Function BuildReferencesChain

internal/outbound/reply.go:89–122

BuildReferencesChain returns the References chain to write on a reply, per RFC 5322 § 3.6.4: - If the parent has a References header, return: parent.References ++ [parentMsgID] - Else if the parent has an In-Reply-To header, return: parent.InReplyTo ++ [parentMsgID] - Else return: [parentMsgID] (th

(rawMessage []byte, parentMsgID string)

// has In-Reply-To pointing at a Message-ID that recipients outside the
// subset have never seen, and Gmail/other clients fork the thread.
func BuildReferencesChain(rawMessage []byte, parentMsgID string) []string {
	if parentMsgID == "" {
		return nil
	}

	var prior []string
	if len(rawMessage) > 0 {
		if msg, err := mail.ReadMessage(bytes.NewReader(rawMessage)); err == nil {
			if refs := strings.TrimSpace(msg.Header.Get("References")); refs != "" {
				prior = parseMessageIDList(refs)
			} else if irt := strings.TrimSpace(msg.Header.Get("In-Reply-To")); irt != "" {
				// In-Reply-To SHOULD contain a single id, but some clients
				// pack multiple. Treat it the same way as References as a
				// pragmatic recovery — better than dropping prior context.
				prior = parseMessageIDList(irt)
			}
		}
		// Parse failures fall through silently — better to send a reply
		// with a shorter chain than to fail the whole send. The reply
		// will still thread for participants who saw the parent.
	}

	chain := make([]string, 0, len(prior)+1)
	for _, id := range prior {
		// Drop the parent if it was already in the prior chain — append it
		// once at the end so the chain ends with the immediate parent
		// (which mirrors what In-Reply-To points at).
		if id != parentMsgID {
			chain = append(chain, id)
		}
	}
	chain = append(chain, parentMsgID)
	return chain
}

// parseMessageIDList splits a References / In-Reply-To header value into
// individual message-ids. The header format is one or more <local@domain>