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 +1,8 @@
 import hashlib
+import json
 import os
+import socket
+import subprocess
 from typing import Sequence
 from typing import Tuple
 
@@ -15,6 +18,32 @@
 healthy = helpers.basic_healthy
 
 
+def is_in_docker() -> bool:
+    try:
+        with open('/proc/1/cgroup') as f:
+            return '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()
+    docker_output = json.loads(
+        subprocess.check_output(['docker', 'inspect', hostname]),
+    )
+    for mount_path in docker_output[0]['HostConfig']['Binds']:
+        src_path, to_path = mount_path.split(':')
+        if to_path == path:
+            return src_path
+        elif path.startswith(to_path):
+            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 +102,7 @@ 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

@@ -1,3 +1,6 @@
+import json
+import subprocess
+from typing import List
 from unittest import mock
 
 from pre_commit.languages import docker
@@ -12,3 +15,44 @@ def invalid_attribute():
         getgid=invalid_attribute,
     ):
         assert docker.get_docker_user() == ()
+
+
+class TestDockerPath:
+    @mock.patch.object(docker, 'is_in_docker', return_value=False)
+    def test_not_in_docker_returns_same(self, _):
+        assert docker.get_docker_path('abc') == 'abc'
+
+    @mock.patch.object(docker, 'is_in_docker', return_value=True)
+    def test_in_docker_no_binds_same_path(self, _):
+        binds_list: List[str] = []
+        output_string = json.dumps([{'HostConfig': {'Binds': binds_list}}])
+        with mock.patch.object(
+            subprocess, 'check_output',
+            return_value=output_string,
+        ):
+            assert docker.get_docker_path('abc') == 'abc'
+
+    @mock.patch.object(docker, 'is_in_docker', return_value=True)
+    def test_in_docker_binds_path_equal(self, _):
+        binds_list = [
+            '/opt/my_code:/project',
+        ]
+        output_string = json.dumps([{'HostConfig': {'Binds': binds_list}}])
+        with mock.patch.object(
+            subprocess, 'check_output',
+            return_value=output_string,
+        ):
+            assert docker.get_docker_path('/project') == '/opt/my_code'
+
+    @mock.patch.object(docker, 'is_in_docker', return_value=True)
+    def test_in_docker_binds_path_complex(self, _):
+        binds_list = [
+            '/opt/my_code:/project',
+        ]
+        output_string = json.dumps([{'HostConfig': {'Binds': binds_list}}])
+        with mock.patch.object(
+            subprocess, 'check_output',
+            return_value=output_string,
+        ):
+            assert docker.get_docker_path('/project/test/something') == \
+                '/opt/my_code/test/something'