Passing the read monitor down to the sstable readers is tricky. The
point of interest - like compaction - are usually very far from the
interfaces that register the monitor, like read_rows. Between the two,
there is usually a mutation_reader, which is and ought to be totally
unaware of the read monitor: technically, a mutation_reader may not even
know it is backed by sstables.
The solution is to create a read_monitor_generator, that can be passed
from the upper layers, like compaction, to the layers that are actually
making the decision of which sstables to create readers for.
Note that we don't need an equivalent piece of infrastructure for
writes, because writes don't happen through hidden layers and have all
the information they need to initialize their monitors.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
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