Avoid too long Git command lines on Windows [WinError 206]

src/darker/__main__.py

@@ -10,6 +10,7 @@
 from pathlib import Path
 from typing import Collection, Generator, List, Optional, Tuple
 
+import darker.black_compat
 from darker.black_diff import (
     BlackConfig,
     filter_python_files,
@@ -572,9 +573,15 @@ def main(  # pylint: disable=too-many-locals,too-many-branches,too-many-statemen
     else:
         # In other modes, only reformat files which have been modified.
         if git_is_repository(root):
-            changed_files_to_reformat = git_get_modified_python_files(
-                files_to_process, revrange, root
-            )
+            # Get the modified files only.
+            repo_root = darker.black_compat.find_project_root([str(root)])
+            changed_files = {
+                (repo_root / file).relative_to(root)
+                for file in git_get_modified_python_files(paths, revrange, repo_root)
+            }
+            # Filter out changed files that are not supposed to be processed
+            changed_files_to_reformat = files_to_process.intersection(changed_files)
+
         else:
             changed_files_to_reformat = files_to_process
         black_exclude = {

src/darker/tests/test_main.py

@@ -5,7 +5,10 @@
 # pylint: disable=use-dict-literal
 
 import logging
+import random
 import re
+import string
+import sys
 from argparse import ArgumentError
 from io import BytesIO
 from pathlib import Path
@@ -28,6 +31,11 @@
 from darker.verification import NotEquivalentError
 
 
+def randomword(length):
+    letters = string.ascii_lowercase
+    return "".join(random.choice(letters) for _i in range(length))
+
+
 def _replace_diff_timestamps(text, replacement="<timestamp>"):
     return re.sub(r"\d{4}-\d\d-\d\d \d\d:\d\d:\d\d\.\d\d\d\d\d\d", replacement, text)
 
@@ -932,3 +940,21 @@ def test_stdout_path_resolution(git_repo, capsys):
 
     assert result == 0
     assert capsys.readouterr().out == 'print("foo")\n'
+
+
+def test_long_command_length(git_repo):
+    # For PR #542 - large character count for changed files
+    # on windows breaks subprocess
+    # Need to exceed 32762 characters
+    files = {}
+    path = "src"
+    for _f in range(0, 4):
+        # Of course windows limits the path length too
+        path = f"{path}/{randomword(30)}"
+
+    for _d in range(0, 210):
+        files[f"{path}/{randomword(30)}.py"] = randomword(10)
+
+    git_repo.add(files, commit="Add all the files")
+    result = darker.__main__.main(["--diff", "--check", "src"])
+    assert result == 0