[Python client] restart//stop hang while info_volume returned

[JeffreyDevloo]

Problem description

Both the StorageRouterClient and LocalStorageRouterClient were hanging when using either restart_volume or stop_object without specifying a timeout. After keyboard interrupting using ctrl-c, nothing happened. I also checked info_volume which yielded me a return value

Tailing the logging in the volumedriver itself: the command were received and processed. It looks like the python client was stuck somewhere.

Restarting the volumedriver while the python cleint was stuck raised the KeyboardInterrupt.

[redlicha]

The underlying issue is most likely that the Python client was waiting in a C++ call for a response from the volumedriver (without timeout). While in a C(++) call, Python signal handlers are not run, only after the call. Cf. https://docs.python.org/3.6/library/signal.html#execution-of-python-signal-handlers (Python 3):

A Python signal handler does not get executed inside the low-level (C) signal handler. Instead, the low-level signal handler sets a flag which tells the virtual machine to execute the corresponding Python signal handler at a later point(for example at the next bytecode instruction). This has consequences:

It makes little sense to catch synchronous errors like SIGFPE or SIGSEGV that are caused by an invalid operation in C code. Python will return from the signal handler to the C code, which is likely to raise the same signal again, causing Python to apparently hang. From Python 3.3 onwards, you can use the faulthandler module to report on synchronous errors. A long-running calculation implemented purely in C (such as regular expression matching on a large body of text) may run uninterrupted for an arbitrary amount of time, regardless of any signals received. The Python signal handlers will be called when the calculation finishes.

[JeffreyDevloo]

This issue was moved to openvstorage/volumedriver-ee#165