Fix #668: add documentation and enable automatically the feature with…

… more than 2 channels

docs/traits.adoc

@@ -157,6 +157,9 @@ More information can be found in the official Kubernetes documentation about htt
 ! knative.endpoint-sinks
 ! Configures a (comma-separated) list of endpoints the Knative consumes.
 
+! knative.filter-source-channels
+! Force the knative endpoint to filter messages based on the `ce-knativehistory` header (Knative experimental feature). It's enabled automatically when there are more than 2 source channels. It's optional (default to false) when there's a single source channel.
+
 !===
 
 | istio

pkg/trait/knative.go

@@ -106,8 +106,10 @@ func (t *knativeTrait) Configure(e *Environment) (bool, error) {
 
 			t.EndpointSinks = strings.Join(items, ",")
 		}
-		if t.FilterSourceChannels == nil && len(strings.Split(t.ChannelSources, ",")) > 1 {
-			// Filter channels when the integration subscribes to more than one
+		if len(strings.Split(t.ChannelSources, ",")) > 1 {
+			// Always filter channels when the integration subscribes to more than one
+			// Using Knative experimental header: https://github.com/knative/eventing/blob/master/pkg/provisioners/message.go#L28
+			// TODO: filter automatically all source channels when the feature becomes stable
 			filter := true
 			t.FilterSourceChannels = &filter
 		}