Handle bad JSON data being returned from the federation API.

[clokep]

This is spamming the logs on matrix.org: https://sentry.matrix.org/sentry/synapse-matrixorg/issues/194939/

I think this works OK since the server on the other end accepts the transaction OK, but returns an empty body back. The report at #9065 says that remote construct servers are doing this. We could also consider an empty body to be an empty JSON object, but that doesn't seem correct. If you're returning a response of applicaton/json I'd expect the body to be valid JSON.

Fixes #9065

[richvdh]

should we not catch JSONDecodeError here?

[clokep]

Arg, I forget half the time there's a specific error for this. Thank you!

[clokep]

Ah, so we need to catch ValueError since we manually throw that when doing JSON validation:

synapse/synapse/util/__init__.py

Lines 30 to 32 in b6ca69e

	def _reject_invalid_json(val):
	"""Do not allow Infinity, -Infinity, or NaN values in JSON."""
	raise ValueError("Invalid JSON value: '%s'" % val)

I had looked at raising a JSONDecodeError there, but you don't really have the necessary information, since we don't have the position of what is being parsed.

A couple of options:

• Leave this as a ValueError
• Change the error raised by _reject_invalid_json to a custom exception InvalidJsonError (which inherits from ValueError) and catch both here.
• Raise JSONDecodeError in _reject_invalid_json by giving a dummy position.

What do you think?

[richvdh]

I think I don't feel very strongly. May as well leave it as-is.

[richvdh]

lgtm