Extra spaces added to lock files - probable issue w/ interpreter selection.

[asherf]

This was introduced in 2.16.0 a1:

While the spaces themselves are not a big deal, there might be an underlying issue with more serious implications.
More information in the slack thread

Relevant PR: #18495
cc: @jsirois @thejcannon

[jsirois]

@thejcannon the issue is #18495 leads to interpreter selection for the Pex CLI selecting Python 2.7 - which is the cause of the JSON trailing whitespace. In @asherf's case, Python 2.7 is not in bounds in any repo ICs and prior to #18495 Python 2.7 was not selected to run the Pex CLI with.

[thejcannon]

Here's where we choose ICs for lockfile generation:

pants/src/python/pants/backend/python/goals/lockfile.py

Line 237 in fb66bd8

interpreter_constraints=InterpreterConstraints(

Here's where we pass it to PexCliProcess:

pants/src/python/pants/backend/python/goals/lockfile.py

Line 138 in fb66bd8

*req.interpreter_constraints.generate_pex_arg_list(),

Neither of which changed in the commit.

Additionally, the changes to pex_cli.py seem to be relatively safe to have done, aside from the obvious "pex is now run using sys.executable" The cmdline args shouldn't have changed.

[jsirois]

If you follow the slack thread all good commit prior, reproducibly bad at that exact commit. So something is up.

[jsirois]

If it's not clear the issue is the Python used to run the Pex CLI, not the arguments passed to it.