fix - uvloop freeze when something other than uvloop invoke process fork and execute non-python process

Problem:
Uvloop for each loop register `atFork` handler that is called after fork is executed by forked child.
This handler works fine when fork was invoked by uvloop. In case fork is invoked by something else (such as external library)
uvloop freeze in this handler because:
- GIL is acquired inside `atFork` handler -> in case forked child does not contain python runtime `atFork` handler freeze at obtaining GIL
- when compiled in debug mode (`make debug`) cython trace calls are inserted inside compiled `atFork` handler -> in case forked child does not contain python runtime `atFork` handler freeze at providing trace call

Solution:
This fix solve described problems by implementing `atFork` handler as C function so that forked child can call it safely whether or not contains python runtime.

Author: grungy-ado

tests/test_process_spawning.py

@@ -0,0 +1,109 @@
+import asyncio
+import ctypes.util
+import logging
+from concurrent.futures import ThreadPoolExecutor
+from threading import Thread
+from unittest import TestCase
+
+import uvloop
+
+
+class ProcessSpawningTestCollection(TestCase):
+
+    def test_spawning_external_process(self):
+        """Test spawning external process (using `popen` system call) that
+        cause loop freeze."""
+
+        async def run(loop):
+            event = asyncio.Event(loop=loop)
+
+            dummy_workers = [simulate_loop_activity(loop, event)
+                             for _ in range(5)]
+            spawn_worker = spawn_external_process(loop, event)
+            done, pending = await asyncio.wait([spawn_worker] + dummy_workers,
+                                               loop=loop)
+            exceptions = [result.exception()
+                          for result in done if result.exception()]
+            if exceptions:
+                raise exceptions[0]
+
+            return True
+
+        async def simulate_loop_activity(loop, done_event):
+            """Simulate loop activity by busy waiting for event."""
+            while True:
+                try:
+                    await asyncio.wait_for(done_event.wait(),
+                                           timeout=0.1, loop=loop)
+                except asyncio.TimeoutError:
+                    pass
+
+                if done_event.is_set():
+                    return None
+
+        async def spawn_external_process(loop, event):
+            executor = ThreadPoolExecutor()
+            try:
+                call = loop.run_in_executor(executor, spawn_process)
+                await asyncio.wait_for(call, loop=loop, timeout=3600)
+            finally:
+                event.set()
+                executor.shutdown(wait=False)
+            return True
+
+        BUFFER_LENGTH = 1025
+        BufferType = ctypes.c_char * (BUFFER_LENGTH - 1)
+
+        def run_echo(popen, fread, pclose):
+            fd = popen('echo test'.encode('ASCII'), 'r'.encode('ASCII'))
+            try:
+                while True:
+                    buffer = BufferType()
+                    data = ctypes.c_void_p(ctypes.addressof(buffer))
+
+                    # -> this call will freeze whole loop in case of bug
+                    read = fread(data, 1, BUFFER_LENGTH, fd)
+                    if not read:
+                        break
+            except Exception:
+                logging.getLogger().exception('read error')
+                raise
+            finally:
+                pclose(fd)
+
+        def spawn_process():
+            """Spawn external process via `popen` system call."""
+
+            stdio = ctypes.CDLL(ctypes.util.find_library('c'))
+
+            # popen system call
+            popen = stdio.popen
+            popen.argtypes = (ctypes.c_char_p, ctypes.c_char_p)
+            popen.restype = ctypes.c_void_p
+
+            # pclose system call
+            pclose = stdio.pclose
+            pclose.argtypes = (ctypes.c_void_p,)
+            pclose.restype = ctypes.c_int
+
+            # fread system call
+            fread = stdio.fread
+            fread.argtypes = (ctypes.c_void_p, ctypes.c_size_t,
+                              ctypes.c_size_t, ctypes.c_void_p)
+            fread.restype = ctypes.c_size_t
+
+            for iteration in range(1000):
+                t = Thread(target=run_echo,
+                           args=(popen, fread, pclose),
+                           daemon=True)
+                t.start()
+                t.join(timeout=10.0)
+                if t.is_alive():
+                    raise Exception('process freeze detected at {}'
+                                    .format(iteration))
+
+            return True
+
+        loop = uvloop.new_event_loop()
+        proc = loop.run_until_complete(run(loop))
+        self.assertTrue(proc)

uvloop/handles/process.pyx

@@ -28,6 +28,7 @@ cdef class UVProcess(UVHandle):
 
         global __forking
         global __forking_loop
+        global __forkHandler
 
         cdef int err
 
@@ -76,6 +77,7 @@ cdef class UVProcess(UVHandle):
             loop.active_process_handler = self
             __forking = 1
             __forking_loop = loop
+            __forkHandler = <OnForkHandler>&__get_fork_handler
 
             PyOS_BeforeFork()
 
@@ -85,6 +87,7 @@ cdef class UVProcess(UVHandle):
 
             __forking = 0
             __forking_loop = None
+            __forkHandler = NULL
             loop.active_process_handler = None
 
             PyOS_AfterFork_Parent()

uvloop/includes/fork_handler.h

@@ -0,0 +1,15 @@
+
+typedef void (*OnForkHandler)();
+
+OnForkHandler __forkHandler = NULL;
+
+/* Auxiliary function to call global fork handler if defined.
+
+Note: Fork handler needs to be in C (not cython) otherwise it would require
+GIL to be present, but some forks can exec non-python processes.
+*/
+void handleAtFork() {
+    if (__forkHandler != NULL) {
+        __forkHandler();
+    }
+}

uvloop/includes/system.pxd

@@ -59,12 +59,12 @@ cdef extern from "unistd.h" nogil:
     void _exit(int status)
 
 
-cdef extern from "pthread.h" nogil:
+cdef extern from "pthread.h":
 
     int pthread_atfork(
-        void (*prepare)() nogil,
-        void (*parent)() nogil,
-        void (*child)() nogil)
+        void (*prepare)(),
+        void (*parent)(),
+        void (*child)())
 
 
 cdef extern from "includes/compat.h" nogil:

uvloop/loop.pyx

@@ -3117,19 +3117,21 @@ cdef vint __atfork_installed = 0
 cdef vint __forking = 0
 cdef Loop __forking_loop = None
 
+cdef extern from "includes/fork_handler.h":
 
-cdef void __atfork_child() nogil:
-    # See CPython/posixmodule.c for details
+    ctypedef void (*OnForkHandler)()
+    cdef OnForkHandler __forkHandler
+    void handleAtFork()
+
+cdef void __get_fork_handler() nogil:
     global __forking
+    global __forking_loop
 
     with gil:
-        if (__forking and
-                __forking_loop is not None and
+        if (__forking and __forking_loop is not None and
                 __forking_loop.active_process_handler is not None):
-
             __forking_loop.active_process_handler._after_fork()
 
-
 cdef __install_atfork():
     global __atfork_installed
     if __atfork_installed:
@@ -3138,7 +3140,7 @@ cdef __install_atfork():
 
     cdef int err
 
-    err = system.pthread_atfork(NULL, NULL, &__atfork_child)
+    err = system.pthread_atfork(NULL, NULL, &handleAtFork)
     if err:
         __atfork_installed = 0
         raise convert_error(-err)