Add select_ok function to detect closed channels during select

[kkroening]

In Go, it's possible to detect closed channels inside a select. It doesn't appear that goless has any way to do this out-of-the-box, since the select function doesn't return an ok indication.

Here's an example:

for {
    select {
    case x, ok := <-ch1:
        fmt.Println("ch1", x, ok)
        if !ok {
            ch1 = nil
        }
    case x, ok := <-ch2:
        fmt.Println("ch2", x, ok)
        if !ok {
            ch2 = nil
        }
    }

    if ch1 == nil && ch2 == nil {
        break
    }
}

I added a select_ok function that behaves the same as select but handles closed channels correctly and returns an ok indication:

while True:
    cases = [goless.rcase(in1), goless.rcase(in2)]
    chosen, value, ok = goless.select_ok(cases)
    if chosen.chan is in1:
        print 'ch1', value, ok
        if !ok:
            in1 = None
    else:
        print 'ch2', value, ok
        if !ok:
            in2 = None
    if in1 is None and in2 is None:
        break

Closed channels cases take precedence over the default case. And if a normal select is performed on a closed channel, a ClosedChannel exception is raised, similar to how Go complains in the same case.

The changes to check for closed channels are a bit hacky: in selecting.py, it directly accesses the channels _closed property. I'm not sure of a better way to do this. I tried changing recv_ready/send_ready to return True if the channel is closed (which would cause the select to choose the corresponding case), but that broke a bunch of stuff. Would love to hear if there's a better way to do this.

[kkroening]

Note: #49 fixes CI for this branch

[rgalanakis]

I need a bit more time to review this... will get back to you soon.

[coveralls]

Coverage decreased (-0.4%) to 93.333% when pulling 9e6ac3d on kkroening:select-ok into 286cd69 on rgalanakis:master.

[coveralls]

Coverage decreased (-0.4%) to 93.333% when pulling a3dfc5b on kkroening:select-ok into 286cd69 on rgalanakis:master.

[kkroening]

Cool, no worries.