bug: Content-Length is required for media uploads [which is against the spec]

[ThaUnknown]

Description

Synapse REQUIRES the Content-Length to be set when uploading media, which is against the spec, which says that only Content-Type is required, and only on version 1.11 and under

Why this is actually an issue:

• Content-Length is a protected header in web browsers, which mean it cannot be configured
• Uploading streamed/generated content is quite important for lowering memory usage

The encrypted attachment spec uses AES-CTR, which [I assume] was picked because it can be multi-threaded, thus it can be streamed, this can let a user specify a file, and start uploading the file while streaming the encryption, without pre-computing and without needing to load the entire file into memory, however POST'ing streams in web doesn't set a Content-Length, and since it's a protected header it can't be set manually.

This is also a problem with streaming the uploads of generated content such as video or audio streams which you realistically don't want to retain in-memory, but upload them on the fly.

Steps to reproduce

• send an authorized POST to /_matrix/media/v3/upload without a Content-Length header
• see instant error

Homeserver

matrix.org

Synapse Version

{"server_version":"1.148.0rc1 (b=matrix-org-hotfixes-priv,047f4e920c)"}

Installation Method

I don't know

Database

matrix.org so unsure

Workers

I don't know

Platform

matrix.org so unsure

Configuration

No response

Relevant log output

-/-

Anything else that would be useful to know?

IMHO the best way to solve this is to make the header optional, rather than required