Previous patch fixes the inaccuracy, but it's done using iterating over
directory on each request (to system.distribution_queue or to check
bytes_to_throw_insert), and like previous patch alredy stated, it may
have pretty huge overhead (especially when you have lots of distributed
files pending).
This patch remove that recalculation (but it will still be done, and
if there is different, there will be a log message), and replace it with
proper account at INSERT time (and after file has been sent, or marked
as broken).
So now system.distribution_queue will show accurate statistics, so tests
does not requires sleep anymore.
But note that with too much distributed pending this will iterate over
all directories.
- the sender will got ATTEMPT_TO_READ_AFTER_EOF (added in
946c275dfb) when the client just go
away, i.e. server had been restarted, and this is incorrect to mark the
file as broken in this case.
- since #18853 the file will be checked on the sender locally, and
in case the file was truncated CANNOT_READ_ALL_DATA will be thrown.
But before #18853 the sender will not receive
ATTEMPT_TO_READ_AFTER_EOF from the client in case of file was truncated
on the sender, since the client will just wait for more data, IOW just hang.
- and I don't see how ATTEMPT_TO_READ_AFTER_EOF can be received while
reading local file.
CurrentMetrics::Increment add amount for specified metric only for the
lifetime of the object, but this is not the intention, since
DistributedFilesToInsert is a gauge and after #10263 it can exit from
the callback (and enter again later, for example after SYSTEM STOP
DISTRIBUTED SEND it will always exit from it, until SYSTEM START
DISTRIBUTED SEND).
So make Increment member of a class (this will also fix possible issues
with substructing value on DROP TABLE).
After #8756 the problem with 1 thread for each (distributed table, disk)
for distributed sends became even worse (since there can be multiple
disks), so use predefined thread pool for this tasks, that can be
controlled with background_distributed_schedule_pool_size knob.
