[Box Backup-dev] COMMIT r476 - box/chris/general/bin/bbackupd

[Chris Wilson]

Hi Ben,

>>  - Use overlapped I/O and IPC to avoid writing to control socket while
>>    another thread is reading from it, which causes race conditions and
>>    deadlocks
>
> Wow. This is complex. Would using critical sections and perhaps PeekNamedPipe 
> be less complicated? I can't believe overlapped I/O and messaging is 
> necessary, and think we should strive for simplicity.
>
> I am curious as to why it's necessary.

Yeah, it is pretty complex, but it wasn't too hard to write (about 7 
hours), amazed me by working almost perfectly first time, and I think it 
is necessary to meet the conditions you once set out.

It breaks down into 2 parts: IPC, and overlapped I/O.

IPC is simply essential. We cannot call API functions on the named pipe in 
two different threads, or the application will deadlock. Read() and 
Write() is one example. All I/O must be done by the handler thread. The 
safest and most obvious way to pass the messages from the main thread 
seems to be a message queue protected by a critical section and signaled 
by an Event.

Overlapped I/O may not be essential, but if you want to avoid polling the 
command socket, as you once said to me, then it is. Overlapped seems to be 
the only way that I can tell from the docs for you to be notified that you 
have a message waiting to be read (especially if you want to combine it 
with waiting for an event from the main thread). Otherwise you have to 
either poll the socket with Peek, as you said, or you end up blocked in 
ReadFile where you can't service the main thread by writing messages to 
the client.

By all means ask Charles to review it, or anyone else you like, but I 
don't think it's excessively complex, and at least the IPC stuff does fix 
some major bugs - the command socket was essentially liable to deadlock 
the whole process before.

Cheers, Chris.
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