Opentracing support

vendor/github.com/opentracing/opentracing-go/gocontext.go

@@ -0,0 +1,57 @@
+package opentracing
+
+import "golang.org/x/net/context"
+
+type contextKey struct{}
+
+var activeSpanKey = contextKey{}
+
+// ContextWithSpan returns a new `context.Context` that holds a reference to
+// `span`'s SpanContext.
+func ContextWithSpan(ctx context.Context, span Span) context.Context {
+	return context.WithValue(ctx, activeSpanKey, span)
+}
+
+// SpanFromContext returns the `Span` previously associated with `ctx`, or
+// `nil` if no such `Span` could be found.
+//
+// NOTE: context.Context != SpanContext: the former is Go's intra-process
+// context propagation mechanism, and the latter houses OpenTracing's per-Span
+// identity and baggage information.
+func SpanFromContext(ctx context.Context) Span {
+	val := ctx.Value(activeSpanKey)
+	if sp, ok := val.(Span); ok {
+		return sp
+	}
+	return nil
+}
+
+// StartSpanFromContext starts and returns a Span with `operationName`, using
+// any Span found within `ctx` as a ChildOfRef. If no such parent could be
+// found, StartSpanFromContext creates a root (parentless) Span.
+//
+// The second return value is a context.Context object built around the
+// returned Span.
+//
+// Example usage:
+//
+//    SomeFunction(ctx context.Context, ...) {
+//        sp, ctx := opentracing.StartSpanFromContext(ctx, "SomeFunction")
+//        defer sp.Finish()
+//        ...
+//    }
+func StartSpanFromContext(ctx context.Context, operationName string, opts ...StartSpanOption) (Span, context.Context) {
+	return startSpanFromContextWithTracer(ctx, GlobalTracer(), operationName, opts...)
+}
+
+// startSpanFromContextWithTracer is factored out for testing purposes.
+func startSpanFromContextWithTracer(ctx context.Context, tracer Tracer, operationName string, opts ...StartSpanOption) (Span, context.Context) {
+	var span Span
+	if parentSpan := SpanFromContext(ctx); parentSpan != nil {
+		opts = append(opts, ChildOf(parentSpan.Context()))
+		span = tracer.StartSpan(operationName, opts...)
+	} else {
+		span = tracer.StartSpan(operationName, opts...)
+	}
+	return span, ContextWithSpan(ctx, span)
+}