fix: fix path mounting when running in Docker

Currently pre-commit mounts the current directory to /src and uses
current directory name as mount base.
However this does not work when pre-commit is run inside the container
on some mounted path already, because mount points are relative to the
host, not to the container.

Fixes #1387

pre_commit/languages/docker.py

@@ -1,5 +1,7 @@
 import hashlib
+import json
 import os
+import socket
 from typing import Sequence
 from typing import Tuple
 
@@ -8,13 +10,43 @@
 from pre_commit.languages import helpers
 from pre_commit.prefix import Prefix
 from pre_commit.util import clean_path_on_failure
+from pre_commit.util import cmd_output_b
 
 ENVIRONMENT_DIR = 'docker'
 PRE_COMMIT_LABEL = 'PRE_COMMIT'
 get_default_version = helpers.basic_get_default_version
 healthy = helpers.basic_healthy
 
 
+def _is_in_docker() -> bool:
+    try:
+        with open('/proc/1/cgroup', 'rb') as f:
+            return b'docker' in f.read()
+    except FileNotFoundError:
+        return False
+
+
+def _get_docker_path(path: str) -> str:
+    if not _is_in_docker():
+        return path
+    hostname = socket.gethostname()
+
+    _, out, _ = cmd_output_b(
+        'docker', 'inspect', hostname,
+    )
+
+    docker_output = json.loads(out)
+    for mount_path in docker_output[0]['HostConfig']['Binds']:
+        src_path, to_path, *_ = mount_path.split(':')
+        if os.path.commonpath((path, to_path)) == to_path:
+            # So there is something in common,
+            # and we can proceed remapping it
+            return path.replace(to_path, src_path)
+    # we're in Docker, but the path is not mounted, cannot really do anything,
+    # so fall back to original path
+    return path
+
+
 def md5(s: str) -> str:  # pragma: win32 no cover
     return hashlib.md5(s.encode()).hexdigest()
 
@@ -73,7 +105,7 @@ def docker_cmd() -> Tuple[str, ...]:  # pragma: win32 no cover
         # https://docs.docker.com/engine/reference/commandline/run/#mount-volumes-from-container-volumes-from
         # The `Z` option tells Docker to label the content with a private
         # unshared label. Only the current container can use a private volume.
-        '-v', f'{os.getcwd()}:/src:rw,Z',
+        '-v', f'{_get_docker_path(os.getcwd())}:/src:rw,Z',
         '--workdir', '/src',
     )
 

tests/languages/docker_test.py

@@ -1,14 +1,144 @@
+import builtins
+import json
+from typing import List
 from unittest import mock
 
+import pytest
+
 from pre_commit.languages import docker
 
 
 def test_docker_fallback_user():
     def invalid_attribute():
         raise AttributeError
+
     with mock.patch.multiple(
-        'os', create=True,
-        getuid=invalid_attribute,
-        getgid=invalid_attribute,
+            'os', create=True,
+            getuid=invalid_attribute,
+            getgid=invalid_attribute,
     ):
         assert docker.get_docker_user() == ()
+
+
+class TestInDocker:
+
+    @pytest.fixture
+    def mock_file_fixture(self):
+        return lambda read_data: mock.patch.object(
+            builtins, 'open',
+            new_callable=mock.mock_open,
+            read_data=read_data,
+        )
+
+    def test_in_docker_no_file(self, mock_file_fixture):
+        with mock_file_fixture(None) as m:
+            m.side_effect = FileNotFoundError
+
+            assert docker._is_in_docker() is False
+            m.assert_called()
+
+    def test_in_docker_docker_in_file(self, mock_file_fixture):
+        with mock_file_fixture(b'tdockert'):
+            assert docker._is_in_docker() is True
+
+    def test_in_docker_docker_not_in_file(self, mock_file_fixture):
+        with mock_file_fixture(b'testtest'):
+            assert docker._is_in_docker() is False
+
+
+class TestDockerPath:
+    @pytest.fixture
+    def in_docker(self):
+        with mock.patch.object(docker, '_is_in_docker', return_value=True):
+            yield
+
+    @pytest.fixture
+    def not_in_docker(self):
+        with mock.patch.object(docker, '_is_in_docker', return_value=False):
+            yield
+
+    def test_not_in_docker_returns_same(self, not_in_docker):
+        assert docker._get_docker_path('abc') == 'abc'
+
+    def test_in_docker_no_binds_same_path(self, in_docker):
+        binds_list: List[str] = []
+        output_string = json.dumps(
+            [{'HostConfig': {'Binds': binds_list}}],
+        ).encode('utf-8')
+        with mock.patch.object(
+                docker, 'cmd_output_b',
+                return_value=(0, output_string, b''),
+        ):
+            assert docker._get_docker_path('abc') == 'abc'
+
+    def test_in_docker_binds_path_equal(self, in_docker):
+        binds_list = [
+            '/opt/my_code:/project',
+        ]
+        output_string = json.dumps(
+            [{'HostConfig': {'Binds': binds_list}}],
+        ).encode('utf-8')
+        with mock.patch.object(
+                docker, 'cmd_output_b',
+                return_value=(0, output_string, b''),
+        ):
+            assert docker._get_docker_path('/project') == '/opt/my_code'
+
+    def test_in_docker_binds_path_complex(self, in_docker):
+        binds_list = [
+            '/opt/my_code:/project',
+        ]
+        output_string = json.dumps(
+            [{'HostConfig': {'Binds': binds_list}}],
+        ).encode('utf-8')
+        with mock.patch.object(
+                docker, 'cmd_output_b',
+                return_value=(0, output_string, b''),
+        ):
+            path = '/project/test/something'
+            expected = '/opt/my_code/test/something'
+            assert docker._get_docker_path(path) == expected
+
+    def test_in_docker_no_substring(self, in_docker):
+        binds_list = [
+            '/opt/my_code:/project',
+        ]
+        output_string = json.dumps(
+            [{'HostConfig': {'Binds': binds_list}}],
+        ).encode('utf-8')
+        with mock.patch.object(
+                docker, 'cmd_output_b',
+                return_value=(0, output_string, b''),
+        ):
+            path = '/projectSUffix/test/something'
+            assert docker._get_docker_path(path) == path
+
+    def test_in_docker_binds_path_many_binds(self, in_docker):
+        binds_list = [
+            '/something_random:/not_related',
+            '/opt/my_code:/project',
+            '/something_random2:/not_related2',
+        ]
+        output_string = json.dumps(
+            [{'HostConfig': {'Binds': binds_list}}],
+        ).encode('utf-8')
+        with mock.patch.object(
+                docker, 'cmd_output_b',
+                return_value=(0, output_string, b''),
+        ):
+            assert docker._get_docker_path('/project') == '/opt/my_code'
+
+    def test_in_docker_two_colons(self, in_docker):
+        binds_list = [
+            '/opt/my_code:/project:qqqrandom',
+        ]
+        output_string = json.dumps(
+            [{'HostConfig': {'Binds': binds_list}}],
+        ).encode('utf-8')
+        with mock.patch.object(
+                docker, 'cmd_output_b',
+                return_value=(0, output_string, b''),
+        ):
+            path = '/project/test/something'
+            expected = '/opt/my_code/test/something'
+            assert docker._get_docker_path(path) == expected