catch potential deadlock

[celrenheit]

catch potential deadlock when the context is done but waiting to send on a channel in the default case

[dgryski]

Do you have an example where this actually happened?

[celrenheit]

I came across twice on an existing code doing:

select {
case <-ctx.Done():
// ...
default:
  ch <- 1
}

instead of

select {
case <-ctx.Done():
// ...
case ch <- 1:
}

This caused a deadlock blocking the exit.

I wasn't sure to open a PR for this, after using it to look for other issues like that, because it might raise false positives.