Fix broken backends: PBS runs a nonexistent file, local auto-parallelization silently no-ops #120

Part of #108 · Phase 3

Three backend defects that make advertised functionality fail. Grouped because each is small and they share test scaffolding.

1. PBS is broken — it runs a file that is never created

clustrix/utils.py:1226 — the generated PBS job script ends with:

python execute_function.py

execute_function.py is never written anywhere in the package. It is the only occurrence of that filename in clustrix/. Compare SSH (utils.py:1330-1452) and SGE (utils.py:1233-1330), which both generate a real inline runner.

Additionally, submit_pbs_job (executor_schedulers.py:157-198) never calls setup_remote_environment, yet the script it submits runs source venv/bin/activate (utils.py:1225) against a venv nothing created.

• PBS generates a real inline runner that writes result.pkl, matching the SSH/SGE pattern
• submit_pbs_job performs environment setup like its siblings
• A test submits a real PBS job and asserts on the returned value

2. Local auto-parallelization is broken

clustrix/decorator.py:778 injects _parallel_{variable} into the kwargs passed to the user's function, then local_executor.py:128 calls self._executor.submit(func, *args, **kwargs). User functions do not accept a _parallel_* keyword, so this raises TypeError — which is then swallowed at decorator.py:708-716 and silently "falls back to sequential".

So the headline auto-parallelization feature silently does nothing, and the user is never told.

Related: decorator.py:375 _create_work_chunks is marked "a simplified implementation", and decorator.py:411 _combine_results says "For now, just return the list".

• Chunk data is passed via a mechanism the user function can actually receive
• The TypeError swallow is removed; a real failure surfaces as a failure
• A test asserts that a parallelized loop actually ran in parallel and produced the correct combined result
• _combine_results implements real combination, or the feature is documented as returning a list

3. cluster_type: "local" is accepted but unsupported

ClusterExecutor has no local branch (executor_core.py:101), so a config that sets cluster_type="local" raises ValueError: Unsupported cluster type: local at execution time. This accounts for 7 of the 127 current test failures.

• Either implement the local branch, or reject it at config-validation time with a message naming the supported types — accepted-then-crashed is the worst of both

Also in scope (small, same area)

• executor_schedulers.py:243 — SGE job-ID parsing uses fragile positional string splitting of qsub output; make it robust and test it against real qsub output shapes
• utils.py:81 — range_obj = eval(range_part), carrying the author's own comment # Dangerous in practice, needs safer evaluation. Replace with ast.literal_eval or a real parser