🐛 fix(seed): isolate shared cache compilation - #3407

The app-data seeder with --symlink-app-data builds one image of each seed wheel in the app-data cache (the per-user directory where virtualenv keeps wheels and extracted images) and symlinks that image into every environment it creates. Building the image ran the destination interpreter as python -m compileall <image> with its default startup. That startup puts the working directory first on sys.path and imports site, which runs the .pth files in the destination's site-packages. A compileall.py in the directory virtualenv was run from replaced the standard library module, and an import line in a .pth file of an existing destination ran before compilation started. 🐛 The compile step now runs with -I -S, so neither the working directory nor the destination's site configuration takes part, and the bytecode in the shared image comes from the standard library's compileall alone.

The compile child started with the destination's full startup

SymlinkPipInstall._generate_new_files ran self._creator.exe -m compileall <image> with no interpreter flags. #1480 added the call when the shared image became read-only, so that the .pyc files exist before an import from a linked environment would try to write them and fail. The -m switch prepends the working directory to sys.path, so import compileall found a compileall.py next to where virtualenv was invoked before the standard library's. Default startup also imports site, which adds the destination's site-packages, executes every line beginning with import in the .pth files there, and then imports sitecustomize and usercustomize. On a re-run over an existing path, or with --system-site-packages, those directories hold whatever was there before. PYTHONPATH, PYTHONHOME and the other PYTHON* variables of the calling process applied as well. FromAppData.run keys the image by installer and wheel and builds it once, and every later environment created from the same cache links to it, so code that ran in the child could change the image before build_image made it read-only. A planted compileall.py that exits at once leaves the image with no bytecode, and the build goes on, since the child's exit status and output are discarded.

The command now carries -I and -S. -I keeps the working directory off sys.path, ignores the PYTHON* variables and skips the user site-packages. -S skips the site import, so no .pth file, sitecustomize or usercustomize runs. sys.path is then the standard library of the interpreter the destination was created from, which is where compileall comes from. The child is still the destination interpreter, so the bytecode still matches the target Python version. Its exit status and output are still not read, as before. The test plants a compileall.py in the working directory and a .pth file in the destination's site-packages, each of which would create a marker file, and checks that pip/__pycache__ is populated and no marker exists.

sequenceDiagram
    participant V as virtualenv
    participant C as destination python
    participant W as working directory
    participant S as destination site-packages
    V->>C: -m compileall image
    rect rgb(255, 214, 214)
    C->>W: sys.path[0]: a compileall.py here wins
    C->>S: import site: .pth import lines run, sitecustomize imported
    Note over C: PYTHONPATH and the other PYTHON* variables applied
    end
    V->>C: -I -S -m compileall image
    rect rgb(216, 243, 220)
    Note over C: sys.path is the stdlib only, site never imported
    C->>C: stdlib compileall writes image/**/__pycache__
    end

Behavior changes

Symlink seeding compiles its shared image without the destination's .pth files, sitecustomize and usercustomize, without the user site-packages, without the working directory on sys.path and without PYTHON* environment variables. A .pth file or sitecustomize that an existing destination relied on during that step no longer runs there. Startup of the completed environment is unchanged, and so is the copy installer, which has no compile step.

Reported by Anthropic's OSS Scanner: ANT-2026-9S579FUR.