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This is meant to limit the amount of messages that event replay needs to examine. Previously, the query required a table scan; table scans on `message` can be quite large, and should really be avoided. The new schema allows replays to be carried out using an index scan.
The premise of this change is that, for each (channel, message), there is a range of event sequence numbers that the (channel, message) may appear in. We'll notate that as `[start, end]` in the general case, but they are:
* for active channels, `[ch.created_sequence, ch.created_sequence]`.
* for deleted channels, `[ch.created_sequence, ch_del.deleted_sequence]`.
* for active messages, `[mg.sent_sequence, mg.sent_sequence]`.
* for deleted messages, `[mg.sent_seqeunce, mg_del.deleted_sequence]`.
(The two "active" ranges may grow in future releases, to support things like channel renames and message editing. That won't change the logic, but those two things will need to update the new `last_sequence` field.)
There are two families of operations that need to retrieve based on these ranges:
* Boot creates a snapshot as of a specific `resume_at` sequence number, and thus must include any record whose `start` falls on or before `resume_at`. We can't exclude records whose `end` is also before it, as their terminal state may be one that is included in boot (eg. active channels).
* Event replay needs to include any events that fall after the same `resume_at`, and thus must include events from any record whose `end` falls strictly after `resume_at`. We can't exclude records whose `start` is also strictly after `resume_at`, as we'd omit them from replay, inappropriately, if we did.
This gives three interesting cases:
1. Record fully after `resume_at`:
event sequence --increasing-->
x-a … x … x+k …
resume_at start end
This record should be omitted by boot, but included for event replay.
2. Record fully before `resume_at`:
event sequence --increasing-->
x … x+k … x+a
start end resume_at
This record should be omitted for event replay, but included for boot.
3. Record overlapping `resume_at`:
event sequence --increasing-->
x … x+a … x+k
start resume_at end
This record needs to be included for both boot and event replay.
However, the bounds of that range were previously stored in two tables (`channel` and `channel_deleted`, or `message` and `message_deleted`, respectively), which sqlite (indeed most SQL implementations) cannot index. This forced a table scan, leading to the program considering every possible (channel, message) during event replay.
This commit adds a `last_sequence` field to channels and messages, which is set to the above values as channels and messages are operated on. This field is indexed, and queries can use it to rapidly identify relevant rows for event replay, cutting down the amount of reading needed to generate events on resume.
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Canonicalization does two things:
* It prevents duplicate names that differ only by case or only by normalization/encoding sequence; and
* It makes certain name-based comparisons "case-insensitive" (generalizing via Unicode's case-folding rules).
This change is complicated, as it means that every name now needs to be stored in two forms. Unfortunately, this is _very likely_ a breaking schema change. The migrations in this commit perform a best-effort attempt to canonicalize existing channel or login names, but it's likely any existing channels or logins with non-ASCII characters will not be canonicalize correctly. Since clients look at all channel names and all login names on boot, and since the code in this commit verifies canonicalization when reading from the database, this will effectively make the server un-usuable until any incorrectly-canonicalized values are either manually canonicalized, or removed
It might be possible to do better with [the `icu` sqlite3 extension][icu], but (a) I'm not convinced of that and (b) this commit is already huge; adding database extension support would make it far larger.
[icu]: https://sqlite.org/src/dir/ext/icu
For some references on why it's worth storing usernames this way, see <https://www.b-list.org/weblog/2018/nov/26/case/> and the refernced talk, as well as <https://www.b-list.org/weblog/2018/feb/11/usernames/>. Bennett's treatment of this issue is, to my eye, much more readable than the referenced Unicode technical reports, and I'm inclined to trust his opinion given that he maintains a widely-used, internet-facing user registration library for Django.
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