"
While periodic mode is a all-bets-off crap-shoot as far as knowing if
data actually reached disk or not, batch mode is supposed to be
somewhat more reliable/deterministic.
Thus, if we get an exception writing/flushing the current buffer,
we should propagate exceptions to all execution paths involved
in this buffer.
Flush queue can now (optionally) propagate exceptions to all clients, and
commit log uses this to ensure that commit log writers in batch mode
all generate exceptions on disk errors.
Also includes some rudimentary tests for flush queue mechanisms.
Note: other main user, sstable flushing, is not affected, as default
mode is still to keep exceptions to individual worker continuations,
not waiters."
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