x/playground: output caching stops working after some time passes

[dmitshur]

It looks like #31889 has regressed. The same snippet can be used to observe it, re-running this program produces successful but different output each time:

https://play.golang.org/p/979rn6Bd7QN

I've noticed this when sharing a snippet (/cc @julieqiu) where a timeout due to a large dependency would happen occasionally. Usually, with output caching, getting it to succeed once makes it possible to share the snippet without risk of timeout because it no longer needs to execute again, but that wasn't the case.

/cc @toothrot @cagedmantis @andybons

[toothrot]

This happens when our memcached instance is unhealthy. I've restarted it and it seems fine now. We should keep this issue open to improve this problem though.

[dmitshur]

It seems the memcached instance (memcached-play-golang per app.yaml) has become unhealthy again. I'll look into restarting it for now.

Edit: Restarting it fixed it.

[dmitshur]

As another data point, this has regressed and required another restart (22 days since last time).