Recently, in merge commit 2718c90448,
we added the ability to cancel pending view-update requests when we detect
that the target node went down. This is important for view updates because
these have a very long timeout (5 minutes), and we wanted to make this
timeout even longer.
However, the implementation caused a race: Between *creating* the update's
request handler (create_write_response_handler()) and actually starting
the request with this handler (mutate_begin()), there is a preemption point
and we may end up deleting the request handler before starting the request.
So mutate_begin() must gracefully handle the case of a missing request
handler, and not crash with a segmentation fault as it did before this patch.
Eventually the lifetime management of request handlers could be refactored
to avoid this delicate fix (which requires more comments to explain than
code), or even better, it would be more correct to cancel individual writes
when a node goes down, not drop the entire handler (see issue #4523).
However, for now, let's not do such invasive changes and just fix bug that
we set out to fix.
Fixes#4386.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20190620123949.22123-1-nyh@scylladb.com>
(cherry picked from commit 6e87bca65d)
b6fa715f7b