PDM PEP 582 detection misses python.use_venv = false as PDM 2.27+ writes it (a TOML string) and in the user config, so agent mode patches an activated or stray venv and leaves __pypackages__ unpatched #609

[agent] Found by the scheduled PDM bug-hunt routine (ledger #312).

Summary

#540 taught Python env discovery that a PDM project with a base interpreter in .pdm-python installs into __pypackages__/<X.Y>/lib (PEP 582) when python.use_venv is off. The setting is read in two ways that miss what PDM actually writes:

1. PDM ≥ 2.27.0 writes the value as a TOML string. pdm config -l python.use_venv false produces use_venv = "false" in pdm.toml (2.26.9 and older write a bare false). pdm_uses_venv reads it with .as_bool(), gets None, and falls back to "on". PDM itself parses both forms as False.
2. The user config is never read. The default scope of pdm config python.use_venv false is the user config (~/.config/pdm/config.toml), which is how PDM's docs enable PEP 582. Discovery only looks at PDM_USE_VENV and the project's pdm.toml / .pdm.toml. This happens on every PDM version.

Either way discovery decides PDM uses a venv. It then takes the generic probes, so an activated VIRTUAL_ENV or a stray ./.venv (for example one an IDE or uv left behind) wins over __pypackages__. PDM ignores both, because the saved interpreter is a base Python and use_venv is off.

Impact

scan --mode agent / get patch a venv the project never runs and print status: success with exit 0. The copy that pdm run imports (__pypackages__) stays unpatched. vex does fail closed here (not_applied), so CI gated on VEX catches it. A plain scan --sync cron does not.

Repro (Linux, real PDM 2.29.2, mock patch API serving urllib3 1.26.18)

mkdir p && cd p
cat > pyproject.toml <<'EOF'
[project]
name = "p"
version = "0.1.0"
requires-python = ">=3.12"
dependencies = ["urllib3==1.26.18"]
[tool.pdm]
distribution = false
EOF
pdm config -l python.use_venv false     # writes use_venv = "false" on PDM >= 2.27
# or: pdm config python.use_venv false  # user config, any PDM version
pdm use -f /usr/bin/python3.12          # "Using __pypackages__ because non-venv Python is used."
pdm install                             # -> __pypackages__/3.12/lib/urllib3
uv venv .venv && uv pip install -p .venv/bin/python urllib3==1.26.18   # stray venv PDM ignores
socket-patch scan --mode agent --json   # exit 0, status success
tail -1 .venv/lib/python3.12/site-packages/urllib3/_version.py    # patched
tail -1 __pypackages__/3.12/lib/urllib3/_version.py               # still original
pdm run python -c 'import urllib3; print(urllib3.__file__)'       # __pypackages__/3.12/lib/urllib3/__init__.py

Activating an unrelated venv (VIRTUAL_ENV=/some/other/venv) instead of the stray .venv gives the same result: that venv gets patched.

Expected vs actual

• Expected (docs/testing/pdm-compatibility.md, "The installed env is the one PDM records"): "A base interpreter means PEP 582 (__pypackages__/<X.Y>/lib) only when python.use_venv is off (PDM_USE_VENV, or [python] use_venv in pdm.toml / .pdm.toml …) … Agent mode patches the env found". So with use_venv off, agent mode should patch __pypackages__.
• Actual: use_venv = "false" in pdm.toml (PDM ≥ 2.27's own output) and use_venv in the user config are both treated as "on", and the activated or stray venv gets patched.

Matrix (Linux, main 045d7ec, each cell run twice)

String-valued writes start at PDM 2.27.0: 2.12.4, 2.20.1, 2.24.2 and 2.26.9 write a bool, and 2.27.0, 2.28.2, 2.29.0 and 2.29.2 write a string. PDM 2.12.4 and 2.29.2 both read use_venv = "false" as False (pdm config python.use_venv). macOS and Windows weren't run. The logic is OS-independent, but the user-config path differs there (~/Library/Application Support/pdm/config.toml, %LOCALAPPDATA%\pdm\pdm\config.toml). The site config (/etc/xdg/pdm/config.toml, cf. #566) and PDM_CONFIG_FILE would be missed the same way.

First bad version

This is new code from #540 (b0a32db). Before that, PEP 582 wasn't crawled at all (documented), so it's a gap in the fix rather than a regression from a release.

Suspect code

• crates/socket-patch-core/src/crawlers/python_crawler.rs:493-510 (pdm_uses_venv): .as_bool() drops string values, so it should parse them the way PDM's ensure_boolean does, like pdm_env_flag. It also never consults the user or site config (PDM_CONFIG_FILE, <user config dir>/pdm/config.toml), which pdm_global_site_packages (:1987) already locates.
• :479-487 is the caller that returns None → generic probes (VIRTUAL_ENV at :367-384, ./.venv at :405-410) win over __pypackages__.