Wait for epair interface to become available

[grembo]

Is your feature request related to a problem? Please describe.
Prior to applying #200 I experienced a few occasions of the epair interface not being available inside of the pot/jail when tinirc executed - it became available a few seconds later though.

I suspect that this was due to some pot start/pot stop overlaps that were removed or at least made a lot less likely by the changes in that pull request. As I can't rule out that this is something that could happen under other circumstances as well (read: it's possible in general), it would be good to have a mechanism to deal with it.

Describe the feature you'd like to have
A simple check with a timeout that waits for the epair interface to show up before ifconfig + route add default are called. Exit with an error code in case it doesn't become available. Update: This could be done in /tmp/tinirc, or maybe even better inside of pot before running pot.cmd (e.g., after sleep 1234 and before jexec $cmd).

I'm not 100% sure if that would cut it though (assuming there is still a possibility that some stray pot stop/cleanup messing with the epair interface) or if it would mask a lower level problem. Like said above, when using #200 I haven't seen it yet, but my test cases weren't particularly complex.

Happy to implement it myself, but would like to have a discussion first.

Describe potential alternatives or workaround you've considered (if any)
Use a handcrafted tinirc and override pot.cmd?!

[grembo]

Looking into the start/stop logic, I understand why this happened in the past (before #200). I'll open a new task, but to leave this open for discussion for the time being.

[pizzamig]

Hi, thanks for reporting this corner case.
In general, I agree that it makes sense to wait that epair interface are available, before to configure them :)

Is the delay in the creation of epair (AKA ifconfig epair create is returning when the interfaces are not yet created) or when the jail start (AKA epairNb shows up in jail context a bit late, hence tinirc fails to configure it)?

[pizzamig]

The FreeBSD rc system has a netwait script to wait for network interface to appear.
We can replicate this logic into tinirc (and probably refactor it's generation, that is currently a bit duplicated)

[grembo]

What I perceived might have been caused by #202, but for robustness, implementing this would still make sense (and is also relatively easy using netwait). I think it should include some sort of timeout.

[pizzamig]

it could be that epair_cleanup deleted the epair couple just created by the new start.
however, a double check, with 1 second delay should be enough (basically 1 second timeout)