validation: Skip VerifyDB checks of level >=3 if dbcache is too small

The previous behavior, skipping some L3 DisconnectBlock calls,
but still attempting to reconnect these blocks at L4, makes
ConnectBlock assert.

The variable skipped_l3_checks is introduced because even with an
insufficient cache for the L3 checks, the L1/L2 checks in the same
loop should still be completed.

Fixes bitcoin#25563.

src/validation.cpp

@@ -3985,6 +3985,7 @@ bool CVerifyDB::VerifyDB(
     int nGoodTransactions = 0;
     BlockValidationState state;
     int reportDone = 0;
+    bool skipped_l3_checks{false};
     LogPrintf("[0%%]..."); /* Continued */
 
     const bool is_snapshot_cs{!chainstate.m_from_snapshot_blockhash};
@@ -4028,30 +4029,36 @@ bool CVerifyDB::VerifyDB(
         // check level 3: check for inconsistencies during memory-only disconnect of tip blocks
         size_t curr_coins_usage = coins.DynamicMemoryUsage() + chainstate.CoinsTip().DynamicMemoryUsage();
 
-        if (nCheckLevel >= 3 && curr_coins_usage <= chainstate.m_coinstip_cache_size_bytes) {
-            assert(coins.GetBestBlock() == pindex->GetBlockHash());
-            DisconnectResult res = chainstate.DisconnectBlock(block, pindex, coins);
-            if (res == DISCONNECT_FAILED) {
-                return error("VerifyDB(): *** irrecoverable inconsistency in block data at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString());
-            }
-            if (res == DISCONNECT_UNCLEAN) {
-                nGoodTransactions = 0;
-                pindexFailure = pindex;
+        if (nCheckLevel >= 3) {
+            if (curr_coins_usage <= chainstate.m_coinstip_cache_size_bytes) {
+                assert(coins.GetBestBlock() == pindex->GetBlockHash());
+                DisconnectResult res = chainstate.DisconnectBlock(block, pindex, coins);
+                if (res == DISCONNECT_FAILED) {
+                    return error("VerifyDB(): *** irrecoverable inconsistency in block data at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString());
+                }
+                if (res == DISCONNECT_UNCLEAN) {
+                    nGoodTransactions = 0;
+                    pindexFailure = pindex;
+                } else {
+                    nGoodTransactions += block.vtx.size();
+                }
             } else {
-                nGoodTransactions += block.vtx.size();
+                skipped_l3_checks = true;
             }
         }
         if (ShutdownRequested()) return true;
     }
     if (pindexFailure) {
         return error("VerifyDB(): *** coin database inconsistencies found (last %i blocks, %i good transactions before that)\n", chainstate.m_chain.Height() - pindexFailure->nHeight + 1, nGoodTransactions);
     }
-
+    if (skipped_l3_checks) {
+        LogPrintf("Skipped verification of level >=3 (insufficient database cache size). Consider increasing -dbcache.\n");
+    }
     // store block count as we move pindex at check level >= 4
     int block_count = chainstate.m_chain.Height() - pindex->nHeight;
 
     // check level 4: try reconnecting blocks
-    if (nCheckLevel >= 4) {
+    if (nCheckLevel >= 4 && !skipped_l3_checks) {
         while (pindex != chainstate.m_chain.Tip()) {
             const int percentageDone = std::max(1, std::min(99, 100 - (int)(((double)(chainstate.m_chain.Height() - pindex->nHeight)) / (double)nCheckDepth * 50)));
             if (reportDone < percentageDone / 10) {