Protect against race conditions when used concurrently

[fingolfin]

It would be nice if PackageManager would protect against race conditions when used concurrently in two processes. That can happen in e.g. automation, when one wants to install a package via PackageManager and by chance this is done concurrently two times.

This can work, but it can also happen that one or even both installation attempts fail.

Other package managers solve this e.g. via PID (process id) based lock files: before installing a package, a lock is "taken". The first process to do so proceeds; other processes trying to install a package see the lock, and then either abort or (more user friendly) print a message like "waiting for lock" and well, wait for it (possibly only up to a certain time out.

[mtorpey]

This sounds like a good idea. Perhaps the place for the lock file is ~/.gap/pkg/pkgman-11834.lock` (where 11834 is the pid) so the lock is per pkg directory.

It'd just need some careful thought on what to do if a process ends without deleting the lockfile – a timeout, or maybe something that checks whether the process still exists. Manual override as well?