feat(util:context) add error logging to context

[btc]

@jbenet
@whyrusleeping

This commit...

is a second attempt at: #68

partially addresses: #66

is the result of discussion at: https://gist.github.com/perfmode/f2951c1ed3a02c484d0b

[btc]

I held off on adding...

func Errors(ctx context.Context) chan<- error {
    v := ctx.Value(ipfsContextErrorLog)
    ch, ok := v.(chan<- error)
    if !ok {
        return nil
    }
    return ch
}

One concern with this is that it leaks the implementation of the error logging. If the parent context was created using context.Background(), then callers may push errors to a nil channel.

@jbenet

[jbenet]

@perfmode yep, good point. i guess this is why WithCancel only gives you the cancel func once. LGTM

[btc]

@whyrusleeping ?

[whyrusleeping]

Other than an impressively long test name, LGTM

[jbenet]

wooo. unlike this: