Indicate what endpoint came back with a JSON response we were unable to parse

[MadLittleMods]

Indicate what endpoint came back with a JSON response we were unable to parse.

Before:

WARNING - POST-11 - Unable to parse JSON: Expecting value: line 1 column 1 (char 0) (b'')

After:

WARNING - POST-11 - Unable to parse JSON from POST /_matrix/client/v3/join/%21ZlmJtelqFroDRJYZaq:hs1?server_name=hs1 response: Expecting value: line 1 column 1 (char 0) (b'')

---

It's possible to figure out which endpoint these warnings were coming from before but you had to follow the request ID POST-11 to the log line that says Completed request [...]. Including this key information next to the JSON parsing error makes it much easier to reason whether it matters or not.

2022-09-29T08:23:25.7875506Z synapse_main | 2022-09-29 08:21:10,336 - synapse.http.matrixfederationclient - 299 - INFO - POST-11 - {GET-O-13} [hs1] Completed request: 200 OK in 0.53 secs, got 450 bytes - GET matrix://hs1/_matrix/federation/v1/make_join/%21ohtKoQiXlPePSycXwp%3Ahs1/%40charlie%3Ahs2?ver=1&ver=2&ver=3&ver=4&ver=5&ver=6&ver=org.matrix.msc2176&ver=7&ver=8&ver=9&ver=org.matrix.msc3787&ver=10&ver=org.matrix.msc2716v4

---

As a note, having no body is normal for the /join endpoint and it can handle it (this was followed up in #14600)

synapse/synapse/rest/client/room.py

Lines 398 to 403 in 0c853e0

	try:
	content = parse_json_object_from_request(request)
	except Exception:
	# Turns out we used to ignore the body entirely, and some clients
	# cheekily send invalid bodies.
	content = {}

Alternatively we could remove these extra logs but they are probably more usually helpful to figure out what went wrong.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[DMRobertson]

Yes please!

[MadLittleMods]

Thanks for the review @DMRobertson 🐕‍🦺