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Store and check shared memory ownership before resizing shared memory objects.
The scenario we are resolving here is the following: FTL is already running and someone(/-thing) deletes all the shared memory objects. If another instance of FTL is then started, it doesn't know a previous process is running and creates new SHM objects for itself. Both processes can - in theory - run just fine without touching each other as they are both actually using different shared memory objects. You are right when you think this is bizarre. The reason is that the first one has the shared memory objects still mapped into memory, i.e., "deleting" them from disk only removes the visible file handles. The next FTL instance creates files with the very same name, however, they are also new and distinct files, i.e., their memory will point elsewhere. Both instances can now run in parallel just fine until one of them needs to resize a shared memory objects. If one of the shared memory events now gets resized to a size larger than it was before BUT smaller than what the other FTL instance is expecting, the other instance will instantaneously crash with a
SIGSEGV
(a bus error to be precise).This PR resolves this by storing the PID of the SHM object creator in the settings object.
Each FTL instance reloads this shared memory instance now before performing any potentially dangerous operation and checks if the shared memory files on disk are still owned by this process. If this is not the case, we are in serious trouble and exit immediately. This should allow the second instance (you could call it the "rightful owner" of the current existing SHM objects) a fairly good chance to never even notice this and continue to operate just fine.
Resolves the root causes behind #1463, #1448 and possibly other similar issues in the past by preventing a certain "mis"configuration to have the bad effect it had for these users.