tracer: close channels on Stop()

[alban]

PerfMap's PollStop() documentation says the channel should be closed
after calling PollStop():
https://github.com/iovisor/gobpf/blob/d0a3e1b/elf/perf.go#L241-L242

This is solving a leaking go routine issue in Scope:
weaveworks/scope#2735 (comment)

---

/cc @iaguis @2opremio @schu

[rade]

btw PollStop is rather non-idiomatic - the typical way to do this kind of signalling is to close() a channel of type struct{}.

[alban]

@rade ok, I filed iovisor/gobpf#69 for this.

[2opremio]

I agree with @rade . I think it's worth fixing it now, but I am fine with fixing it later if you prefer.

[alban]

idiomatic close in gobpf PR iovisor/gobpf#70

I'll update this PR shortly.

[alban]

I updated the gobpf vendoring. I needed to change the select{} to avoid a panic ("panic: runtime error: index out of range"), see comment in the code.

It was working fine for a while but now I got another panic:

panic: send on closed channel

goroutine 75 [running]:
github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/vendor/github.com/iovisor/gobpf/elf.(*PerfMap).PollStart.func1(0xc420270370, 0xc422eff000, 0xc422eedfe0)
	/go/src/github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/vendor/github.com/iovisor/gobpf/elf/perf.go:229 +0x4f7
created by github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/vendor/github.com/iovisor/gobpf/elf.(*PerfMap).PollStart
	/go/src/github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/vendor/github.com/iovisor/gobpf/elf/perf.go:238 +0xf1

I am checking this...

[alban]

channel closing principle:

One general principle of using Go channels is don't close a channel from the receiver side

This PR should be rethought..

[rade]

@alban you beat me to quoting that :)

[alban]

Waiting for iovisor/gobpf#71

[alban]

Closing in favor of #50