fix(linux): give Ghostscript a clean library path in the frozen build

[nelsonduarte]

Summary

Step 1 of keeping the frozen Linux build's bundled libraries away from the system tools it starts. This step covers Ghostscript in Compress.

On Linux the PyInstaller one-file bootloader prepends its extraction directory (sys._MEIPASS, /tmp/_MEI...) to LD_LIBRARY_PATH and keeps the previous value, if there was one, in LD_LIBRARY_PATH_ORIG. Every child inherits that, so the system gs started by Compress resolved its shared libraries against the bundle first. The new module docstring records the measurement: with the v1.15.1 PDFApps-Linux.tar.gz on Ubuntu 26.04 the system gs 10.06.0 loaded the bundle's libstdc++.so.6, exited 1 with "GLIBCXX_3.4.32 not found", and Compress dropped its Ghostscript pass without a message.

Changes

• app/_subprocess_env.py (new, no Qt and no app.* imports): system_tool_env(environ=None, *, frozen=None, platform=None, meipass=None).
  • Returns None (inherit unchanged) outside a frozen build, on any platform other than Linux, with no _MEIPASS, or when there is nothing to undo.
  • Otherwise returns a copy of the environment with the bundle taken out of LD_LIBRARY_PATH: when the first entry is inside _MEIPASS and LD_LIBRARY_PATH_ORIG exists, the value from before launch is put back, even when empty; otherwise every entry inside _MEIPASS is dropped, and the variable is removed when nothing is left. LD_LIBRARY_PATH_ORIG is removed in both cases.
  • A sibling directory such as /tmp/_MEIabcd is not treated as inside /tmp/_MEIabc. Neither the input mapping nor os.environ is modified.
• app/_gs_paths.py: an environ argument goes through _gs_lib_dirs and _derive_gs_lib_dirs down to _gs_version, so the gs --version probe runs with the same environment as the gs child. A probe run with the inherited LD_LIBRARY_PATH would fail and stay cached for the session.
• app/utils.py (_compress_pdf, pass A): base_env = system_tool_env(), then GS_LIB on top of it with _gs_subprocess_env(gs, environ=base_env), falling back to base_env when there is no GS_LIB to add. The imports of both helpers move out of the try block, so an ImportError is no longer swallowed by its except Exception.

Tests

• tests/test_subprocess_env.py (new): platforms, restore from _ORIG, sibling directories, path normalisation, no mutation, defaults read from sys at call time, and an import allowlist for the module.
• tests/test_gs_lib_paths.py: the version probe inherits by default and otherwise gets the child's environment; pass A builds GS_LIB on the system-tool environment and still passes that environment when there is no GS_LIB; a missing helper module raises instead of being swallowed.
• Full suite at verification time: 1057 passed.

Not in this PR (later steps)

• the tesseract calls;
• xdg-open in app/base.py;
• CONTRIBUTING;
• the relaunch on a language change, which hands the old _MEI directory on in LD_LIBRARY_PATH_ORIG;
• the test for an empty meipass.

🤖 Generated with Claude Code