Before this series, Alternator's Query and Scan operations convert an
entire result page to JSON without yielding. For a page of maximum
size (1MB) and tiny rows, this can cause a significant stall - the
test included in this PR reported stalls of 14-26ms on my laptop.
The problem is the describe_items() function, which does this conversion
immediately, without yielding. This patch changes this function to
return a future, and use a new result_set::visit_gently() method
that does what visit() does, but with yields when needed.
This PR improves #17995, but does not completely fix is as the stalls in the
are not completely eliminated. But on my laptop it usually reduces the stalls
to around 5ms. It appears that the remaining stalls some from other places
not fixed in this PR, such as perhaps query_page::handle_result(), and will need
to be fixed by additional patches.
Closesscylladb/scylladb#18036
* github.com:scylladb/scylladb:
alternator: reduce stall for Query and Scan with large pages
result_set: introduce visit_gently()
alternator: coroutinize do_query() function
96a3544739