[PATCH 3 of 4 V2] chgcache: implement the background preloading thread

[Yuya Nishihara]

On Thu, 2 Mar 2017 13:32:23 -0800, Jun Wu wrote:
> Excerpts from Yuya Nishihara's message of 2017-03-03 00:51:12 +0900:
> > I hope so, but googling "fork thread" or "fork thread python" made me feel
> > bad. I haven't read them carefully, but they seemed to say "don't do it."
> 
> Good catch!
> 
> I had a look at CPython source code. The most related function is
> PyOS_AfterFork(void), which will be executed by the child. It does not
> handle all lock issues (like malloc() in libc).
> 
> I think we can use os.fork() in master and avoid threading to solve the
> issue:
> 
>   worker              master        forked master
>     | send(repopath) -> |
>                         | fork() -----> |
>                                         | preload() 
>                                         | replace master's socket
>     | connect() ----------------------> |
> 
> This solves other problems - like parsers.c has some slow functions that may
> block accept().

Hmm, that seems relatively simple and worth trying. We might have to rewind
stack before fork() so the forked master wouldn't end up exhausting Python
stack.