It doesn't do much more than act as a glorified persistent
cursor at the moment, but the plumbing will enable calling
out to eg: lua or an arbitrary subprocess to process batches
in the future.
This commit adjusts the logger:
* When a log segment is finised, the `w` permission bits are removed.
This is used to signal to the tailer that there will be no additional
data written to that file.
* On startup, kumod will remove the `w` bits from all files in the
configured log directories so that any that were left on by a prior
incarnation will be considered closed when the new generation is
started up.
* Add a `tailer` utility that will essentially `cat` all of the log
files in a given directory in the appropriate order, then go into
log tailing mode, where it will wait for new data to show up.
* When reading data, if it hits EOF, the tailer will wait around until
more data shows up, or until the `w` bit is removed from the file
that it is reading.
The idea is that, in a later commit, the tailer will be able to persist
a checkpoint file to record where it got to, and be able to dispatch
chunks of data to eg: a webhook push utility, advancing the checkpoint
only when the utility indicates success.
For now, running `cargo run -p tailer -- --tail /var/tmp/kumo-logs` is
a convenient way to tail the logs as they are written.
Wez Furlong