Get multiprocessing working in kitty

Monkeypatch the stdlib multiprocessing module to
use kitty as its python interpreter for spawning worker
processes.

kitty/multiprocessing.py

@@ -0,0 +1,66 @@
+#!/usr/bin/env python
+# vim:fileencoding=utf-8
+# License: GPLv3 Copyright: 2020, Kovid Goyal <kovid at kovidgoyal.net>
+
+# Monkeypatch the stdlib multiprocessing module to work with the embedded python
+# in kitty, when using the spawn launcher.
+
+
+from concurrent.futures import ProcessPoolExecutor
+from multiprocessing import util  # type: ignore
+from multiprocessing import context, get_all_start_methods, get_context, spawn
+from typing import Any, Callable, List, Optional, Tuple, Union
+
+from .constants import kitty_exe
+
+orig_spawn_passfds = util.spawnv_passfds
+orig_executable = spawn.get_executable()
+
+
+def spawnv_passfds(path: str, args: List[str], passfds: List[int]) -> Any:
+    idx = args.index('-c')
+    patched_args = [spawn.get_executable(), '+runpy'] + args[idx + 1:]
+    return orig_spawn_passfds(kitty_exe(), patched_args, passfds)
+
+
+def monkey_patch_multiprocessing() -> None:
+    # Use kitty to run the worker process used by multiprocessing
+    spawn.set_executable(kitty_exe())
+    util.spawnv_passfds = spawnv_passfds
+
+
+def unmonkey_patch_multiprocessing() -> None:
+    spawn.set_executable(orig_executable)
+    util.spawnv_passfds = orig_spawn_passfds
+
+
+def get_process_pool_executor(
+    prefer_fork: bool = False,
+    max_workers: Optional[int] = None,
+    initializer: Optional[Callable] = None,
+    initargs: Tuple[Any, ...] = ()
+) -> ProcessPoolExecutor:
+    if prefer_fork and 'fork' in get_all_start_methods():
+        ctx: Union[context.DefaultContext, context.ForkContext] = get_context('fork')
+    else:
+        monkey_patch_multiprocessing()
+        ctx = get_context()
+    try:
+        return ProcessPoolExecutor(max_workers=max_workers, initializer=initializer, initargs=initargs, mp_context=ctx)
+    except TypeError:
+        return ProcessPoolExecutor(max_workers=max_workers, initializer=initializer, initargs=initargs)
+
+
+def test_spawn() -> None:
+    monkey_patch_multiprocessing()
+    try:
+        from multiprocessing import get_context
+        ctx = get_context('spawn')
+        q = ctx.Queue()
+        p = ctx.Process(target=q.put, args=('hello',))
+        p.start()
+        x = q.get(timeout=2)
+        assert x == 'hello'
+        p.join()
+    finally:
+        unmonkey_patch_multiprocessing()