add TraceID(ctx context.Context) (#41)

README.md

@@ -41,6 +41,14 @@ passing `context.Background()`. We must also be sure to end this span, which wil
 span, ctx, err := tracer.CreateLocalSpan(context.Background())
 ```
 
+## Get Global TraceID
+
+Get the `TraceID` of the `activeSpan` in the `Context`.
+
+```go
+go2sky.TraceID(ctx)
+```
+
 ## Create a sub span
 
 A sub span created as the children of root span links to its parent with `Context`.

segment.go

@@ -18,6 +18,8 @@
 package go2sky
 
 import (
+	"fmt"
+	"strings"
 	"sync/atomic"
 
 	"github.com/SkyAPM/go2sky/internal/idgen"
@@ -54,6 +56,14 @@ type SegmentContext struct {
 	spanIDGenerator *int32
 }
 
+func (ctx SegmentContext) GetReadableGlobalTraceID() string {
+	ii := make([]string, len(ctx.TraceID))
+	for i, v := range ctx.TraceID {
+		ii[i] = fmt.Sprint(v)
+	}
+	return strings.Join(ii, ".")
+}
+
 // ReportedSpan is accessed by Reporter to load reported data
 type ReportedSpan interface {
 	Context() *SegmentContext

trace.go

@@ -30,7 +30,11 @@ import (
 	"github.com/SkyAPM/go2sky/propagation"
 )
 
-const errParameter = tool.Error("parameter are nil")
+const (
+	errParameter = tool.Error("parameter are nil")
+	EmptyTraceID = "N/A"
+	NoopTraceID  = "[Ignored Trace]"
+)
 
 // Tracer is go2sky tracer implementation.
 type Tracer struct {
@@ -235,3 +239,15 @@ type Reporter interface {
 	Send(spans []ReportedSpan)
 	Close()
 }
+
+func TraceID(ctx context.Context) string {
+	activeSpan := ctx.Value(ctxKeyInstance)
+	if activeSpan == nil {
+		return EmptyTraceID
+	}
+	span, ok := activeSpan.(segmentSpan)
+	if ok {
+		return span.context().GetReadableGlobalTraceID()
+	}
+	return NoopTraceID
+}

trace_test.go

@@ -107,6 +107,43 @@ func verifySpans(t *testing.T, span ReportedSpan, subSpan ReportedSpan) {
 	}
 }
 
+func TestTrace_TraceID(t *testing.T) {
+	// activeSpan == nil
+	traceID := TraceID(context.Background())
+	verifyTraceID(t, EmptyTraceID, traceID)
+
+	// activeSpan == NoopSpan
+	reporter := &mockRegisterReporter{
+		success: false,
+	}
+	tracer, _ := NewTracer("service", WithReporter(reporter))
+	_, ctx, err := tracer.CreateLocalSpan(context.Background())
+	if err != nil {
+		t.Error(err)
+	}
+	traceID = TraceID(ctx)
+	verifyTraceID(t, NoopTraceID, traceID)
+
+	// activeSpan == segmentSpan
+	reporter = &mockRegisterReporter{
+		success: true,
+	}
+	tracer, _ = NewTracer("service", WithReporter(reporter))
+	tracer.WaitUntilRegister()
+	span, ctx, err := tracer.CreateLocalSpan(context.Background())
+	if err != nil {
+		t.Error(err)
+	}
+	traceID = TraceID(ctx)
+	verifyTraceID(t, span.(segmentSpan).context().GetReadableGlobalTraceID(), traceID)
+}
+
+func verifyTraceID(t *testing.T, expectTraceID string, actualTraceID string) {
+	if expectTraceID != actualTraceID {
+		t.Errorf("expectTraceID: %v, actualTraceID: %v", expectTraceID, actualTraceID)
+	}
+}
+
 type mockRegisterReporter struct {
 	success bool
 	wg      sync.WaitGroup