Support pre-commit from inside submodules

pre_commit/git.py

@@ -24,10 +24,18 @@ def get_root():
         )
 
 
+def get_git_dir(git_root):
+    return os.path.normpath(os.path.join(
+        git_root,
+        cmd_output('git', 'rev-parse', '--git-dir', cwd=git_root)[1].strip(),
+    ))
+
+
 def is_in_merge_conflict():
+    git_dir = get_git_dir('.')
     return (
-        os.path.exists(os.path.join('.git', 'MERGE_MSG')) and
-        os.path.exists(os.path.join('.git', 'MERGE_HEAD'))
+        os.path.exists(os.path.join(git_dir, 'MERGE_MSG')) and
+        os.path.exists(os.path.join(git_dir, 'MERGE_HEAD'))
     )
 
 
@@ -46,7 +54,7 @@ def get_conflicted_files():
     logger.info('Checking merge-conflict files only.')
     # Need to get the conflicted files from the MERGE_MSG because they could
     # have resolved the conflict by choosing one side or the other
-    merge_msg = open(os.path.join('.git', 'MERGE_MSG')).read()
+    merge_msg = open(os.path.join(get_git_dir('.'), 'MERGE_MSG')).read()
     merge_conflict_filenames = parse_merge_msg_for_conflicts(merge_msg)
 
     # This will get the rest of the changes made after the merge.

pre_commit/main.py

@@ -25,6 +25,13 @@ os.environ.pop('__PYVENV_LAUNCHER__', None)
 # https://github.com/pre-commit/pre-commit/issues/300
 # In git 2.6.3 (maybe others), git exports this while running pre-commit hooks
 os.environ.pop('GIT_WORK_TREE', None)
+# In git 1.9.1 (maybe others), git exports these while running pre-commit hooks
+# in submodules.  In the general case this causes problems.
+# These are covered by test_install_in_submodule_and_run
+# Causes git clone to clone wrong thing
+os.environ.pop('GIT_DIR', None)
+# Causes 'error invalid object ...' during commit
+os.environ.pop('GIT_INDEX_FILE', None)
 
 
 def main(argv=None):

pre_commit/runner.py

@@ -30,6 +30,10 @@ class Runner(object):
         os.chdir(root)
         return cls(root)
 
+    @cached_property
+    def git_dir(self):
+        return git.get_git_dir(self.git_root)
+
     @cached_property
     def config_file_path(self):
         return os.path.join(self.git_root, C.CONFIG_FILE)
@@ -44,7 +48,7 @@ class Runner(object):
         return repositories
 
     def get_hook_path(self, hook_type):
-        return os.path.join(self.git_root, '.git', 'hooks', hook_type)
+        return os.path.join(self.git_dir, 'hooks', hook_type)
 
     @cached_property
     def pre_commit_path(self):