### Background
When SSRF detection fails, the exception bubbles all the way up, causing a log alert. This isn't actionable, and should instead be ignored. The existing `rescue` does already ignore network errors, but fails to account for SSRF exceptions coming from `FinalDestination`.
### What is this change?
This PR does two things.
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Firstly, it introduces a common root exception class, `FinalDestination::SSRFError` for SSRF errors. This serves two functions: 1) it makes it easier to rescue both errors at once, which is generally what one wants to do and 2) prevents having to dig deep into the class hierarchy for the constant.
This change is fully backwards compatible thanks to how inheritance and exception handling works.
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Secondly, it rescues this new exception in `UserAvatar.import_url_for_user`, which is causing sporadic errors to be logged in production. After this SSRF errors are handled the same as network errors.
The current default timeout is hardcoded to 2 seconds which is proving
too low for certain cases, and resulting in sporadic timeouts due to slow DNS queries.
There is an issue where chat message processing breaks due to
unhandles `SocketError` exceptions originating in the SSRF check,
specifically in `FinalDestination::Resolver`.
This change gives `FinalDestination::SSRFDetector` a new error class
to wrap the `SocketError` in, and haves the `RetrieveTitle` class
handle that error gracefully.
We were adding to the resolver's work queue before setting up the `@lookup` and `@parent` information. That could lead to the lookup being performed on the wrong (or `nil`) hostname. This also lead to some flakiness in specs.
Ted Johansson
Alan Guo Xiang Tan
Sam
Leonardo Mosquera
David Taylor