[Bugfix] apm.DefaultTracer misbehaves when transport configuration is…

… invalid (#1618)

* incase of invalid tracer config disabling it instead of returning discard transport

* The default value of 'active'  is true, setting it to 'false' to support the case where invalid tracer config

* added a test case to check if tracer is being set to inactive incase of invalid configuration

* removed unwanted variable and linter fix

* fix precheck

* ran gofmt and golangci-lint to format tracer.go and tracer_test.go

* ensure tracecontext example has a discard tracer

---------

Co-authored-by: dmathieu <damien.mathieu@elastic.co>

example_tracecontext_test.go

@@ -23,9 +23,17 @@ import (
 	"os"
 
 	"go.elastic.co/apm/v2"
+	"go.elastic.co/apm/v2/apmtest"
 )
 
 func ExampleTransaction_EnsureParent() {
+	// When environment variables aren't set, the default tracer is inactive.
+	// For the sake of this example, we want a discarded tracer instead, so we do
+	// have trace and span IDs generated.
+	//
+	// This setup is now something you should be using in your own code.
+	apm.SetDefaultTracer(apmtest.NewDiscardTracer())
+
 	tx := apm.DefaultTracer().StartTransactionOptions("name", "type", apm.TransactionOptions{
 		TraceContext: apm.TraceContext{
 			Trace: apm.TraceID{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15},

tracer.go

@@ -53,10 +53,8 @@ var (
 // DefaultTracer returns the default global Tracer, set the first time the
 // function is called, or after calling SetDefaultTracer(nil).
 //
-// The default tracer is configured via environment variables, and will always
-// be non-nil. If any of the environment variables are invalid, the
-// corresponding errors will be logged to stderr and the default values will be
-// used instead.
+// The default tracer is configured via environment variables, and if the those are invalid
+// the tracer will be disabled.
 func DefaultTracer() *Tracer {
 	tracerMu.RLock()
 	if defaultTracer != nil {
@@ -314,7 +312,7 @@ func (opts *TracerOptions) initDefaults(continueOnError bool) error {
 	if opts.Transport == nil {
 		initialTransport, err := initialTransport(opts.ServiceName, opts.ServiceVersion)
 		if failed(err) {
-			opts.Transport = transport.NewDiscardTransport(err)
+			active = false
 		} else {
 			opts.Transport = initialTransport
 		}

tracer_test.go

@@ -597,17 +597,10 @@ func TestTracerDefaultTransport(t *testing.T) {
 		require.Error(t, err)
 		assert.EqualError(t, err, "failed to parse ELASTIC_APM_SERVER_TIMEOUT: invalid duration never")
 
-		// Implicitly created Tracers will have a discard tracer.
+		// Implicitly created Tracers will have Inactive tracer in case of invalid configuration
 		apm.SetDefaultTracer(nil)
 		tracer = apm.DefaultTracer()
-
-		tracer.StartTransaction("name", "type").End()
-		tracer.Flush(nil)
-		assert.Equal(t, apm.TracerStats{
-			Errors: apm.TracerStatsErrors{
-				SendStream: 1,
-			},
-		}, tracer.Stats())
+		assert.Equal(t, false, tracer.Active())
 	})
 }