This adds support for the WIP (at the time of this commit)
draft/chathistory extension, based on the draft at [1] and the
additional comments at [2].
This gets the history by parsing the chat logs, and is therefore only
enabled when the logs are enabled and the log path is configured.
Getting the history only from the logs adds some restrictions:
- we cannot get history by msgid (those are not logged)
- we cannot get the users masks (maybe they could be inferred from the
JOIN etc, but it is not worth the effort and would not work every
time)
The regular soju network history is not sent to clients that support
draft/chathistory, so that they can fetch what they need by manually
calling CHATHISTORY.
The only supported command is BEFORE for now, because that is the only
required command for an app that offers an "infinite history scrollback"
feature.
Regarding implementation, rather than reading the file from the end in
reverse, we simply start from the beginning of each log file, store each
PRIVMSG into a ring, then add the last lines of that ring into the
history we'll return later. The message parsing implementation must be
kept somewhat fast because an app could potentially request thousands of
messages in several files. Here we are using simple sscanf and indexOf
rather than regexps.
In case some log files do not contain any message (for example because
the user had not joined a channel at that time), we try up to a 100 days
of empty log files before giving up.
[1]: https://github.com/prawnsalad/ircv3-specifications/pull/3/files
[2]: https://github.com/ircv3/ircv3-specifications/pull/393/files#r350210018
