qa: comment about potential freeze when a functional test ends

neo/tests/functional/__init__.py

@@ -171,6 +171,14 @@ class Process(object):
                     from coverage import Coverage
                     coverage = Coverage(coverage_data_path)
                     coverage.start()
+                # XXX: Sometimes, the handler is not called immediately.
+                #      The process is stuck at an unknown place and the test
+                #      never ends. strace unlocks:
+                #        strace: Process 5520 attached
+                #        close(25)                               = 0
+                #        getpid()                                = 5520
+                #        kill(5520, SIGSTOP)                     = 0
+                #        ...
                 signal.signal(signal.SIGUSR2, save_coverage)
                 os.close(self._coverage_fd)
                 os.write(w, '\0')