This is based on https://github.com/element-hq/synapse/pull/18416, which
got reverted (#19614) due to it incorrectly rejecting to-device messages
to users with many devices (and thus breaking message sending).
Fix https://github.com/element-hq/synapse/issues/17035
A to-device message content looks like:
```jsonc
{
"@user:domain": {"device1": {...}, "device2": {...}},
...
}
```
The previous PR would split up into multiple EDUs, each with a subset of
the users. However, if one user's entry was too large it would not
further split it up and then error out.
The main change in this PR is to allow splitting up a single user into
multiple EDUs.
Other changes:
1. Rename to `SOFT_MAX_EDU_SIZE` to indicate that we sometimes send EDUs
with larger size than that, and its more a target than a hard limit.
2. Check early if any to-device message (to a specific device) is too
large to send, even if we're not going to send it over federation. This
ensures that we catch issues where clients try to send too large
to-device.
This still means that if a client send a large individual to-device
message it will fail, but I don't believe we ever send such large
to-device messages (normally they're in the range of a few KB).
---
I ended up changing the implementation a bunch to make it easy to reuse
the code to split up dictionaries. Instead of repeatedly splitting up
the EDU until each bit fits into the size, we instead record the size of
each entry in the dict and instead split up based on cumulative size.
This means we call `encode_canonical_json` on each entry rather than
once on the entire struct, but its not significantly slower to do so.
--
cc @MatMaul @MadLittleMods
---------
Co-authored-by: Mathieu Velten <matmaul@gmail.com>
Co-authored-by: mcalinghee <mcalinghee.dev@gmail.com>
Co-authored-by: Eric Eastwood <madlittlemods@gmail.com>
Reverts element-hq/synapse#18416
Unfortunately, this causes failures on `/sendToDevice` endpoint in
normal circumstances. If a single user has, say, a hundred devices then
we easily go over the limit. This blocks message sending entirely in
encrypted rooms.
cc @MadLittleMods @MatMaul
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
If ther are more than 100 to-device messages pending for a device
`/sync` will only return the first 100, however the next batch token was
incorrectly calculated and so all other pending messages would be
dropped.
This is due to `txn.rowcount` only returning the number of rows that
*changed*, rather than the number *selected* in SQLite.
