fts: When indexing virtual mailbox, index each real mailbox entirely

Index all the unindexed messages in them at once, instead of jumping between
real mailboxes and indexing them in small pieces.

src/plugins/fts/fts-storage.c

@@ -486,6 +486,7 @@ static void fts_mail_index(struct mail *_mail)
 {
 	struct fts_transaction_context *ft = FTS_CONTEXT_REQUIRE(_mail->transaction);
 	struct fts_mailbox_list *flist = FTS_LIST_CONTEXT_REQUIRE(_mail->box->list);
+	struct mail_private *pmail = (struct mail_private *)_mail;
 
 	if (ft->failed)
 		return;
@@ -496,9 +497,30 @@ static void fts_mail_index(struct mail *_mail)
 			return;
 		}
 	}
+	if (pmail->vmail != NULL) {
+		/* Indexing via virtual mailbox: Index all the mails in this
+		   same real mailbox. */
+		uint32_t msgs_count =
+			mail_index_view_get_messages_count(_mail->box->view);
+
+		fts_backend_update_set_mailbox(flist->update_ctx, _mail->box);
+		if (ft->next_index_seq > msgs_count) {
+			/* everything indexed already */
+		} else if (fts_mail_precache_range(_mail->transaction,
+						   flist->update_ctx,
+						   ft->next_index_seq,
+						   msgs_count,
+						   &ft->precache_extra_count) < 0) {
+			ft->failed = TRUE;
+		} else {
+			ft->next_index_seq = msgs_count+1;
+		}
+		return;
+	}
+
 	if (ft->next_index_seq < _mail->seq) {
-		/* most likely a virtual mailbox. we'll first need to
-		   index all mails up to the current one. */
+		/* we'll first need to index all the missing mails up to the
+		   current one. */
 		fts_backend_update_set_mailbox(flist->update_ctx, _mail->box);
 		if (fts_mail_precache_range(_mail->transaction,
 					    flist->update_ctx,