context.go

package stats

import (
	"context"
	"sync"
)

// ContextWithTags returns a new child context with the given tags.  If the
// parent context already has tags set on it, they are _not_ propegated into
// the context children.
func ContextWithTags(ctx context.Context, tags ...Tag) context.Context {
	// initialize the context reference and return a new context
	return context.WithValue(ctx, contextKeyReqTags, &tagSlice{
		tags: tags,
	})
}

// ContextAddTags adds the given tags to the given context, if the tags have
// been set on any of the ancestor contexts.  ContextAddTags returns true
// if tags were successfully appended to the context, and false otherwise.
//
// The proper way to set tags on a context if you don't know whether or not
// tags already exist on the context is to first call ContextAddTags, and if
// that returns false, then call ContextWithTags instead.
func ContextAddTags(ctx context.Context, tags ...Tag) bool {
	if x := getTagSlice(ctx); x != nil {
		x.lock.Lock()
		x.tags = append(x.tags, tags...)
		x.lock.Unlock()
		return true
	}
	return false
}

// ContextTags returns a copy of the tags on the context if they exist and nil
// if they don't exist.
func ContextTags(ctx context.Context) []Tag {
	if x := getTagSlice(ctx); x != nil {
		x.lock.Lock()
		ret := make([]Tag, len(x.tags))
		copy(ret, x.tags)
		x.lock.Unlock()
		return ret
	}
	return nil
}

func getTagSlice(ctx context.Context) *tagSlice {
	if tags, ok := ctx.Value(contextKeyReqTags).(*tagSlice); ok {
		return tags
	}
	return nil
}

type tagSlice struct {
	tags []Tag
	lock sync.Mutex
}

// tagsKey is a value for use with context.WithValue. It's used as
// a pointer so it fits in an interface{} without allocation. This technique
// for defining context keys was copied from Go 1.7's new use of context in net/http.
type tagsKey struct{}

// String implements the fmt.Stringer interface.
func (k tagsKey) String() string {
	return "stats_tags_context_key"
}

// contextKeyReqTags is contextKey for tags.
var (
	contextKeyReqTags = tagsKey{}
)