1 files changed, 113 insertions, 0 deletions
diff --git a/pre_commit/staged_files_only.py b/pre_commit/staged_files_only.py
new file mode 100644
index 0000000..e1f81ba
--- /dev/null
+++ b/pre_commit/staged_files_only.py
@@ -0,0 +1,113 @@
+from __future__ import annotations
+
+import contextlib
+import logging
+import os.path
+import time
+from collections.abc import Generator
+
+from pre_commit import git
+from pre_commit.errors import FatalError
+from pre_commit.util import CalledProcessError
+from pre_commit.util import cmd_output
+from pre_commit.util import cmd_output_b
+from pre_commit.xargs import xargs
+
+
+logger = logging.getLogger('pre_commit')
+
+# without forcing submodule.recurse=0, changes in nested submodules will be
+# discarded if `submodule.recurse=1` is configured
+# we choose this instead of `--no-recurse-submodules` because it works on
+# versions of git before that option was added to `git checkout`
+_CHECKOUT_CMD = ('git', '-c', 'submodule.recurse=0', 'checkout', '--', '.')
+
+
+def _git_apply(patch: str) -> None:
+    args = ('apply', '--whitespace=nowarn', patch)
+    try:
+        cmd_output_b('git', *args)
+    except CalledProcessError:
+        # Retry with autocrlf=false -- see #570
+        cmd_output_b('git', '-c', 'core.autocrlf=false', *args)
+
+
+@contextlib.contextmanager
+def _intent_to_add_cleared() -> Generator[None, None, None]:
+    intent_to_add = git.intent_to_add_files()
+    if intent_to_add:
+        logger.warning('Unstaged intent-to-add files detected.')
+
+        xargs(('git', 'rm', '--cached', '--'), intent_to_add)
+        try:
+            yield
+        finally:
+            xargs(('git', 'add', '--intent-to-add', '--'), intent_to_add)
+    else:
+        yield
+
+
+@contextlib.contextmanager
+def _unstaged_changes_cleared(patch_dir: str) -> Generator[None, None, None]:
+    tree = cmd_output('git', 'write-tree')[1].strip()
+    diff_cmd = (
+        'git', 'diff-index', '--ignore-submodules', '--binary',
+        '--exit-code', '--no-color', '--no-ext-diff', tree, '--',
+    )
+    retcode, diff_stdout, diff_stderr = cmd_output_b(*diff_cmd, check=False)
+    if retcode == 0:
+        # There weren't any staged files so we don't need to do anything
+        # special
+        yield
+    elif retcode == 1 and not diff_stdout.strip():
+        # due to behaviour (probably a bug?) in git with crlf endings and
+        # autocrlf set to either `true` or `input` sometimes git will refuse
+        # to show a crlf-only diff to us :(
+        yield
+    elif retcode == 1 and diff_stdout.strip():
+        patch_filename = f'patch{int(time.time())}-{os.getpid()}'
+        patch_filename = os.path.join(patch_dir, patch_filename)
+        logger.warning('Unstaged files detected.')
+        logger.info(f'Stashing unstaged files to {patch_filename}.')
+        # Save the current unstaged changes as a patch
+        os.makedirs(patch_dir, exist_ok=True)
+        with open(patch_filename, 'wb') as patch_file:
+            patch_file.write(diff_stdout)
+
+        # prevent recursive post-checkout hooks (#1418)
+        no_checkout_env = dict(os.environ, _PRE_COMMIT_SKIP_POST_CHECKOUT='1')
+
+        try:
+            cmd_output_b(*_CHECKOUT_CMD, env=no_checkout_env)
+            yield
+        finally:
+            # Try to apply the patch we saved
+            try:
+                _git_apply(patch_filename)
+            except CalledProcessError:
+                logger.warning(
+                    'Stashed changes conflicted with hook auto-fixes... '
+                    'Rolling back fixes...',
+                )
+                # We failed to apply the patch, presumably due to fixes made
+                # by hooks.
+                # Roll back the changes made by hooks.
+                cmd_output_b(*_CHECKOUT_CMD, env=no_checkout_env)
+                _git_apply(patch_filename)
+
+            logger.info(f'Restored changes from {patch_filename}.')
+    else:  # pragma: win32 no cover
+        # some error occurred while requesting the diff
+        e = CalledProcessError(retcode, diff_cmd, b'', diff_stderr)
+        raise FatalError(
+            f'pre-commit failed to diff -- perhaps due to permissions?\n\n{e}',
+        )
+
+
+@contextlib.contextmanager
+def staged_files_only(patch_dir: str) -> Generator[None, None, None]:
+    """Clear any unstaged changes from the git working directory inside this
+    context.
+    """
+    with _intent_to_add_cleared(), _unstaged_changes_cleared(patch_dir):
+        yield