Work around stability errors due to optional trailing commas

src/black/__init__.py

@@ -1016,7 +1016,17 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
 
     if not fast:
         assert_equivalent(src_contents, dst_contents)
-        assert_stable(src_contents, dst_contents, mode=mode)
+
+        # Forced second pass to work around optional trailing commas (becoming
+        # forced trailing commas on pass 2) interacting differently with optional
+        # parentheses.  Admittedly ugly.
+        dst_contents_pass2 = format_str(dst_contents, mode=mode)
+        if dst_contents != dst_contents_pass2:
+            dst_contents = dst_contents_pass2
+            assert_equivalent(src_contents, dst_contents, pass_num=2)
+            assert_stable(src_contents, dst_contents, mode=mode)
+        # Note: no need to explicitly call `assert_stable` if `dst_contents` was
+        # the same as `dst_contents_pass2`.
     return dst_contents
 
 
@@ -6489,7 +6499,7 @@ def _stringify_ast(
     yield f"{'  ' * depth})  # /{node.__class__.__name__}"
 
 
-def assert_equivalent(src: str, dst: str) -> None:
+def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
     """Raise AssertionError if `src` and `dst` aren't equivalent."""
     try:
         src_ast = parse_ast(src)
@@ -6504,9 +6514,9 @@ def assert_equivalent(src: str, dst: str) -> None:
     except Exception as exc:
         log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)
         raise AssertionError(
-            f"INTERNAL ERROR: Black produced invalid code: {exc}. Please report a bug"
-            " on https://github.com/psf/black/issues.  This invalid output might be"
-            f" helpful: {log}"
+            f"INTERNAL ERROR: Black produced invalid code on pass {pass_num}: {exc}. "
+            "Please report a bug on https://github.com/psf/black/issues.  "
+            f"This invalid output might be helpful: {log}"
         ) from None
 
     src_ast_str = "\n".join(_stringify_ast(src_ast))
@@ -6515,8 +6525,8 @@ def assert_equivalent(src: str, dst: str) -> None:
         log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))
         raise AssertionError(
             "INTERNAL ERROR: Black produced code that is not equivalent to the"
-            " source.  Please report a bug on https://github.com/psf/black/issues. "
-            f" This diff might be helpful: {log}"
+            f" source on pass {pass_num}.  Please report a bug on "
+            f"https://github.com/psf/black/issues.  This diff might be helpful: {log}"
         ) from None
 
 

tests/test_black.py

@@ -254,6 +254,24 @@ def test_trailing_comma_optional_parens_stability3(self) -> None:
         actual = fs(source)
         black.assert_stable(source, actual, DEFAULT_MODE)
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_trailing_comma_optional_parens_stability1_pass2(self) -> None:
+        source, _expected = read_data("trailing_comma_optional_parens1")
+        actual = fs(fs(source))  # this is what `format_file_contents` does with --safe
+        black.assert_stable(source, actual, DEFAULT_MODE)
+
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_trailing_comma_optional_parens_stability2_pass2(self) -> None:
+        source, _expected = read_data("trailing_comma_optional_parens2")
+        actual = fs(fs(source))  # this is what `format_file_contents` does with --safe
+        black.assert_stable(source, actual, DEFAULT_MODE)
+
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_trailing_comma_optional_parens_stability3_pass2(self) -> None:
+        source, _expected = read_data("trailing_comma_optional_parens3")
+        actual = fs(fs(source))  # this is what `format_file_contents` does with --safe
+        black.assert_stable(source, actual, DEFAULT_MODE)
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_pep_572(self) -> None:
         source, expected = read_data("pep_572")