mdbox: Assume that empty uid maps found during sync are harmless

Instead of failing the sync and causing index rebuild, just skip over
the empty uid maps.  Chances are that they these records came from
various plugins that create fake mails.

src/lib-storage/index/dbox-multi/mdbox-sync.c

@@ -24,6 +24,7 @@
 #include "mailbox-recent-flags.h"
 
 
+/* returns -1 on error, 1 on success, 0 if guid is empty/missing */
 static int
 dbox_sync_verify_expunge_guid(struct mdbox_sync_context *ctx, uint32_t seq,
 			      const guid_128_t guid_128)
@@ -34,9 +35,13 @@ dbox_sync_verify_expunge_guid(struct mdbox_sync_context *ctx, uint32_t seq,
 	mail_index_lookup_uid(ctx->sync_view, seq, &uid);
 	mail_index_lookup_ext(ctx->sync_view, seq,
 			      ctx->mbox->guid_ext_id, &data, NULL);
+
+	if ((data == NULL) || guid_128_is_empty(data))
+		return 0;
+
 	if (guid_128_is_empty(guid_128) ||
 	    memcmp(data, guid_128, GUID_128_SIZE) == 0)
-		return 0;
+		return 1;
 
 	mailbox_set_critical(&ctx->mbox->box,
 		"Expunged GUID mismatch for UID %u: %s vs %s",
@@ -49,14 +54,16 @@ static int mdbox_sync_expunge(struct mdbox_sync_context *ctx, uint32_t seq,
 			      const guid_128_t guid_128)
 {
 	uint32_t map_uid;
+	int ret;
 
 	if (seq_range_array_add(&ctx->expunged_seqs, seq)) {
 		/* already marked as expunged in this sync */
 		return 0;
 	}
 
-	if (dbox_sync_verify_expunge_guid(ctx, seq, guid_128) < 0)
-		return -1;
+	ret = dbox_sync_verify_expunge_guid(ctx, seq, guid_128);
+	if (ret <= 0)
+		return ret;
 	if (mdbox_mail_lookup(ctx->mbox, ctx->sync_view, seq, &map_uid) < 0)
 		return -1;
 	if (mdbox_map_update_refcount(ctx->map_trans, map_uid, -1) < 0)
@@ -107,7 +114,10 @@ static int dbox_sync_mark_expunges(struct mdbox_sync_context *ctx)
 		mail_index_lookup_uid(ctx->sync_view, seq, &uid);
 		mail_index_lookup_ext(ctx->sync_view, seq,
 				      ctx->mbox->guid_ext_id, &data, NULL);
-		mail_index_expunge_guid(trans, seq, data);
+		if ((data == NULL) || guid_128_is_empty(data))
+			mail_index_expunge(trans, seq);
+		else
+			mail_index_expunge_guid(trans, seq, data);
 	}
 	if (mail_index_transaction_commit(&trans) < 0)
 		return -1;