Panic in span End() when created with NewSpan

[delthas]

When creating a span using the now deprecated NewSpan function, calling End() on it panics. Panic trace:

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x9ebf4e]

goroutine 39 [running]:
testing.tRunner.func1(0xc42017e3c0)
	/usr/local/go/src/testing/testing.go:742 +0x454
panic(0xfd56e0, 0x164dfc0)
	/usr/local/go/src/runtime/panic.go:505 +0x24a
go.opencensus.io/trace.(*Span).End.func1()
	[XXX]/go.opencensus.io/trace/trace.go:269 +0x3e
sync.(*Once).Do(0xc42051b208, 0xc4204b4c90)
	/usr/local/go/src/sync/once.go:44 +0xd2
go.opencensus.io/trace.(*Span).End(0xc42051b1d0)
	[XXX]/go/src/go.opencensus.io/trace/trace.go:268 +0xa0

The simplest way to fix this is probably to replace s.executionTracerSpanEnd() by if s.executionTracerSpanEnd != nil { s.executionTracerSpanEnd() }.

This is a serious bug caused by #715 .

[odeke-em]

/cc @rakyll @Ramonza

[odeke-em]

Thank you for reporting this @delthas and welcome to OpenCensus!

@rakyll just fixed this bug, please get the latest from master.

[delthas]

Thanks. I would have done the PR myself but signing the Contributor License Agreement is quite a hassle.

I'm using this library in production, is there a way I can freeze the version of this library to avoid my infrastructure going down because of panics introduced in the code?

[odeke-em]

@delthas not a problem, thank you for reporting it and for using OpenCensus!

Awesome, that's great to know. So in order to freeze a version, perhaps you can use a package management tool, as per https://github.com/golang/go/wiki/PackageManagementTools?

Also it would be great to keep in touch with companies/individuals using OpenCensus in production, if you are comfortable doing so, please send me an email at [email protected] telling me how you are using it and in the future, we'll compile a list and post it publicly on the wiki here perhaps or somewhere.

[odeke-em]

PS: for clarity, I asked for an email because I haven't yet discussed with the OpenCensus team on how to liase with companies using OpenCensus i.e. publicly via wiki? Specialized contact? I didn't want to make a wiki suggestion without first discussing with the team on liason policy
/cc @bogdandrutu

[rakyll]

@delthas, you should pin to a specific release version at https://github.com/census-instrumentation/opencensus-go/releases and have reproducible builds by using a dependency management tool such as dep (https://github.com/golang/dep).