do not let both-sided uidvalidity change deter us

the algorithm is symmetrical, comparing the msgids that belong to the
paired uids. so it doesn't matter for recovering one side if the other
side's uidvalidity also changed. it does however impact our ability to
say on which side the change was genuine.

the pointless limitation was presumably a vestige from an earlier
iteration.

amends 77acc26 and 594e60b.

src/sync.c

@@ -511,7 +511,7 @@ box_opened2( sync_vars_t *svars, int t )
 	channel_conf_t *chan;
 	sync_rec_t *srec;
 	uint_array_alloc_t mexcs;
-	uint opts[2], fails, minwuid;
+	uint opts[2], minwuid;
 
 	svars->state[t] |= ST_SELECTED;
 	if (!(svars->state[t^1] & ST_SELECTED))
@@ -520,23 +520,13 @@ box_opened2( sync_vars_t *svars, int t )
 	ctx[1] = svars->ctx[1];
 	chan = svars->chan;
 
-	fails = 0;
-	for (t = 0; t < 2; t++)
-		if (svars->uidval[t] != UIDVAL_BAD && svars->uidval[t] != svars->newuidval[t])
-			fails++;
-	// If only one side changed UIDVALIDITY, we will try to re-approve it further down.
-	if (fails == 2) {
-		error( "Error: channel %s: UIDVALIDITY of both far side %s and near side %s changed.\n",
-		       svars->chan->name, svars->orig_name[F], svars->orig_name[N]);
+	if (!lock_state( svars )) {
 	  bail:
 		svars->ret = SYNC_FAIL;
 		sync_bail( svars );
 		return;
 	}
 
-	if (!lock_state( svars ))
-		goto bail;
-
 	int any_dummies[2] = { 0, 0 };
 	int any_purges[2] = { 0, 0 };
 	int any_upgrades[2] = { 0, 0 };
@@ -574,9 +564,11 @@ box_opened2( sync_vars_t *svars, int t )
 	}
 
 	opts[F] = opts[N] = 0;
-	if (fails)
-		opts[F] = opts[N] = OPEN_PAIRED | OPEN_PAIRED_IDS;
 	for (t = 0; t < 2; t++) {
+		if (svars->uidval[t] != UIDVAL_BAD && svars->uidval[t] != svars->newuidval[t]) {
+			opts[F] |= OPEN_PAIRED | OPEN_PAIRED_IDS;
+			opts[N] |= OPEN_PAIRED | OPEN_PAIRED_IDS;
+		}
 		if (any_purges[t]) {
 			debug( "resuming %d %s purge(s)\n", any_purges[t], str_fn[t] );
 			opts[t] |= OPEN_SETFLAGS;
@@ -859,8 +851,15 @@ box_loaded( int sts, message_t *msgs, int total_msgs, int recent_msgs, void *aux
 				if (!srec->msg[t^1])
 					continue;  // Partner disappeared.
 				if (!srec->msg[t^1]->msgid || strcmp( srec->msg[F]->msgid, srec->msg[N]->msgid )) {
-					error( "Error: channel %s, %s box %s: UIDVALIDITY genuinely changed (at UID %u).\n",
+					if (svars->uidval[t^1] != svars->newuidval[t^1]) {
+						error( "Error: channel %s, %s box %s (at UID %u):"
+						       " Unable to recover from both-sided UIDVALIDITY change,"
+						       " as it is genuine on at least one side.\n",
+						       svars->chan->name, str_fn[t], svars->orig_name[t], srec->uid[t] );
+					} else {
+						error( "Error: channel %s, %s box %s (at UID %u): UIDVALIDITY genuinely changed.\n",
 						       svars->chan->name, str_fn[t], svars->orig_name[t], srec->uid[t] );
+					}
 				  uvchg:
 					svars->ret |= SYNC_FAIL;
 					cancel_sync( svars );