[SSSD] [PATCH] sss_cache: Remove fastcache even if sssd is not running.

[Michal Židek]

https://fedorahosted.org/sssd/ticket/1584

First, I thought the race condition between sssd_nss and sss_cache 
should be solved by some sort of file locking mechanism, but when 
started working on it, the places where we needed to check for the file 
being locked or free were too many and spread among monitor, nss and 
sss_cache tool processes and it was not clear how the access is controlled.

So I decided to do it this way:
1. sss_cache tries to send_sighup to monitor as usual
2. if signal_sssd returns that sssd is not running, proceed with 
memcache invalidation
3. As a part of memcache invalidation:
     - it first opens the mc file
     - then it checks if sssd is running (with pgrep)
     - if sssd is running it stops the the process of invalidation
     - if sssd is still not running it proceeds with the invalidation

See that if sssd starts after (or during) the pgrep check (so we will 
not catch it as running, but will assume it is off) it is not  a 
problem, because we have file descriptor associated with file that was 
present before sssd was running (we open the file before pgrep call). 
sssd_nss alwas creates a new memory cache file on startup, so we will 
only mark the old one as recycled (and not the new one), that we do not 
care about (because it will be deleted by sssd_nss and marking that as 
recycled is not a problem, I think -- the worst thing that can happen is 
race between nss and cache tool while both are marking the OLD file as 
recycled, but I can not figure out situation where this could be 
dangerous, because there are no "temp" states that could be harmful. 
Both processes are changing the value of status, but both to the same 
value).

Another thing that I like about this is that we do not have to care 
about communication sssd vs sssd_nss vs sss_cache but only control sssd 
vs sss_cache, which is much easier to understand.

Can someone see any problems that I missed?

NOTE: The function sss_mc_set_recycled is copied from different module. 
I did not want to make this function non static and put it to a header 
file because it is not intended to be used directly.

I tested it and it works fine for me.

The patch is attached.

Thanks
Michal