Ensure tox-created directories contain CACHEDIR.TAG

Fixes #3334

docs/changelog/3342.feature.rst

@@ -0,0 +1,7 @@
+Tox now creates ``CACHEDIR.TAG`` files in work directories it creates,
+so that tools like ``tar`` can exclude them from e.g. backups where
+ephemeral directories are not desired.
+
+Tag files are not created in directories Tox does not itself create.
+
+- by :user:`akx`

src/tox/tox_env/api.py

@@ -17,7 +17,7 @@
 from tox.execute.request import ExecuteRequest
 from tox.tox_env.errors import Fail, Recreate, Skip
 from tox.tox_env.info import Info
-from tox.util.path import ensure_empty_dir
+from tox.util.path import ensure_cachedir_dir, ensure_empty_dir
 
 if TYPE_CHECKING:
     from tox.config.cli.parser import Parsed
@@ -315,16 +315,27 @@ def _setup_with_env(self) -> None:  # noqa: B027 # empty abstract base class
     def _done_with_setup(self) -> None:  # noqa: B027 # empty abstract base class
         """Called when setup is done."""
 
+    def _maybe_ensure_workdir(self) -> None:
+        if not self.work_dir.is_dir():
+            # Populate the workdir with a CACHEDIR.TAG file only if we would
+            # be creating it now. If it already exists, do not touch it.
+            ensure_cachedir_dir(self.work_dir)
+
     def _handle_env_tmp_dir(self) -> None:
         """Ensure exists and empty."""
         env_tmp_dir = self.env_tmp_dir
         if env_tmp_dir.exists() and next(env_tmp_dir.iterdir(), None) is not None:
             LOGGER.debug("clear env temp folder %s", env_tmp_dir)
             ensure_empty_dir(env_tmp_dir)
-        env_tmp_dir.mkdir(parents=True, exist_ok=True)
+        if env_tmp_dir.parent == self.work_dir:
+            self._maybe_ensure_workdir()
+        ensure_cachedir_dir(env_tmp_dir)
 
     def _handle_core_tmp_dir(self) -> None:
-        self.temp_dir.mkdir(parents=True, exist_ok=True)
+        temp_dir = self.temp_dir
+        if temp_dir.parent == self.work_dir:
+            self._maybe_ensure_workdir()
+        ensure_cachedir_dir(temp_dir)
 
     def _clean(self, transitive: bool = False) -> None:  # noqa: ARG002, FBT001, FBT002
         if self._run_state["clean"]:  # pragma: no branch
@@ -333,6 +344,7 @@ def _clean(self, transitive: bool = False) -> None:  # noqa: ARG002, FBT001, FBT
         if env_dir.exists():
             LOGGER.warning("remove tox env folder %s", env_dir)
             ensure_empty_dir(env_dir, except_filename="file.lock")
+            ensure_cachedir_dir(env_dir)
         self._log_id = 0  # we deleted logs, so start over counter
         self.cache.reset()
         self._run_state.update({"setup": False, "clean": True})

src/tox/tox_env/package.py

@@ -10,6 +10,8 @@
 
 from filelock import FileLock
 
+from tox.util.path import ensure_cachedir_dir
+
 from .api import ToxEnv, ToxEnvCreateArgs
 
 if TYPE_CHECKING:
@@ -67,9 +69,12 @@ def __getattribute__(self, name: str) -> Any:
 
     def register_config(self) -> None:
         super().register_config()
-        file_lock_path: Path = self.env_dir / "file.lock"
+        env_dir = self.env_dir
+        if env_dir.parent == self.work_dir:
+            self._maybe_ensure_workdir()
+        ensure_cachedir_dir(env_dir)
+        file_lock_path: Path = env_dir / "file.lock"
         self._file_lock = FileLock(file_lock_path)
-        file_lock_path.parent.mkdir(parents=True, exist_ok=True)
         self.core.add_config(
             keys=["package_root", "setupdir"],
             of_type=Path,

src/tox/tox_env/python/api.py

@@ -13,6 +13,7 @@
 
 from tox.tox_env.api import ToxEnv, ToxEnvCreateArgs
 from tox.tox_env.errors import Fail, Recreate, Skip
+from tox.util.path import ensure_cachedir_dir
 
 if TYPE_CHECKING:
     from tox.config.main import Config
@@ -235,6 +236,7 @@ def ensure_python_env(self) -> None:
         with self.cache.compare(conf, Python.__name__) as (eq, old):
             if old is None:  # does not exist -> create
                 self.create_python_env()
+                ensure_cachedir_dir(self.env_dir)
             elif eq is False:  # pragma: no branch # exists but changed -> recreate
                 raise Recreate(self._diff_msg(conf, old))
 

src/tox/util/path.py

@@ -6,6 +6,12 @@
 if TYPE_CHECKING:
     from pathlib import Path
 
+CACHEDIR_TAG_CONTENT = b"""Signature: 8a477f597d28d172789f06886806bc55
+# This file is a cache directory tag created by the Tox automation project (https://tox.wiki/).
+# For information about cache directory tags, see:
+#	http://www.brynosaurus.com/cachedir/
+"""
+
 
 def ensure_empty_dir(path: Path, except_filename: str | None = None) -> None:
     if path.exists():
@@ -24,6 +30,18 @@ def ensure_empty_dir(path: Path, except_filename: str | None = None) -> None:
         path.mkdir(parents=True)
 
 
+def ensure_cachedir_dir(path: Path) -> None:
+    """
+    Ensure that the given path is a directory, exists and
+    contains a `CACHEDIR.TAG` file.
+    """
+    path.mkdir(parents=True, exist_ok=True)
+    cachetag = path / "CACHEDIR.TAG"
+    if not cachetag.is_file():
+        cachetag.write_bytes(CACHEDIR_TAG_CONTENT)
+
+
 __all__ = [
+    "ensure_cachedir_dir",
     "ensure_empty_dir",
 ]