feat: Add BeforeSendTransaction hook

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Features
+
+* Add `beforeSendTransaction` hook ([#517](https://github.com/getsentry/sentry-go/pull/517))
+
 ## 0.16.0
 
 The Sentry SDK team is happy to announce the immediate availability of Sentry Go SDK v0.16.0.
@@ -18,10 +24,10 @@ Due to ongoing work towards a stable API for `v1.0.0`, we sadly had to include *
       ```
 - Unify TracesSampler [#498](https://github.com/getsentry/sentry-go/pull/498)
     - `TracesSampler` was changed to a callback that must return a `float64` between `0.0` and `1.0`.
-       
+
        For example, you can apply a sample rate of `1.0` (100%) to all `/api` transactions, and a sample rate of `0.5` (50%) to all other transactions.
        You can read more about this in our [SDK docs](https://docs.sentry.io/platforms/go/configuration/filtering/#using-sampling-to-filter-transaction-events).
-       
+
        ```go
        sentry.Init(sentry.ClientOptions{
            TracesSampler: sentry.TracesSampler(func(ctx sentry.SamplingContext) float64 {

client.go

@@ -140,8 +140,10 @@ type ClientOptions struct {
 	SendDefaultPII bool
 	// BeforeSend is called before error events are sent to Sentry.
 	// Use it to mutate the event or return nil to discard the event.
-	// See EventProcessor if you need to mutate transactions.
 	BeforeSend func(event *Event, hint *EventHint) *Event
+	// BeforeSendTransaction is called before transaction events are sent to Sentry.
+	// Use it to mutate the transaction or return nil to discard the transaction.
+	BeforeSendTransaction func(event *Event, hint *EventHint) *Event
 	// Before breadcrumb add callback.
 	BeforeBreadcrumb func(breadcrumb *Breadcrumb, hint *BreadcrumbHint) *Breadcrumb
 	// Integrations to be installed on the current Client, receives default
@@ -570,11 +572,18 @@ func (client *Client) processEvent(event *Event, hint *EventHint, scope EventMod
 		return nil
 	}
 
-	// As per spec, transactions do not go through BeforeSend.
-	if event.Type != transactionType && options.BeforeSend != nil {
-		if hint == nil {
-			hint = &EventHint{}
+	// Apply beforeSend* processors
+	if hint == nil {
+		hint = &EventHint{}
+	}
+	if event.Type == transactionType && options.BeforeSendTransaction != nil {
+		// Transaction events
+		if event = options.BeforeSendTransaction(event, hint); event == nil {
+			Logger.Println("Transaction dropped due to BeforeSendTransaction callback.")
+			return nil
 		}
+	} else if event.Type != transactionType && options.BeforeSend != nil {
+		// All other events
 		if event = options.BeforeSend(event, hint); event == nil {
 			Logger.Println("Event dropped due to BeforeSend callback.")
 			return nil

client_test.go

@@ -383,6 +383,60 @@ func TestBeforeSendGetAccessToEventHint(t *testing.T) {
 	assertEqual(t, transport.lastEvent.Message, "customComplexError: Foo 42")
 }
 
+func TestBeforeSendTransactionCanDropTransaction(t *testing.T) {
+	transport := &TransportMock{}
+	ctx := NewTestContext(ClientOptions{
+		EnableTracing:    true,
+		TracesSampleRate: 1.0,
+		Transport:        transport,
+		BeforeSend: func(event *Event, hint *EventHint) *Event {
+			t.Error("beforeSend should not be called")
+			return event
+		},
+		BeforeSendTransaction: func(event *Event, hint *EventHint) *Event {
+			assertEqual(t, event.Transaction, "Foo")
+			return nil
+		},
+	})
+
+	transaction := StartTransaction(ctx,
+		"Foo",
+	)
+	transaction.Finish()
+
+	if transport.lastEvent != nil {
+		t.Error("expected event to be dropped")
+	}
+}
+
+func TestBeforeSendTransactionIsCalled(t *testing.T) {
+	transport := &TransportMock{}
+	ctx := NewTestContext(ClientOptions{
+		EnableTracing:    true,
+		TracesSampleRate: 1.0,
+		Transport:        transport,
+		BeforeSend: func(event *Event, hint *EventHint) *Event {
+			t.Error("beforeSend should not be called")
+			return event
+		},
+		BeforeSendTransaction: func(event *Event, hint *EventHint) *Event {
+			assertEqual(t, event.Transaction, "Foo")
+			event.Transaction = "Bar"
+			return event
+		},
+	})
+
+	transaction := StartTransaction(ctx,
+		"Foo",
+	)
+	transaction.Finish()
+
+	lastEvent := transport.lastEvent
+	assertEqual(t, lastEvent.Transaction, "Bar")
+	// Make sure it's the same span
+	assertEqual(t, lastEvent.Contexts["trace"]["span_id"], transaction.SpanID)
+}
+
 func TestSampleRate(t *testing.T) {
 	tests := []struct {
 		SampleRate float64