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Fix docker-in-docker detection for cgroups v2 #3535

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Merged
asottile merged 1 commit into from
Nov 8, 2025
+201 −69
Merged

Fix docker-in-docker detection for cgroups v2 #3535

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40 changes: 22 additions & 18 deletions pre_commit/languages/docker.py
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Original file line number Diff line number Diff line change
@@ -1,9 +1,11 @@
from __future__ import annotations

import contextlib
import functools
import hashlib
import json
import os
import re
from collections.abc import Sequence

from pre_commit import lang_base
Expand All @@ -17,31 +19,33 @@
health_check = lang_base.basic_health_check
in_env = lang_base.no_env # no special environment for docker

_HOSTNAME_MOUNT_RE = re.compile(
rb"""
/containers
(?:/overlay-containers)?
/([a-z0-9]{64})
(?:/userdata)?
/hostname
""",
re.VERBOSE,
)

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_container_id() -> str | None:
with contextlib.suppress(FileNotFoundError):
with open('/proc/1/mountinfo', 'rb') as f:
for line in f:
m = _HOSTNAME_MOUNT_RE.search(line)
if m:
return m[1].decode()

def _get_container_id() -> str:
# It's assumed that we already check /proc/1/cgroup in _is_in_docker. The
# cpuset cgroup controller existed since cgroups were introduced so this
# way of getting the container ID is pretty reliable.
with open('/proc/1/cgroup', 'rb') as f:
for line in f.readlines():
if line.split(b':')[1] == b'cpuset':
return os.path.basename(line.split(b':')[2]).strip().decode()
raise RuntimeError('Failed to find the container ID in /proc/1/cgroup.')
return None


def _get_docker_path(path: str) -> str:
if not _is_in_docker():
return path

container_id = _get_container_id()
if container_id is None:
return path

try:
_, out, _ = cmd_output_b('docker', 'inspect', container_id)
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