Code review comment for lp:~alan-griffiths/mir/fix-1526248

> I recall a long time ago that locking during exec() caused deadlocks there.
> How sure are you that's now fixed?
>
> - lk.unlock();
> exec(surface);

I've left the tests of this area looping over lunch and been running exploratory tests.

This isn't mathematical proof that there isn't a deadlock scenario, but deadlock is hypothetical and this fixes a demonstrable problem:

Unlocking is clearly wrong as it allows another thread to call erase() followed by deleting "surface" between unlock() and exec(). Holding a shared lock prevents that while still allowing non-mutating access.