fix issue 26

kaitai/stream.go

@@ -55,27 +55,28 @@ func (k *Stream) EOF() (bool, error) {
 }
 
 // Size returns the number of bytes of the stream.
-func (k *Stream) Size() (int64, error) {
+func (k *Stream) Size() (size int64, err error) {
 	// Go has no internal ReadSeeker function to get current ReadSeeker size,
 	// thus we use the following trick.
 	// Remember our current position
 	curPos, err := k.Pos()
 	if err != nil {
 		return 0, err
 	}
+	// Deferred Seek back to the current position
+	defer func() {
+		if _, serr := k.Seek(curPos, io.SeekStart); serr != nil {
+			err = fmt.Errorf("failed to seek to the initial position %v: %w", curPos, serr)
+		}
+	}()
 	// Seek to the end of the File object
 	_, err = k.Seek(0, io.SeekEnd)
 	if err != nil {
 		return 0, err
 	}
-	// Remember position, which is equal to the full length
-	fullSize, err := k.Pos()
-	if err != nil {
-		return fullSize, err
-	}
-	// Seek back to the current position
-	_, err = k.Seek(curPos, io.SeekStart)
-	return fullSize, err
+
+	// Return the current position, which is equal to the full length
+	return k.Pos()
 }
 
 // Pos returns the current position of the stream.

kaitai/stream_test.go

@@ -58,7 +58,8 @@ func TestStream_Size(t *testing.T) {
 		want    int64
 		wantErr bool
 	}{
-		{"Size", NewStream(bytes.NewReader([]byte("test"))), 4, false},
+		{"Zero size", NewStream(bytes.NewReader([]byte{})), 0, false},
+		{"Small size", NewStream(bytes.NewReader([]byte("test"))), 4, false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -74,6 +75,99 @@ func TestStream_Size(t *testing.T) {
 	}
 }
 
+type artificialError struct{}
+
+func (e artificialError) Error() string {
+	return "artificial error when seeking with io.SeekCurrent after seeking to end"
+}
+
+type failingReader struct {
+	pos      int64
+	mustFail func(fr failingReader, offset int64, whence int) bool
+}
+
+func (fr *failingReader) Read(p []byte) (n int, err error) { return 0, nil }
+func (fr *failingReader) Seek(offset int64, whence int) (int64, error) {
+	if fr.mustFail(*fr, offset, whence) {
+		return 0, artificialError{}
+	}
+
+	switch {
+	case whence == io.SeekCurrent:
+		return fr.pos, nil
+	case whence == io.SeekStart:
+		fr.pos = offset
+	default: // whence == io.SeekEnd
+		fr.pos = -1
+	}
+
+	return fr.pos, nil
+}
+
+// No regression test for issue #26
+func TestErrorHandlingInStream_Size(t *testing.T) {
+	tests := map[string]struct {
+		initialPos       int64
+		failingCondition func(fr failingReader, offset int64, whence int) bool
+		errorCheck       func(err error) bool
+		wantFinalPos     int64
+	}{
+		"fails to get initial position": {
+			initialPos: 5,
+			failingCondition: func(fr failingReader, offset int64, whence int) bool {
+				return whence == io.SeekCurrent && offset == 0
+			},
+			errorCheck: func(err error) bool {
+				_, ok := err.(artificialError)
+				return ok
+			},
+			wantFinalPos: 5,
+		},
+		"seek to the end fails": {
+			initialPos: 5,
+			failingCondition: func(fr failingReader, offset int64, whence int) bool {
+				return whence == io.SeekEnd
+			},
+			errorCheck: func(err error) bool {
+				_, ok := err.(artificialError)
+				return ok
+			},
+			wantFinalPos: 5,
+		},
+		"deferred seek to the initial pos fails": {
+			initialPos: 5,
+			failingCondition: func(fr failingReader, offset int64, whence int) bool {
+				return whence == io.SeekStart && fr.pos == -1
+			},
+			errorCheck: func(err error) bool {
+				_, ok := err.(artificialError)
+				return !ok
+			},
+			wantFinalPos: -1,
+		},
+	}
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			fr := &failingReader{tt.initialPos, tt.failingCondition}
+			s := NewStream(fr)
+			_, err := s.Size()
+
+			if err == nil {
+				t.Fatal("Expected error, got nothing")
+			}
+
+			if !tt.errorCheck(err) {
+				t.Fatalf("Expected error of type %T, got one of type %T", artificialError{}, err)
+			}
+
+			if fr.pos != tt.wantFinalPos {
+				t.Fatalf("Expected position to be %v, got %v", tt.wantFinalPos, fr.pos)
+			}
+		})
+	}
+
+}
+
 func TestStream_Pos(t *testing.T) {
 	tests := []struct {
 		name    string