cmd/vet: flag use of .Timeout() after Set*Deadline()

[networkimprov]

In #31449 it was decided to make deadline expiration return a newly-defined error, as checking net.OpError.Timeout() also detects keepalive failures. Keepalives were enabled by default for TCP clients in 1.12 and servers in 1.13, breaking some code setting deadlines longer than the keepalive period (which varies with platform).

This means that .Timeout() use after Set*Deadline() should be changed to look for the new error, since that is not a reliable way to check for deadline-expired.

I suggest go vet flag use of .Timeout() after Set*Deadline(). That will generate reports for existing code, but perhaps that's better than leaving code broken due to the keepalive problem.

Triage: milestone should be 1.15
@gopherbot add NeedsDecision

[ianlancetaylor]

In order to make a decision here we need to see examples of existing code that should change. We also need to understand whether this vet check will have any false positives, and what such false positives look like in existing code.

[jayconrod]

Moving to Backlog milestone. There are a lot of open 1.15 issues, and unless someone is committing to working on this, it doesn't seem likely to happen in that timeframe.