Recognize ERROR_INVALID_HANDLE as indicating the IOCP was closed

There's a race condition when we signal the IOCP thread to exit by
closing the IOCP: the thread might be blocked in
GetQueuedCompletionStatusEx, or it might (rarely) be in the rest of
the loop. The effect of this is that there are two possible error
messages we might get that both indicate that the IOCP was
closed. Before we were only detecting one of them; now we detect
both.

This has the side-effect of making it so that during the test suite we
deterministically only take the regular-exit path, rather than
sometimes taking the blow-up-on-unrecognized-error path. Which is
good because codecov was getting cranky about that non-deterministic
coverage and yelling at people for no reason (see python-triogh-81).

trio/_core/_io_windows.py

@@ -268,6 +268,18 @@ def do_select():
                     queue.put_nowait(info)
 
     def _iocp_thread_fn(self):
+        # This thread sits calling GetQueuedCompletionStatusEx forever. To
+        # signal that it should shut down, the main thread just closes the
+        # IOCP, which causes GetQueuedCompletionStatusEx to return with an
+        # error:
+        IOCP_CLOSED_ERRORS = {
+            # If the IOCP is closed while we're blocked in
+            # GetQueuedCompletionStatusEx, then we get this error:
+            ErrorCodes.ERROR_ABANDONED_WAIT_0,
+            # If the IOCP is already closed when we initiate a
+            # GetQueuedCompletionStatusEx, then we get this error:
+            ErrorCodes.ERROR_INVALID_HANDLE,
+        }
         while True:
             max_events = 1
             batch = ffi.new("OVERLAPPED_ENTRY[]", max_events)
@@ -277,7 +289,7 @@ def _iocp_thread_fn(self):
                 _check(kernel32.GetQueuedCompletionStatusEx(
                     self._iocp, batch, max_events, received, 0xffffffff, 0))
             except OSError as exc:
-                if exc.winerror == ErrorCodes.ERROR_ABANDONED_WAIT_0:
+                if exc.winerror in IOCP_CLOSED_ERRORS:
                     # The IOCP handle was closed; time to shut down.
                     return
                 else:

trio/_core/_windows_cffi.py

@@ -118,3 +118,4 @@ class ErrorCodes(enum.IntEnum):
     ERROR_IO_PENDING = 997
     ERROR_OPERATION_ABORTED = 995
     ERROR_ABANDONED_WAIT_0 = 735
+    ERROR_INVALID_HANDLE = 6