Run Function Executor process in its own process group on POSIX systems

This allows us to kill all processes in Function Executor process group
instead of only killing the Function Executor process. This addresses the
use case when customer code forks processes. We now clean up the forked
processes too.

Testing:

make check
make test

python-sdk/indexify/executor/function_executor/server/subprocess_function_executor_server_factory.py

@@ -1,4 +1,6 @@
 import asyncio
+import os
+import signal
 from typing import Any, Optional
 
 from .function_executor_server_factory import (
@@ -46,6 +48,8 @@ async def create(
             proc: asyncio.subprocess.Process = await asyncio.create_subprocess_exec(
                 "indexify-cli",
                 *args,
+                # TODO: pass `process_group=0` instead of the depricated `preexec_fn` once we only support Python 3.11+.
+                preexec_fn=_new_process_group,
             )
             return SubprocessFunctionExecutorServer(
                 process=proc,
@@ -77,6 +81,10 @@ async def destroy(
                 # The process already exited and was waited() sucessfully.
                 return
 
+            if os.name == "posix":
+                # On POSIX systems, we can kill the whole process group so processes forked by customer code are also killed.
+                # This should be done before proc.kill because PG processes get their own PG when their PG leader dies.
+                os.killpg(proc.pid, signal.SIGKILL)
             proc.kill()
             await proc.wait()
         except Exception as e:
@@ -100,3 +108,8 @@ def _release_port(self, port: int) -> None:
 
 def _server_address(port: int) -> str:
     return f"localhost:{port}"
+
+
+def _new_process_group() -> None:
+    """Creates a new process group with ID equal to the current process PID. POSIX only."""
+    os.setpgid(0, 0)