cv2 import appends a trailing ":" to LD_LIBRARY_PATH — empty entry resolves to CWD and breaks child processes

[superbigcup325]

Environment

• opencv-python: reproduced on 4.11.0.86, 4.12.0.88, 4.13.0.92, 4.14.0.94 and 5.0.0.93 (PyPI manylinux x86_64 wheels; earliest tested 4.11.0.86, Jan 2025)
• Python 3.13.15 / 3.14.7, Linux x86_64 (glibc; CachyOS/Arch, kernel 6.x)

Minimal reproduction

python -m venv venv && venv/bin/pip install opencv-python==5.0.0.93
venv/bin/python - <<'EOF'
import os, subprocess

print("before import:", repr(os.environ.get("LD_LIBRARY_PATH")))
import cv2
print("cv2", cv2.__version__, "after import:", repr(os.environ.get("LD_LIBRARY_PATH")))
print("child sees:", subprocess.run(["sh", "-c", "printf '%s' \"$LD_LIBRARY_PATH\""],
      capture_output=True, text=True).stdout)
EOF

Observed

before import: None
cv2 5.0.0 after import: '/…/site-packages/cv2/../../lib64:'
child sees: '/…/site-packages/cv2/../../lib64:'

cv2/__init__.py (POSIX branch, cv2/__init__.py:146-147 in current wheels):

# amending of LD_LIBRARY_PATH works for sub-processes only
os.environ['LD_LIBRARY_PATH'] = ':'.join(l_vars['BINARIES_PATHS']) + ':' + os.environ.get('LD_LIBRARY_PATH', '')

Three problems in one line:

1. Unconditional trailing : — when LD_LIBRARY_PATH was previously unset the
   variable is created with a dangling separator. Per ld.so(8) semantics an empty
   entry resolves to the current working directory of every child process.
2. Global environment mutation — the comment itself notes this "works for
   sub-processes only", yet the write persists in os.environ for the whole
   process lifetime, so every subprocess/os.system/webbrowser call made by
   the host application inherits the modified path. This is invisible side-channel
   state an importer cannot reasonably expect from import cv2.
3. The injected directory typically does not exist — with standard manylinux
   wheel layouts the bundled libraries live in opencv_python.libs/ (located via
   $ORIGIN RPATH), and <site-packages>/cv2/../../lib64 is absent in venv,
   project, and standalone-packaged layouts, so the write buys nothing while
   adding the hazard.

Real-world impact

We ship a Nuitka-standalone application whose launcher cds into the release
folder. With cv2 imported, LD_LIBRARY_PATH ends with : → the release folder
(the CWD) enters the child loader path → children that load system OpenSSL
(xdg-open → kde-open, i.e. "open a URL in the default browser") resolve the
bundled, older libssl.so.3/libcrypto.so.3 instead of the system ones and die
with:

kde-open: libssl.so.3: version `OPENSSL_3.2.0' not found (required by /usr/lib/libcurl.so.4)

Any application that imports cv2 and then spawns helper processes from a
directory containing same-named libraries is affected the same way.

Suggested fix

• Do not append a dangling separator; only write the variable when there is
  something to add and prepend/append with proper joining:

  paths = l_vars['BINARIES_PATHS']
  if paths:
      old = os.environ.get('LD_LIBRARY_PATH')
      os.environ['LD_LIBRARY_PATH'] = ':'.join(paths) + (':' + old if old else '')

• Longer term, prefer not mutating the global environment at all: the bundled
  libraries are already found through $ORIGIN RPATH; if an env-based fallback
  is really needed, consider documenting it and cleaning up after import, or
  exposing an opt-in.

Happy to send a PR to opencv-python/opencv-python (the loader __init__.py)
if you agree with the direction.