close()
 		return err
 	}

tx_test.go

@@ -338,6 +338,57 @@ func TestTx_CopyFile(t *testing.T) {
 	})
 }
 
+type failWriterError struct{}
+
+func (failWriterError) Error() string {
+	return "error injected for tests"
+}
+
+type failWriter struct {
+	// fail after this many bytes
+	After int
+}
+
+func (f *failWriter) Write(p []byte) (n int, err error) {
+	n = len(p)
+	if n > f.After {
+		n = f.After
+		err = failWriterError{}
+	}
+	f.After -= n
+	return n, err
+}
+
+// Ensure that Copy handles write errors right.
+func TestTx_CopyFile_Error_Meta(t *testing.T) {
+	withOpenDB(func(db *DB, path string) {
+		db.Update(func(tx *Tx) error {
+			tx.CreateBucket([]byte("widgets"))
+			tx.Bucket([]byte("widgets")).Put([]byte("foo"), []byte("bar"))
+			tx.Bucket([]byte("widgets")).Put([]byte("baz"), []byte("bat"))
+			return nil
+		})
+
+		err := db.View(func(tx *Tx) error { return tx.Copy(&failWriter{}) })
+		assert.EqualError(t, err, "meta copy: error injected for tests")
+	})
+}
+
+// Ensure that Copy handles write errors right.
+func TestTx_CopyFile_Error_Normal(t *testing.T) {
+	withOpenDB(func(db *DB, path string) {
+		db.Update(func(tx *Tx) error {
+			tx.CreateBucket([]byte("widgets"))
+			tx.Bucket([]byte("widgets")).Put([]byte("foo"), []byte("bar"))
+			tx.Bucket([]byte("widgets")).Put([]byte("baz"), []byte("bat"))
+			return nil
+		})
+
+		err := db.View(func(tx *Tx) error { return tx.Copy(&failWriter{3 * db.pageSize}) })
+		assert.EqualError(t, err, "error injected for tests")
+	})
+}
+
 func ExampleTx_Rollback() {
 	// Open the database.
 	db, _ := Open(tempfile(), 0666)