Support GN build language

GN is a meta-build system that generates build files for Ninja
See also https://gn.googlesource.com/gn/

pre_commit/languages/gn.py

@@ -0,0 +1,101 @@
+from __future__ import annotations
+
+import contextlib
+import functools
+import os.path
+import platform
+import shutil
+import tempfile
+import urllib.error
+import urllib.request
+import zipfile
+from collections.abc import Generator
+from collections.abc import Sequence
+from typing import ContextManager
+from typing import IO
+
+import pre_commit.constants as C
+from pre_commit import lang_base
+from pre_commit.envcontext import envcontext
+from pre_commit.envcontext import PatchesT
+from pre_commit.envcontext import Var
+from pre_commit.prefix import Prefix
+from pre_commit.util import make_executable
+
+ENVIRONMENT_DIR = 'gn_env'
+health_check = lang_base.basic_health_check
+run_hook = lang_base.basic_run_hook
+
+_ARCH_ALIASES = {
+    'x86_64': 'amd64',
+    'aarch64': 'arm64',
+    'armv8': 'arm64',
+}
+_ARCH = platform.machine().lower()
+_ARCH = _ARCH_ALIASES.get(_ARCH, _ARCH)
+
+
+def _open_archive(bio: IO[bytes]) -> ContextManager[zipfile.ZipFile]:
+    return zipfile.ZipFile(bio)
+
+
+def _get_gn_url() -> str:
+    os_name = platform.system().lower()
+    if os_name == 'linux':
+        os_name = 'linux-' + _ARCH
+    elif os_name == 'darwin':
+        os_name = 'mac-' + _ARCH
+    elif os_name == 'windows':
+        os_name = 'windows-' + _ARCH
+    return 'https://chrome-infra-packages.appspot.com/dl/gn/gn/' + \
+        f'{os_name}/+/latest'
+
+
+def _install_gn(dest: str) -> None:
+    try:
+        resp = urllib.request.urlopen(_get_gn_url())
+    except urllib.error.HTTPError as e:  # pragma: no cover
+        os_name = platform.system().lower()
+        raise ValueError(
+            f'Could not find GN for your system (os={os_name}; arch={_ARCH})',
+        ) from e
+    else:
+        with tempfile.TemporaryFile() as f:
+            shutil.copyfileobj(resp, f)
+            f.seek(0)
+            with _open_archive(f) as archive:
+                archive.extractall(dest)
+                make_executable(f'{dest}/gn')
+
+
+@functools.lru_cache(maxsize=1)
+def get_default_version() -> str:
+    if lang_base.exe_exists('gn'):
+        return 'system'
+    else:
+        return C.DEFAULT
+
+
+def get_env_patch(venv: str, version: str) -> PatchesT:
+    if version == 'system':
+        return ()
+
+    return (('PATH', (venv, os.pathsep, Var('PATH'))),)
+
+
+@contextlib.contextmanager
+def in_env(prefix: Prefix, version: str) -> Generator[None, None, None]:
+    env_dir = lang_base.environment_dir(prefix, ENVIRONMENT_DIR, version)
+    with envcontext(get_env_patch(env_dir, version)):
+        yield
+
+
+def install_environment(
+    prefix: Prefix,
+    version: str,
+    additional_dependencies: Sequence[str],
+) -> None:
+    if version == 'system':
+        return
+    env_dir = lang_base.environment_dir(prefix, ENVIRONMENT_DIR, version)
+    _install_gn(env_dir)