Allow trace plugins to be registered from outside the package.

Fixes #248

go/trace/trace.go

@@ -3,7 +3,8 @@
 // license that can be found in the LICENSE file.
 
 // Package trace contains a helper interface that allows various tracing
-// tools to be plugged in to components using this interface.
+// tools to be plugged in to components using this interface. If no plugin is
+// registered, the default one makes all trace calls into no-ops.
 package trace
 
 import (
@@ -57,13 +58,23 @@ func NewSpanFromContext(ctx context.Context) Span {
 	return NewSpan(nil)
 }
 
-// spanFactory should be changed by a plugin during init() to a factory that
-// creates an actual Span implementation for that plugin's tracing framework.
-var spanFactory interface {
+// SpanFactory is an interface for creating spans or extracting them from Contexts.
+type SpanFactory interface {
 	New(parent Span) Span
 	FromContext(ctx context.Context) (Span, bool)
 	NewContext(parent context.Context, span Span) context.Context
-} = fakeSpanFactory{}
+}
+
+var spanFactory SpanFactory = fakeSpanFactory{}
+
+// RegisterSpanFactory should be called by a plugin during init() to install a
+// factory that creates Spans for that plugin's tracing framework. Each call to
+// RegisterSpanFactory will overwrite any previous setting. If no factory is
+// registered, the default fake factory will produce Spans whose methods are all
+// no-ops.
+func RegisterSpanFactory(sf SpanFactory) {
+	spanFactory = sf
+}
 
 type fakeSpanFactory struct{}
 

go/trace/trace_test.go

@@ -0,0 +1,25 @@
+// Copyright 2014, Google Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"testing"
+
+	"golang.org/x/net/context"
+)
+
+func TestFakeSpan(t *testing.T) {
+	ctx := context.Background()
+	RegisterSpanFactory(fakeSpanFactory{})
+
+	// It should be safe to call all the usual methods as if a plugin were installed.
+	span := NewSpanFromContext(ctx)
+	span.StartLocal("label")
+	span.StartClient("label")
+	span.StartServer("label")
+	span.Annotate("key", "value")
+	span.Finish()
+	NewContext(ctx, span)
+}