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vendor/github.com/jtolds/gls/README.md

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+gls
+===
+
+Goroutine local storage
+
+### IMPORTANT NOTE ###
+
+It is my duty to point you to https://blog.golang.org/context, which is how 
+Google solves all of the problems you'd perhaps consider using this package
+for at scale. 
+
+One downside to Google's approach is that *all* of your functions must have
+a new first argument, but after clearing that hurdle everything else is much
+better.
+
+If you aren't interested in this warning, read on.
+
+### Huhwaht? Why? ###
+
+Every so often, a thread shows up on the
+[golang-nuts](https://groups.google.com/d/forum/golang-nuts) asking for some
+form of goroutine-local-storage, or some kind of goroutine id, or some kind of
+context. There are a few valid use cases for goroutine-local-storage, one of
+the most prominent being log line context. One poster was interested in being
+able to log an HTTP request context id in every log line in the same goroutine
+as the incoming HTTP request, without having to change every library and
+function call he was interested in logging.
+
+This would be pretty useful. Provided that you could get some kind of
+goroutine-local-storage, you could call
+[log.SetOutput](http://golang.org/pkg/log/#SetOutput) with your own logging
+writer that checks goroutine-local-storage for some context information and
+adds that context to your log lines.
+
+But alas, Andrew Gerrand's typically diplomatic answer to the question of
+goroutine-local variables was:
+
+> We wouldn't even be having this discussion if thread local storage wasn't
+> useful. But every feature comes at a cost, and in my opinion the cost of
+> threadlocals far outweighs their benefits. They're just not a good fit for
+> Go.
+
+So, yeah, that makes sense. That's a pretty good reason for why the language
+won't support a specific and (relatively) unuseful feature that requires some
+runtime changes, just for the sake of a little bit of log improvement.
+
+But does Go require runtime changes?
+
+### How it works ###
+
+Go has pretty fantastic introspective and reflective features, but one thing Go
+doesn't give you is any kind of access to the stack pointer, or frame pointer,
+or goroutine id, or anything contextual about your current stack. It gives you
+access to your list of callers, but only along with program counters, which are
+fixed at compile time.
+
+But it does give you the stack.
+
+So, we define 16 special functions and embed base-16 tags into the stack using
+the call order of those 16 functions. Then, we can read our tags back out of
+the stack looking at the callers list.
+
+We then use these tags as an index into a traditional map for implementing
+this library.
+
+### What are people saying? ###
+
+"Wow, that's horrifying."
+
+"This is the most terrible thing I have seen in a very long time."
+
+"Where is it getting a context from? Is this serializing all the requests? 
+What the heck is the client being bound to? What are these tags? Why does he 
+need callers? Oh god no. No no no."
+
+### Docs ###
+
+Please see the docs at http://godoc.org/github.com/jtolds/gls
+
+### Related ###
+
+If you're okay relying on the string format of the current runtime stacktrace 
+including a unique goroutine id (not guaranteed by the spec or anything, but 
+very unlikely to change within a Go release), you might be able to squeeze 
+out a bit more performance by using this similar library, inspired by some 
+code Brad Fitzpatrick wrote for debugging his HTTP/2 library: 
+https://github.com/tylerb/gls (in contrast, jtolds/gls doesn't require 
+any knowledge of the string format of the runtime stacktrace, which 
+probably adds unnecessary overhead).