Lektor cached venv is broken on macOS with Python installed via Homebrew

[Igor Hatarist]

I'm using the latest version (Lektor-3.4.0b9.dev7+gbfce91a.d20230827 as of now).

python3 is installed via homebrew (python@3.11) and is symlinked:

/usr/local/bin/python3 -> /usr/local/Cellar/python@3.11/3.11.5/bin/python3 -> ... -> /usr/local/Cellar/python@3.11/3.11.5/Frameworks/Python.framework/Versions/3.11/bin/python3.11

I have created a venv using python3 -m venv env, everything worked fine in that venv until I installed a plugin (lektor plugins add lektor-index-pages). After that, trying to run the lektor server started raising an exception:

...
  File "/Users/igor/.virtualenvs/lektor/lib/python3.11/site-packages/lektor/packages.py", line 125, in create
    EnvBuilder(upgrade_deps=upgrade_deps, **options).create(self.path)
...
subprocess.CalledProcessError: Command '['/Users/igor/Library/Caches/Lektor/venvs/d63a0228b40929c7189d2c99d18e960b/bin/python3.11', '-m', 'ensurepip', '--upgrade', '--default-pip']' died with <Signals.SIGABRT: 6>.

When I tried to run python3 using that path, I saw this error:

dyld[34820]: Library not loaded: @loader_path/../../../../Python.framework/Versions/3.11/Python
  Referenced from: <430E089A-35B0-314B-9FD1-1E60F9CE5756> /Users/igor/Library/Caches/Lektor/venvs/d63a0228b40929c7189d2c99d18e960b/bin/python3.11
  Reason: tried: '/Users/igor/Library/Caches/Lektor/venvs/d63a0228b40929c7189d2c99d18e960b/bin/../../../../Python.framework/Versions/3.11/Python' (no such file), '/Library/Frameworks/Python.framework/Versions/3.11/Python' (no such file), '/System/Library/Frameworks/Python.framework/Versions/3.11/Python' (no such file, not in dyld cache)
[1]    34820 abort

So, looks like the homebrew version of Python was built with relative paths in mind and when EnvBuilder copies that binary to another path, things go south.

I have successfully fixed that for myself by editing the EnvBuilder options in lektor/packages.py:123, adding an symlinks=True option there:

options: Dict[str, Any] = {"clear": True, "with_pip": with_pip, "symlinks": True}

This has solved the issue for me - now, when Lektor creates a cached venv, it symlinks the used python binary in there and everything seems to work fine.

I've never heard of Lektor until today and I don't feel comfortable with making a pull request since my fix could break something else. Sorry!

[Jeff Dairiki]

Thank you for the report!

At this point, I only vaguely understand how symlinking vs copying might make a difference, but...

It appears the venv CLI (python -m venv), by default, uses symlinks everywhere except Windows. (Some versions/configurations of Windows do not fully support symlinks, or do not allow their creation without administrator-level access.)

In contrast, venv.EnvBuilder (as you note) defaults to not using symlinks anywhere.

So, I think an appropriate fix on our side would be just to duplicate the venv CLI's default logic to determine when to use symlinks. (I.e. pass symlinks=os.name != "nt" to the EnvBuilder constructor.)

If you're so inclined, feel free to make a PR to do that. Otherwise, I'll work one up in the next few days.

[Jeff Dairiki]

@hatarist This should now be fixed in the master branch. If not, please re-open the issue.

Thank you for the report and the analysis!