Calle Wilund 43f51e9639 commitlog: Ensure we don't run continuation (task switch) with queues modified ...

Fixes #9955

In #9348 we handled the problem of failing to delete segment files on disk, and
the need to recompute disk footprint to keep data flow consistent across intermittent
failures. However, because _reserve_segments and _recycled_segments are queues, we
have to empty them to inspect the contents. One would think it is ok for these
queues to be empty for a while, whilst we do some recaclulating, including
disk listing -> continuation switching. But then one (i.e. I) misses the fact
that these queues use the pop_eventually mechanism, which does _not_ handle
a scenario where we push something into an empty queue, thus triggering the
future that resumes a waiting task, but then pop the element immediately, before
the waiting task is run. In fact, _iff_ one does this, not only will things break,
they will in fact start creating undefined behaviour, because the underlying
std::queue<T, circular_buffer> will _not_ do any bounds checks on the pop/push
operations -> we will pop an empty queue, immediately making it non-empty, but
using undefined memory (with luck null/zeroes).

Strictly speakging, seastar::queue::pop_eventually should be fixed to handle
the scenario, but nontheless we can fix the usage here as well, by simply copy
objects and do the calculation "in background" while we potentially start
popping queue again.

Closes #9966

2022-01-26 13:51:01 +02:00

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