[Samba] How to do nonblocking writes to CIFS networkshares?

[Sebastian Rasmussen]

Hi!

I'm new to this mailing list, so please help out if I'm adressing the 
wrong forum. :)

I'm working on an application accessing a mounted networkshare. Under 
normal circumstances there is no problem accessing reading and writing 
to the networkshare using POSIX open(2)/read(2)/write(2) etc. However if 
there is intermittent connectivity problems to the networkshare my 
application experiences calls blocking for long periods of time when 
accessing the missing networkshare. I have e.g. observed write(2) calls 
being blocked for up to 5 minutes.

Worth noting is that my samba 4.1.13-based networkshare is running on a 
Linux 3.14.15 server and is mounted from a Linux-3.14 client system like so:
mount -t cifs -o nounix,noserverino,nobrl //192.168.0.42/myshare 
/tmp/mountpoint

In order not to block in these POSIX calls I attempted to open(2) the 
file for writing using O_NONBLOCK and then use select(2) to try to 
determine if the file descriptor was writable before attemping to call 
write(2). The idea being that if the fd claims not to be writable the 
application then has a chance to both wait in the select(2) call for a 
reasonable time and also try to timeout and let the application fail 
gracefully, should it detect that it cannot write to the fd. I assume 
that any previously successful writes may not have been transferred to 
the networkshare, but instead have been cached in the page cache to be 
transferred if the networkshare responds again (within the ~5 minut 
timeout).

My attempt resulted in the application being able to write lots of data 
to the share while it was unavailable, and the timeout in select(2) was 
never triggered. This was a surprise to me as I expected the fd to 
indicate that it was blocking at some point and that the timeout would 
be triggered.

Now, I need some advice: Have I glossed over something fundamental about 
how CIFS networkshares work? Is it possible to write applications that 
will not block indefinitely (5 minuts would be considered indefinitely 
in my application)? If yes, then what alternatives exists to O_NONBLOCK 
to do this?

  / Sebastian

PS. I contemplated fusesmb but it doesn't look like it handles 
O_NONBLOCK or similar either, so I think that is not viable.