UID MOVE to remote server returns bad COPYUID response

[elliefm]

This came up on the info list: https://cyrus.topicbox.com/groups/info/T25c7531b76d23ba0-Mb4348afa7dbb30b2bc95f978/move-mails-between-folders-on-different-backend-servers

If a copy/move is local, then the COPYUID response is calculated locally, and everything's fine.

If the target of the copy/move is remote, then the messages are APPENDed to the target, and the APPENDUID from the remote server's response is copied into the COPYUID response to the client:

cyrus-imapd/imap/imapd.c

Lines 6698 to 6707 in 7f17801

	if (res == PROXY_OK) {
	if (myrights & ACL_READ) {
	appenduid = strchr(s->last_result.s, '[');
	/* skip over APPENDUID */
	if (appenduid) {
	appenduid += strlen("[appenduid ");
	b = strchr(appenduid, ']');
	if (b) *b = '\0';
	prot_printf(imapd_out, "%s OK [COPYUID %s] %s\r\n", tag,
	appenduid, error_message(IMAP_OK_COMPLETED));

A similar pattern occurs in imap_proxy.c:

cyrus-imapd/imap/imap_proxy.c

Lines 1170 to 1179 in 7f17801

	if (res == PROXY_OK) {
	if (myrights & ACL_READ) {
	appenduid = strchr(s->last_result.s, '[');
	/* skip over APPENDUID */
	if (appenduid) {
	appenduid += strlen("[appenduid ");
	b = strchr(appenduid, ']');
	if (b) *b = '\0';
	prot_printf(imapd_out, "%s OK [COPYUID %s] %s\r\n", tag,
	appenduid, error_message(IMAP_OK_COMPLETED));

But this behaviour makes no sense: the APPENDUID response is specified as having two fields, whereas the COPYUID response has three: https://datatracker.ietf.org/doc/html/rfc4315#section-3

[elliefm]

If I'm reading the spec right, I think [APPENDUID uidvalidity newuids] would correspond with [COPYUID uidvalidity olduids newuids].

That is to say, if we were to use the remote values from APPENDUID and copy in the locally-known one, the locally-known one goes in the middle. So we'd need to parse and reassemble the APPENDUID response properly, not just concatenate the locally-known value.