Commit aba475fe1d accidentally fixed a race, which happens in
the following sequence of events:
1) storage service starts drain() via API for example
2) main's abort source is triggered, calling compaction_manager's do_stop()
via subscription.
2.1) do_stop() initiates the stop but doesn't wait for it.
2.2) compaction_manager's state is set to stopped, such that
compaction_manager::stop() called in defer_verbose_shutdown()
will wait for the stop and not start a new one.
3) drain() calls compaction_manager::drain() changing the state from
stopped to disabled.
4) main calls compaction_manager::stop() (as described in 2.2) and
incorrectly tries to stop the manager again, because the state was
changed in step 3.
aba475fe1d accidentally fixed this problem because drain() will no
longer take place if it detects the shutdown process was initiated
(it does so by ignoring drain request if abort source's subscription
was unlinked).
This shows us that looking at the state to determine if stop should be
performed is fragile, because once the state changes from A to B,
manager doesn't know the state was A. To make it robust, we can instead
check if the future that stores stop's promise is engaged, meaning that
the stop was already initiated and we don't have to start a new one.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#11711
14d6459efc