fix: detect free-threaded Python from venv and uv before setting PYTHONMALLOC

[not-matthias]

Problem

Free-threaded CPython (3.13t+) refuses to start with PYTHONMALLOC=malloc. The valgrind executor already skips that override when it detects a free-threaded interpreter, but the detection only probed python on the PATH. It missed the interpreter the benchmark actually runs under when that interpreter lives in a virtual environment or is selected by uv, so runs using astral-sh/setup-uv with python-version: 3.13t still failed.

Change

Probe every interpreter the benchmark command could resolve to, and skip the override if any of them is free-threaded:

• python / python3 on the PATH
• $VIRTUAL_ENV/bin/python
• .venv/bin/python in the benchmark working directory
• the interpreter uv python find $UV_PYTHON resolves to (setup-uv exports UV_PYTHON from its python-version input). uv downloads interpreters lazily, so when the request is not installed yet the request string itself (3.13t, +freethreaded) decides.

The probe now reads sys.abiflags instead of sysconfig.get_config_var('Py_GIL_DISABLED'). import sysconfig fails when _PYTHON_SYSCONFIGDATA_NAME is set for a different interpreter (common in Nix shells), which turned the old check into a silent false negative for exactly the venvs it needs to detect.

Verification

Release-mode smoke runs against real interpreters created with uv venv:

scenario	free-threaded detected
no venv, PATH python 3.12	no
plain 3.12 .venv in cwd	no
3.14t .venv in cwd	yes
UV_PYTHON=3.14t (installed), no venv	yes
UV_PYTHON=3.13t (not installed), no venv	yes (request fallback)
UV_PYTHON=3.12, no venv	no
VIRTUAL_ENV=<3.14t venv>, cwd has no venv	yes

Plus rstest cases for the UV_PYTHON request-string classifier.

[codspeed]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 17 untouched benchmarks

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_{Comparing cod-296-fails-with-python313-freethreaded (5af022c) with main (88c994e)}

[not-matthias]

NOTE: This PR tries to fix the known cases when a free-threaded python can be used and try to not use the environment variable. However, there are many more cases that can bypass our checks (e.g. using custom compiled python).

In the future, we can explore removing PYTHONMALLOC, but this would be a breaking change so it should only be included in the next runner release (as to why this partial solution exists)

[greptile-apps]

Confidence Score: 4/5

The PR is not safe to merge until free-threaded detection stops depending on an import that can fail under the interpreter environments this change is meant to support.

Findings

1.  Sysconfig Probe Breaks Detection ▶

Fix with agent prompt

### Issue 1
src/executor/valgrind/helpers/python.rs:7-9
If `_PYTHON_SYSCONFIGDATA_NAME` refers to configuration data that the candidate interpreter does not provide, importing `sysconfig` fails. The probe then treats that free-threaded interpreter as a regular build, and `measure` sets `PYTHONMALLOC=malloc`, causing Python to refuse to start. Keep the `sys.abiflags` probe from the previous revision because it does not import interpreter-specific configuration data.

```suggestion
/// Free-threaded builds carry the `t` ABI flag. `sys.abiflags` needs no `sysconfig`
/// import, which fails when `_PYTHON_SYSCONFIGDATA_NAME` names a module the
/// interpreter does not ship.
const GIL_DISABLED_PROBE: &str = "import sys; print(int('t' in sys.abiflags))";
```

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Summary

The PR expands free-threaded Python detection to cover PATH interpreters, conventional virtual environments, and UV_PYTHON, and adds an experimental option to suppress the allocator override. The newest revision, however, restores the environment-sensitive probe that the PR was intended to replace.

• Propagates the allocator-override setting through CLI and executor configuration.
• Resolves and probes Python candidates associated with virtual environments and uv.
• Classifies free-threaded uv request strings when no installed interpreter can be resolved.
• Reintroduces a sysconfig import that can make free-threaded detection fail in Nix or cross-interpreter environments.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Build candidate interpreter list] --> B[Run free-threading probe]
    B --> C{Probe succeeds and reports GIL disabled?}
    C -- Yes --> D[Do not set PYTHONMALLOC]
    C -- No or import failure --> E{Override explicitly disabled?}
    E -- Yes --> D
    E -- No --> F[Set PYTHONMALLOC=malloc]
    F --> G[Free-threaded Python refuses to start after a false negative]

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_{Reviews (2) · Last reviewed commit: "feat(simulation): add Python allocator o..."}

[GuillaumeLagrange]

olgtm

[not-matthias]

@greptileai review