[opentracing] add SpanContext implementation

opentracing/context.go

@@ -3,10 +3,36 @@ package opentracing
 // SpanContext represents Span state that must propagate to descendant Spans
 // and across process boundaries.
 type SpanContext struct {
+	traceID  uint64
+	spanID   uint64
+	parentID uint64
+	sampled  bool
+	baggage  map[string]string
 }
 
 // ForeachBaggageItem grants access to all baggage items stored in the
 // SpanContext
-func (n SpanContext) ForeachBaggageItem(handler func(k, v string) bool) {
-	// TODO: implementation required
+func (c SpanContext) ForeachBaggageItem(handler func(k, v string) bool) {
+	for k, v := range c.baggage {
+		if !handler(k, v) {
+			break
+		}
+	}
+}
+
+// WithBaggageItem returns an entirely new SpanContext with the
+// given key:value baggage pair set.
+func (c SpanContext) WithBaggageItem(key, val string) SpanContext {
+	var newBaggage map[string]string
+	if c.baggage == nil {
+		newBaggage = map[string]string{key: val}
+	} else {
+		newBaggage = make(map[string]string, len(c.baggage)+1)
+		for k, v := range c.baggage {
+			newBaggage[k] = v
+		}
+		newBaggage[key] = val
+	}
+	// Use positional parameters so the compiler will help catch new fields.
+	return SpanContext{c.traceID, c.spanID, c.parentID, c.sampled, newBaggage}
 }

opentracing/context_test.go

@@ -0,0 +1,29 @@
+package opentracing
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSpanContextBaggage(t *testing.T) {
+	assert := assert.New(t)
+
+	ctx := SpanContext{}
+	ctx = ctx.WithBaggageItem("key", "value")
+	assert.Equal("value", ctx.baggage["key"])
+}
+
+func TestSpanContextIterator(t *testing.T) {
+	assert := assert.New(t)
+
+	baggageIterator := make(map[string]string)
+	ctx := SpanContext{baggage: map[string]string{"key": "value"}}
+	ctx.ForeachBaggageItem(func(k, v string) bool {
+		baggageIterator[k] = v
+		return true
+	})
+
+	assert.Len(baggageIterator, 1)
+	assert.Equal("value", baggageIterator["key"])
+}