Merge pull request #1 from Vinelab/graceful_inject

Define errors out of existence

drivers/noop/tracer.go

@@ -66,11 +66,6 @@ func (tracer *Tracer) Extract(carrier interface{}, format string) (tracing.SpanC
 // Inject implicitly serializes current span context using the format descriptor that
 // tells how to encode trace info in the carrier parameters
 func (tracer *Tracer) Inject(carrier interface{}, format string) error {
-	span := tracer.currentSpan
-	if span == nil {
-		return tracing.ErrMissingTraceContext
-	}
-
 	return nil
 }
 

drivers/zipkin/tracer.go

@@ -172,7 +172,7 @@ func (tracer *Tracer) Extract(carrier interface{}, format string) (tracing.SpanC
 func (tracer *Tracer) Inject(carrier interface{}, format string) error {
 	span := tracer.currentSpan
 	if span == nil {
-		return tracing.ErrMissingTraceContext
+		return nil
 	}
 
 	injector, ok := tracer.injectionFormats[format]

errors.go

@@ -1,16 +1,9 @@
 package tracing
 
 import (
-	"errors"
 	"fmt"
 )
 
-var (
-	// ErrMissingTraceContext is returned when trying to implicitly inject SpanContext
-	// and there is no active span
-	ErrMissingTraceContext = errors.New("cannot obtain context because there is no active span")
-)
-
 // UnregisteredFormatError is returned when you tried to inject/extract trace context
 // using unregistered format or when there is a mismatch between format and extractor
 type UnregisteredFormatError struct {