url preview fails on IP addresses on py3

[richvdh]

2018-11-20 11:58:46,190 - synapse.access.http.8085 - 233 - INFO - GET-10890 - 81.2.100.169 - 8085 - Received request: GET /_matrix/media/r0/preview_url?url=http://8.8.8.8
2018-11-20 11:58:46,269 - synapse.http.client - 96 - INFO - GET-10890 - Sending request GET http://8.8.8.8
2018-11-20 11:58:46,270 - synapse.http.client - 473 - INFO - GET-10890 - Getting endpoint for b'http://8.8.8.8'
2018-11-20 11:58:46,270 - synapse.http.client - 118 - INFO - GET-10890 - Error sending request to  GET http://8.8.8.8: TypeError a bytes-like object is required, not 'str'
2018-11-20 11:58:46,270 - synapse.rest.media.v1.preview_url_resource - 323 - WARNING - GET-10890 - Error downloading http://8.8.8.8: TypeError("a bytes-like object is required, not 'str'",)
2018-11-20 11:58:46,270 - synapse.http.server - 85 - INFO - GET-10890 - <XForwardedForRequest at 0x7ff7e8948a58 method='GET' uri='/_matrix/media/r0/preview_url?url=http://8.8.8.8' clientproto='HTTP/1.1' site=8085> SynapseError: 500 - Failed to download content: ["TypeError: a bytes-like object is required, not 'str'\n"]
2018-11-20 11:58:46,271 - synapse.access.http.8085 - 302 - INFO - GET-10890 - 81.2.100.169 - 8085 - {@richvdh:matrix.org} Processed request: 0.081sec/0.000sec (0.000sec, 0.000sec) (0.000sec/0.078sec/1) 138B 500 "GET /_matrix/media/r0/preview_url?url=http://8.8.8.8 HTTP/1.1" "curl/7.58.0" [0 dbevts]

[richvdh]

The problem here is:

• twisted's URI object is defined such that its host is a bytes
• twisted.web.client.Agent feeds the uri into the endpointForURI method of an EndpointFactory
• SpiderEndpointFactory feeds uri.host into SpiderEndpoint's constructor
• SpinderEndpoint.connect feeds it it into reactor.resolve, and thence to netaddr.ip.IPAddress.
• reactor.resolve returns its input if the input is an IP address
• netaddr.ip.IPAddress's constructor explodes horribly if given a bytes instead of a str.

It's a bit of a catalogue of fail, but the upshot seems to be that only previews of IP literal hostnames are affected, so this is probably not a hard blocker for 0.34.

[hawkowl]

Fixed by #4215