child post-fork gRPC shutdown to wait briefly (#36873)

gRPC shutdown may happen asynchronously with `work_serializer_dispatch`, so wait briefly until shutdown completes in the child's post-fork.

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Closes #36873

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36873 from yousukseung:python-fork-shutdown-wait 7908a6912a
PiperOrigin-RevId: 642066286

src/python/grpcio/grpc/_cython/_cygrpc/fork_posix.pyx.pxi

@@ -79,11 +79,15 @@ cdef void __postfork_child() noexcept nogil:
             _LOGGER.error('Exiting child due to raised exception')
             _LOGGER.error(sys.exc_info()[0])
             os._exit(os.EX_USAGE)
-
-    if grpc_is_initialized() > 0:
-        with gil:
-            _LOGGER.error('Failed to shutdown gRPC Core after fork()')
-            os._exit(os.EX_USAGE)
+        # Give ~2s to shutdown asynchronously.
+        wait_ms = 10
+        while wait_ms < 1500:
+            if grpc_is_initialized() == 0:
+                return
+            time.sleep(wait_ms / 1000)
+            wait_ms = wait_ms * 2
+        _LOGGER.error('Failed to shutdown gRPC Core after fork()')
+        os._exit(os.EX_USAGE)
 
 
 def fork_handlers_and_grpc_init():
@@ -153,7 +157,7 @@ def get_fork_epoch():
 def is_fork_support_enabled():
     return _GRPC_ENABLE_FORK_SUPPORT
 
-    
+
 def fork_register_channel(channel):
     if _GRPC_ENABLE_FORK_SUPPORT:
         _fork_state.channels.add(channel)