Semaphore name needs to be randomized, otherwise clash with other users is unavoidable

[peci1]

On some of our cluster nodes, when I run Ignition Blueprint/ign-launch1 (the SubT simulator), I get an error:

[Err] [Manager.cc:300] Error initializing semaphore: Permission denied

That points to

https://github.com/ignitionrobotics/ign-launch/blob/9f4a9298b2f549cd0df66d30f9c5942198593ea5/src/Manager.cc#L298

which can fail in case the semaphore with name

https://github.com/ignitionrobotics/ign-launch/blob/9f4a9298b2f549cd0df66d30f9c5942198593ea5/src/Manager.cc#L50

already exists and is created by another user with permissions that don't allow modification of the semaphore (i.e. 644).

I've checked that on the failing nodes, the semaphore indeed exists and is created by someone else:

$ ls -al /dev/shm/sem.*
-rw-r--r-- 1 otheruser rob 32 Aug 17 22:36 /dev/shm/sem.child_semaphore

In such a case, I can't use Ignition launch at all on that cluster node.

It'd be much better to name the semaphore either randomly, or predictably incorporating the username. Some other apps create semaphores named e.g.:

-rw-------. 1 user group 32 Aug 27 17:42 /dev/shm/sem.loky-447964-ctehrdwd
-rw-------. 1 user group 32 Aug 27 17:42 /dev/shm/sem.loky-447964-dfqrdehz
-rw-------. 1 user group 32 Aug 27 17:42 /dev/shm/sem.loky-447964-ed6_sluy
-rw-------. 1 user group 32 Nov 20  2019 /dev/shm/sem.mp-29fggu5g
-rw-------. 1 user group 32 Dec 16  2019 /dev/shm/sem.mp-2croga40

[peci1]

FYI @mjcarroll added the semaphore stuff according to git blame.

[mjcarroll]

Busted :)

I think having a randomized suffix makes perfect sense. Is this something you are interested in contributing?

[peci1]

I can definitely prepare a PR appending a random string/number to the semaphore name. But as I don't know the architecture of ign-launch, I can't foresee the consequences (if there are any).

[mjcarroll]

The semaphore was added to fix the way that we were originally using SIGCHLD. The commit of interest is here: 8ecae13

It was originally supposed to be an unnamed semaphore, but MacOS explicitly has to be named. Because the original implementation was unnamed, I don't see any issue with adding a randomized string. The semaphore is not being accessed across multiple launch instances or being used in a way that the name is needed.

[peci1]

Okay, then I can prepare the PR. One thing to think through - shouldn't we go with unnamed semaphores on non-Mac platforms?

[mjcarroll]

I opted to used named semaphores for all platforms to prevent having to maintain platform specific code.

[peci1]

Fix posted in #57.

[peci1]

Could I ask for a release of ign-launch1 which includes this fix? It'd simplify our testing...

[chapulina]

Could I ask for a release

See #61