[ccache] cache coherency across multiple users

[devsk]

A quick question: man page suggests that it is possible to share ccache across 
multiple developers by following a set of rules for permissions, umask and 
setting up of ccache dirs env vars etc. What it doesn't talk about is what 
happens if two developers are compiling at the same time and there is a cache 
miss and ccache needs to write the newly created object to the ccache directory.

What happens to two (or more) write calls arriving at the same time? Is there 
some sort of locking in place to avoid getting garbage in the resultant object 
file?

What about the case where one gets a hit and wants to read while the other gets 
a miss and wants to overwrite the object with the new object file?

And the case, where cache size management needs to throw away an existing object 
file to replace with another object file? Another developer's run may be reading 
it while this run is throwing it away.

Thanks for your time!
-devsk