Lennart Poettering
fb687fe62a
Previously, if the Synchronize() varlink call is issued we'd wait for journald to become idle before returning success. That is problematic however: on a busy system journald might never become idle. Hence, let's beef up the logic to ensure that we do not wait longer than necessary: i.e. we make sure we process any data enqueued before the sync request was submitted, but not more. Implementing this isn't trivial unfortunately. To deal with this reasonably, we need to determine somehow for incoming log messages whether they are from before or after the point in time where the sync requested was received. For AF_UNIX/SOCK_DGRAM we can use SO_TIMESTAMP to directly compare timestamps of incoming messages with the timestamp of the sync request (unfortunately only CLOCK_REALTIME). For AF_UNIX/SOCK_STREAM we can call SIOCINQ at the moment we initiate the sync, and then continue processing incoming traffic, counting down the bytes until the SIOCINQ returned bytes have been processed. All further data must have been enqueued later hence. With those two mechanisms in place we can relatively reliably synchronize the journal. This also adds a boolean argument "offline" to the Synchronize() call, which controls whether to offline the journal after processing the pending messages. it defaults to true, for compat with the status quo ante. But for most cases the offlining is probably not necessary, and is cheaper to do without, hence allow not to do it.
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