Erik Johnston 5bf825c016 Limit to-device EDU sizes (#19617)
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>
2026-06-02 15:57:55 +00:00
2026-05-27 12:27:43 +01:00
2026-05-29 10:57:33 +00:00
2026-05-27 12:27:43 +01:00
2026-06-02 15:57:55 +00:00
2025-07-08 11:57:23 +01:00
2026-05-27 14:27:37 +01:00
2024-11-20 15:01:56 +00:00
2023-12-13 16:15:22 +00:00
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   :height: 60px

**Element Synapse - Matrix homeserver implementation**

|support| |development| |documentation| |license| |pypi| |python|

Synapse is an open source `Matrix <https://matrix.org>`__ homeserver
implementation, written and maintained by `Element <https://element.io>`_.
`Matrix <https://github.com/matrix-org>`__ is the open standard for secure and
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==================

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It gets shipped as part of the **Element Server Suite (ESS)** which provides the
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ESS is a Matrix distribution from Element with focus on quality and ease of use.
It ships a full Matrix stack tailored to the respective use case.

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🛠️ Standalone installation and configuration
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- `Synapse configuration for federation <https://element-hq.github.io/synapse/latest/federate.html>`_
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🎯 Troubleshooting and support
==============================

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-----------------------

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--------------------

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Copyright and Licensing
=======================

  | Copyright 20142017 OpenMarket Ltd
  | Copyright 2017 Vector Creations Ltd
  | Copyright 20172025 New Vector Ltd
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