🐛 fix(app-data): build the default folder only when none is given

docs/changelog/+read-only-app-data-default.bugfix.rst

@@ -0,0 +1,4 @@
+Build the default application data folder only when no folder was given through ``--app-data``, ``VIRTUALENV_APP_DATA``
+or the configuration file, so ``--read-only-app-data`` with an explicit folder works when the default folder is missing,
+and writable runs stop creating an unused one. Folders from ``VIRTUALENV_APP_DATA`` and the configuration file now get
+the same checks as ``--app-data``.

src/virtualenv/run/__init__.py

@@ -165,11 +165,11 @@ def load_app_data(
     options, _ = parser.parse_known_args(args, namespace=options)
 
     # here we need a write-able application data (e.g. the zipapp might need this for discovery cache)
-    parser.add_argument(
+    app_data: Final = parser.add_argument(
         "--app-data",
         help="a data folder used as cache by the virtualenv",
         type=partial(make_app_data, read_only=options.read_only_app_data, env=options.env),
-        default=make_app_data(None, read_only=options.read_only_app_data, env=options.env),
+        default=None,
     )
     parser.add_argument(
         "--reset-app-data",
@@ -182,6 +182,10 @@ def load_app_data(
         help="trigger a manual update of the embedded wheels",
     )
     options, _ = parser.parse_known_args(args, namespace=options)
+    if options.app_data is None:
+        # build the default only when no folder was given, since building it creates the folder or, when read-only,
+        # requires it to exist; it is also stored on the action so that --help still shows it
+        options.app_data = app_data.default = make_app_data(None, read_only=options.read_only_app_data, env=options.env)
     if options.reset_app_data:
         options.app_data.reset()
     return options

tests/unit/test_run.py

@@ -1,17 +1,22 @@
 from __future__ import annotations
 
 import logging
+import re
 import shutil
 import sys
+from functools import partial
+from pathlib import Path
 from typing import TYPE_CHECKING, Final
 
 import pytest
 
 from virtualenv import __version__
+from virtualenv.app_data import AppDataDiskFolder, ReadOnlyAppData
+from virtualenv.config.cli.parser import VirtualEnvOptions
 from virtualenv.run import cli_run, session_via_cli
 
 if TYPE_CHECKING:
-    from pathlib import Path
+    from collections.abc import Callable
 
     from pytest_mock import MockerFixture
 
@@ -30,6 +35,94 @@ def test_missing_option_value_does_not_create_app_data(tmp_path: Path, option: s
     assert not (tmp_path / "cache").exists()
 
 
+def test_read_only_app_data_ignores_missing_default(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, give_app_data: Callable[[Path], list[str]]
+) -> None:
+    cache: Final = tmp_path / "cache"
+    session_via_cli(["--app-data", str(cache), str(tmp_path / "warm")])  # discovery cannot fill a read-only cache
+    monkeypatch.setenv("VIRTUALENV_OVERRIDE_APP_DATA", str(tmp_path / "default"))
+    options: Final = VirtualEnvOptions()
+    session_via_cli(["--read-only-app-data", *give_app_data(cache), str(tmp_path / "env")], options)
+    assert (type(options.app_data), str(options.app_data)) == (ReadOnlyAppData, str(cache))
+
+
+@pytest.mark.parametrize(
+    "prepare_default",
+    [
+        pytest.param(lambda _: None, id="missing"),
+        # a file in the way makes the default fall back to temporary app-data
+        pytest.param(Path.touch, id="blocked"),
+    ],
+)
+def test_given_app_data_leaves_default_alone(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+    give_app_data: Callable[[Path], list[str]],
+    prepare_default: Callable[[Path], None],
+) -> None:
+    default_folder: Final = tmp_path / "default"
+    prepare_default(default_folder)
+    monkeypatch.setenv("VIRTUALENV_OVERRIDE_APP_DATA", str(default_folder / "cache"))
+    cache: Final = tmp_path / "cache"
+    options: Final = VirtualEnvOptions()
+    session_via_cli([*give_app_data(cache), str(tmp_path / "env")], options)
+    assert (type(options.app_data), str(options.app_data), (default_folder / "cache").exists()) == (
+        AppDataDiskFolder,
+        str(cache),
+        False,
+    )
+
+
+def test_read_only_app_data_rejects_missing_given_folder(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, give_app_data: Callable[[Path], list[str]]
+) -> None:
+    (tmp_path / "default").mkdir()
+    monkeypatch.setenv("VIRTUALENV_OVERRIDE_APP_DATA", str(tmp_path / "default"))
+    cache: Final = tmp_path / "cache"
+    args: Final = ["--read-only-app-data", *give_app_data(cache), str(tmp_path / "env")]
+    with pytest.raises(RuntimeError, match=rf"^read-only app data directory {re.escape(str(cache))} does not exist$"):
+        session_via_cli(args)
+
+
+def test_help_shows_default_app_data(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    monkeypatch.setenv("COLUMNS", "1000")
+    monkeypatch.setenv("VIRTUALENV_OVERRIDE_APP_DATA", str(tmp_path))
+    with pytest.raises(SystemExit) as context:
+        cli_run(args=["--help"])
+    assert (
+        context.value.code,
+        f"a data folder used as cache by the virtualenv (default: {tmp_path})" in capsys.readouterr().out,
+    ) == (0, True)
+
+
+def _on_command_line(_: pytest.MonkeyPatch, folder: Path) -> list[str]:
+    return ["--app-data", str(folder)]
+
+
+def _in_environment(monkeypatch: pytest.MonkeyPatch, folder: Path) -> list[str]:
+    monkeypatch.setenv("VIRTUALENV_APP_DATA", str(folder))
+    return []
+
+
+def _in_config_file(monkeypatch: pytest.MonkeyPatch, folder: Path) -> list[str]:
+    (ini := folder.parent / "virtualenv.ini").write_text(f"[virtualenv]\napp_data = {folder}\n", encoding="utf-8")
+    monkeypatch.setenv("VIRTUALENV_CONFIG_FILE", str(ini))
+    return []
+
+
+@pytest.fixture(
+    params=[
+        pytest.param(_on_command_line, id="cli"),
+        pytest.param(_in_environment, id="env-var"),
+        pytest.param(_in_config_file, id="ini"),
+    ]
+)
+def give_app_data(request: pytest.FixtureRequest, monkeypatch: pytest.MonkeyPatch) -> Callable[[Path], list[str]]:
+    return partial(request.param, monkeypatch)
+
+
 def test_help(capsys) -> None:
     with pytest.raises(SystemExit) as context:
         cli_run(args=["-h", "-vvv"])