nntp: prevent event_read from firing twice in a row
When a client starts pipelining requests to us which trigger
long responses, we need to keep socket readiness checks disabled
and only enable them when our socket rbuf is drained.
Failure to do this caused aborted clients with
"BUG: nested long response" when Danga::Socket calls event_read
for read-readiness after our "next_tick" sub fires in the
same event loop iteration.
Reported-by: Jonathan Corbet <corbet@lwn.net>
cf. https://public-inbox.org/meta/
20181013124658.
23b9f9d2@lwn.net/