reply: handle the Sender header

Sometimes emails will have a Sender: header, that is different from the
From: header. The only use of this in the wild I have seen to date is
when meeting invitation is forwarded by somebody. The From: header will
be the person organising the meeting, and the Sender: will be the person
forwarding. Naturally, when one replies (e.g. with on accept), it should
go to the meeting oragniser, but sometimes one would want to include the
Sender in such a reply.

When executing :reply determine the To: address in order of Reply-To:,
From:, Sender:. When executing :reply -a, include the Sender: in Cc:.

Implements: https://todo.sr.ht/~rjarry/aerc/259
Link: https://www.rfc-editor.org/rfc/rfc4021#section-2.1.3
Changelog-added: Replying to all will include the Sender in Cc.
Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Reviewed-by: Tim Culverhouse <tim@timculverhouse.com>
Reviewed-by: Tristan Partin <tristan@partin.io>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/msg/reply.go

@@ -86,11 +86,13 @@ func (r reply) Execute(args []string) error {
 	)
 
 	recSet := newAddrSet() // used for de-duping
-
-	if len(msg.Envelope.ReplyTo) != 0 {
+	switch {
+	case len(msg.Envelope.ReplyTo) != 0:
 		to = msg.Envelope.ReplyTo
-	} else {
+	case len(msg.Envelope.From) != 0:
 		to = msg.Envelope.From
+	default:
+		to = msg.Envelope.Sender
 	}
 
 	if !config.Compose.ReplyToSelf {
@@ -131,6 +133,13 @@ func (r reply) Execute(args []string) error {
 			}
 			cc = append(cc, addr)
 		}
+		for _, addr := range msg.Envelope.Sender {
+			// dedupe stuff from the to/from headers
+			if recSet.Contains(addr) {
+				continue
+			}
+			cc = append(cc, addr)
+		}
 		recSet.AddList(cc)
 	}
 

lib/rfc822/message.go

@@ -163,6 +163,7 @@ func parseEnvelope(h *mail.Header) *models.Envelope {
 		MessageId: msgID,
 		From:      parseAddressList(h, "from"),
 		ReplyTo:   parseAddressList(h, "reply-to"),
+		Sender:    parseAddressList(h, "sender"),
 		To:        parseAddressList(h, "to"),
 		Cc:        parseAddressList(h, "cc"),
 		Bcc:       parseAddressList(h, "bcc"),

models/models.go

@@ -229,6 +229,7 @@ type Envelope struct {
 	Subject   string
 	From      []*mail.Address
 	ReplyTo   []*mail.Address
+	Sender    []*mail.Address
 	To        []*mail.Address
 	Cc        []*mail.Address
 	Bcc       []*mail.Address