extension: BeforeMessageStored event to rewrite envelope (#417)

* extension: add InboundMessage type
* manager: fires BeforeMessageStored event
* manager: Reacts to BeforeMessageStored event response
* manager: Apply BeforeMessageStored response fields to message

Signed-off-by: James Hillyerd <james@hillyerd.com>

pkg/extension/broker.go

@@ -6,7 +6,7 @@ import (
 
 // EventBroker maintains a list of listeners interested in a specific type
 // of event.
-type EventBroker[E any, R comparable] struct {
+type EventBroker[E any, R interface{}] struct {
 	sync.RWMutex
 	listenerNames []string     // Ordered listener names.
 	listenerFuncs []func(E) *R // Ordered listener functions.

pkg/extension/event/events.go

@@ -11,6 +11,15 @@ type AddressParts struct {
 	Domain string
 }
 
+// InboundMessage contains the basic header and mailbox data for a message being received.
+type InboundMessage struct {
+	Mailboxes []string
+	From      mail.Address
+	To        []mail.Address
+	Subject   string
+	Size      int64
+}
+
 // MessageMetadata contains the basic header data for a message event.
 type MessageMetadata struct {
 	Mailbox string

pkg/extension/host.go

@@ -23,6 +23,7 @@ type Events struct {
 	AfterMessageDeleted AsyncEventBroker[event.MessageMetadata]
 	AfterMessageStored  AsyncEventBroker[event.MessageMetadata]
 	BeforeMailAccepted  EventBroker[event.AddressParts, bool]
+	BeforeMessageStored EventBroker[event.InboundMessage, event.InboundMessage]
 }
 
 // Void indicates the event emitter will ignore any value returned by listeners.