ContextForChannel uses a goroutine to transform a channel close to
a context cancel. However, this exposes a synchronization issue if
we want to unify the underlying implementation between contextless
and with context - a ConditionFunc that closes the channel today
expects the behavior that no subsequent conditions will be invoked
(we have a test in wait_test.go TestUntilReturnsImmediately that
verifies this expectation). We can't unify the implementation
without ensuring this property holds.
To do that this commit changes from the goroutine propagation to
implementing context.Context and using stopCh as the Done(). We
then implement Err() by returning context.Canceled and stub the
other methods. Since our context cannot be explicitly cancelled
by users, we cease to return the cancelFn and callers that need
that behavior must wrap the context as normal.
This should be invisible to clients - they would already observe
the same behavior from the context, and the existing error
behavior of Poll* is preserved (which ignores ctx.Err()).
As a side effect, one less goroutine is created making it more
efficient.
95051a63b3