Add a maximum size for well-known lookups.

[clokep]

I noticed we already had a way to enforce a maximum size when reading a response from Twisted. There's no reason for a .well-known lookup to be large so we should bail after a short amount of data. I chose 50kb in here, but that's somewhat arbitrary, the maximum reasonable size for a well-known response is:

len("""{
    "m.server": "<maximum domain length>:65535"
}
""")

Note that the maximum domain length is 253 characters. So this leads to roughly 28 + 253 < 300 bytes. We can bikeshed the maximum size if you'd like.

Note that I don't think there's anything else useful to put in there.

[tulir]

Note that I don't think there's anything else useful to put in there.

Some people might handle all /.well-known/matrix/* paths as one and return the same JSON object regardless of the file name. 50kb limit should still be fine though

[clokep]

I wanted to make this async and handle the Deferred logic internally, but the call from the MatrixFederationHttpClient adds a timeout, which involves having both a timeout and a reactor. The timeout could be made optional easily, but needed both or neither is kind of meh.

[richvdh]

lgtm

[ghost]

Reviewed changes