[Python 3.14 proxy worker] Missing sys.modules["__main__"] breaks multiprocessing spawn

[coding-totoro]

Description

The Azure Functions Python 3.14 proxy worker removes __main__ from
sys.modules before invoking application code.

As a result, Python's standard multiprocessing spawn context fails during
Process.start(), before the child process or its target function begins
executing.

This is not specific to any third-party package. The minimal reproduction uses
only azure-functions and the Python standard library.

Environment

• Azure Functions Python runtime: 3.14
• Python: 3.14.6
• Azure Functions Core Tools: 4.12.0
• Functions runtime: 4.1048.200.26180
• Programming model: Python v2
• azure-functions application package: 2.2.0
• Reproduced locally inside the Functions worker
• The same failure was observed after deploying the application to Azure Functions on Python 3.14

The same application code worked on the Python 3.12 Functions runtime.

Minimal reproduction

import multiprocessing
import sys

import azure.functions as func

app = func.FunctionApp()


def child_process() -> None:
    return None


@app.route(route="multiprocessing-repro")
def multiprocessing_repro(req: func.HttpRequest) -> func.HttpResponse:
    main_present = "__main__" in sys.modules

    context = multiprocessing.get_context("spawn")
    process = context.Process(target=child_process)
    process.start()
    process.join()

    return func.HttpResponse(
        f"__main__ present: {main_present}; exit code: {process.exitcode}"
    )

Start the application using Core Tools with Python 3.14 and invoke
/api/multiprocessing-repro.

Actual behavior

"__main__" in sys.modules is false during the function invocation, and
Process.start() raises:

Traceback (most recent call last):
  File ".../multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File ".../multiprocessing/context.py", line 289, in _Popen
    return Popen(process_obj)
  File ".../multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File ".../multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File ".../multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File ".../multiprocessing/spawn.py", line 164, in get_preparation_data
    main_module = sys.modules["__main__"]
KeyError: "__main__"

The failure occurs before the child process starts.

Expected behavior

The Functions worker should retain a valid __main__ module, or otherwise
initialize the application environment so that standard-library
multiprocessing spawn works inside a function invocation.

Investigation

The Python 3.14 worker uses proxy_worker. Its dependency cleanup appears to
remove modules from sys.modules based on their file location. It excludes
modules whose names begin with proxy_worker, but does not appear to exclude
__main__.

That appears to leave the invocation environment without the module required
by multiprocessing.spawn.get_preparation_data().

Launching an explicit Python subprocess with an importable module works in the
same Functions-host process, confirming that the failure is specifically in
the multiprocessing spawn preparation path.

Impact

Any application or dependency that uses the standard-library spawn context can
fail under the Python 3.14 Functions worker. In our application this prevented
all isolated document conversions from starting, regardless of document type.

Related issue

Related historical issue: #1094 reported the same missing-__main__ worker
condition on Python 3.9 through a different caller.

This report concerns the Python 3.14 proxy worker and reproduces through:

multiprocessing.get_context("spawn").Process(...).start()

It is therefore related behavior, but not the same caller or worker
implementation.