audit_syslog_storage_helper::syslog_send_helper uses Seastar's
net::datagram_channel to write to syslog device (usually /dev/log).
However, datagram_channel.send() is not fiber-safe (ref seastar#2690),
so unserialized use of send() results in packets overwriting its state.
This, in turn, causes a corruption of audit logs, as well as assertion
failures.
To workaround the problem, a new semaphore is introduced in
audit_syslog_storage_helper. As storage_helper is a member of sharded
audit service, the semaphore allows for one datagram_channel.send() on
each shard. Each audit_syslog_storage_helper stores its own
datagram_channel, therefore concurrent sends to datagram_channel are
eliminated.
This change:
- Introduce semaphore with count=1 in audit_syslog_storage_helper.
- Added 1 hour timeout to the semaphore, so semaphore stalls are
failed just as all other syslog auditing failures.
Fixes: scylladb#22973
c12f976389