Tracing: traces now correctly form a tree (#1170)

* First cut of trace

* Dial it back yo

* Defensive programming

debug/trace/memory/memory.go

@@ -49,27 +49,27 @@ func (t *Tracer) Start(ctx context.Context, name string) (context.Context, *trac
 
 	// return span if no context
 	if ctx == nil {
-		return context.Background(), span
+		return trace.ToContext(context.Background(), span.Trace, span.Id), span
 	}
-
-	s, ok := trace.FromContext(ctx)
+	traceID, parentSpanID, ok := trace.FromContext(ctx)
+	// If the trace can not be found in the header,
+	// that means this is where the trace is created.
 	if !ok {
-		return ctx, span
+		return trace.ToContext(ctx, span.Trace, span.Id), span
 	}
 
 	// set trace id
-	span.Trace = s.Trace
+	span.Trace = traceID
 	// set parent
-	span.Parent = s.Id
+	span.Parent = parentSpanID
 
-	// return the sapn
-	return ctx, span
+	// return the span
+	return trace.ToContext(ctx, span.Trace, span.Id), span
 }
 
 func (t *Tracer) Finish(s *trace.Span) error {
 	// set finished time
 	s.Duration = time.Since(s.Started)
-
 	// save the span
 	t.buffer.Put(s)
 
@@ -84,7 +84,7 @@ func NewTracer(opts ...trace.Option) trace.Tracer {
 
 	return &Tracer{
 		opts: options,
-		// the last 64 requests
-		buffer: ring.New(64),
+		// the last 256 requests
+		buffer: ring.New(256),
 	}
 }

debug/trace/trace.go

@@ -3,6 +3,7 @@ package trace
 
 import (
 	"context"
+	"github.com/micro/go-micro/v2/metadata"
 	"time"
 )
 
@@ -34,17 +35,32 @@ type Span struct {
 	Metadata map[string]string
 }
 
-type spanKey struct{}
+const (
+	traceIDKey = "Micro-Trace-Id"
+	spanIDKey  = "Micro-Span-Id"
+)
 
 // FromContext returns a span from context
-func FromContext(ctx context.Context) (*Span, bool) {
-	s, ok := ctx.Value(spanKey{}).(*Span)
-	return s, ok
+func FromContext(ctx context.Context) (traceID string, parentSpanID string, isFound bool) {
+	traceID, traceOk := metadata.Get(ctx, traceIDKey)
+	microID, microOk := metadata.Get(ctx, "Micro-Id")
+	if !traceOk && !microOk {
+		isFound = false
+		return
+	}
+	if !traceOk {
+		traceID = microID
+	}
+	parentSpanID, ok := metadata.Get(ctx, spanIDKey)
+	return traceID, parentSpanID, ok
 }
 
-// NewContext creates a new context with the span
-func NewContext(ctx context.Context, s *Span) context.Context {
-	return context.WithValue(ctx, spanKey{}, s)
+// ToContext saves the trace and span ids in the context
+func ToContext(ctx context.Context, traceID, parentSpanID string) context.Context {
+	return metadata.MergeContext(ctx, map[string]string{
+		traceIDKey: traceID,
+		spanIDKey:  parentSpanID,
+	}, true)
 }
 
 var (

metadata/metadata.go

@@ -63,6 +63,9 @@ func NewContext(ctx context.Context, md Metadata) context.Context {
 
 // MergeContext merges metadata to existing metadata, overwriting if specified
 func MergeContext(ctx context.Context, patchMd Metadata, overwrite bool) context.Context {
+	if ctx == nil {
+		ctx = context.Background()
+	}
 	md, _ := ctx.Value(metaKey{}).(Metadata)
 	cmd := make(Metadata)
 	for k, v := range md {