Fix handling of empty files

black.py

@@ -607,6 +607,9 @@ def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
     """
     srcbuf = io.BytesIO(src)
     encoding, lines = tokenize.detect_encoding(srcbuf.readline)
+    if not lines:
+        return "", encoding, "\n"
+
     newline = "\r\n" if b"\r\n" == lines[0][-2:] else "\n"
     srcbuf.seek(0)
     with io.TextIOWrapper(srcbuf, encoding) as tiow:
@@ -623,7 +626,7 @@ def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
 def lib2to3_parse(src_txt: str) -> Node:
     """Given a string with source, return the lib2to3 Node."""
     grammar = pygram.python_grammar_no_print_statement
-    if src_txt[-1] != "\n":
+    if src_txt[-1:] != "\n":
         src_txt += "\n"
     for grammar in GRAMMARS:
         drv = driver.Driver(grammar, pytree.convert)

tests/test_black.py

@@ -100,6 +100,25 @@ def assertFormatEqual(self, expected: str, actual: str) -> None:
                 black.err(str(ve))
         self.assertEqual(expected, actual)
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_empty(self) -> None:
+        source = expected = ""
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, line_length=ll)
+
+    def test_empty_ff(self) -> None:
+        expected = ""
+        tmp_file = Path(black.dump_to_file())
+        try:
+            self.assertFalse(ff(tmp_file, write_back=black.WriteBack.YES))
+            with open(tmp_file, encoding="utf8") as f:
+                actual = f.read()
+        finally:
+            os.unlink(tmp_file)
+        self.assertFormatEqual(expected, actual)
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_self(self) -> None:
         source, expected = read_data("test_black")