db_sync: mc>pc guard with last_peer_count — detect stalled sync when pc doesn't change

src/db_sync.c

@@ -39,6 +39,7 @@ struct SI_PEER {
     uint64_t node_id;
     uint32_t synced_pos;
     uint32_t verified_pos;
+    uint32_t last_peer_count;
     uint8_t  sync_state;
     uint64_t sync_start_tb;
 };
@@ -1049,12 +1050,6 @@ static void db_handle_send_data(struct DB_SYNC_INSTANCE* si, uint64_t src, const
     }
 
     uint64_t lch8 = 0; if (mc > 0) db_chain_hash8_at(si, mc - 1, &lch8);
-    if (received == 0 && resp_count == 0) {
-        if (sp) { uint32_t nsp = mc > 0 ? mc - 1 : 0; if (sp->synced_pos < nsp) sp->synced_pos = nsp; sp->sync_state = 2; sp->sync_start_tb = 0; }
-        DEBUG_INFO(DEBUG_CATEGORY_DB_SYNC, "sync [%s:%04llX] → SEND_DATA: 0 new records, skip SYNC_DONE (synced=%u)",
-                   SI_SHRT(si), (unsigned long long)(src >> 16), sp ? sp->synced_pos : 0);
-        return;
-    }
     uint8_t sd[13];
     sd[0] = DB_MSG_SYNC_DONE; memcpy(sd + 1, &mc, 4); memcpy(sd + 5, &lch8, 8);
     db_sync_send(si, src, sd, 13);
@@ -1103,6 +1098,14 @@ static void db_handle_sync_done(struct DB_SYNC_INSTANCE* si, uint64_t src, const
     }
 
     if (mc > pc) {
+        if (sp && sp->synced_pos + 1 >= mc && sp->last_peer_count == pc) {
+            if (sp) sp->sync_state = 2;
+            DEBUG_INFO(DEBUG_CATEGORY_DB_SYNC,
+                       "sync [%s:%04llX] ← SYNC_DONE: count diff my=%u peer=%u synced=%u — same pc, accept",
+                       SI_SHRT(si), (unsigned long long)(src >> 16), mc, pc, sp ? sp->synced_pos : 0);
+            return;
+        }
+        if (sp) sp->last_peer_count = pc;
         DEBUG_INFO(DEBUG_CATEGORY_DB_SYNC,
                    "sync [%s:%04llX] ← SYNC_DONE: count diff my=%u peer=%u — sending tail from=%u",
                    SI_SHRT(si), (unsigned long long)(src >> 16), mc, pc, pc);