New database check routine

-checklevel gets a new meaning:
0: verify blocks can be read from disk (like before)
1: verify (contextless) block validity (like before)
2: verify undo files can be read and have good checksums
3: verify coin database is consistent with the last few blocks
   (close to level 6 before)
4: verify all validity rules of the last few blocks

Level 3 is the new default, as it's reasonably fast. As level 3 and
4 are implemented using an in-memory rollback of the database, they
are limited to as many blocks as possible without exceeding the
limits set by -dbcache. The default of -dbcache=25 allows for some
150-200 blocks to be rolled back.

In case an error is found, the application quits with a message
instructing the user to restart with -reindex. Better instructions,
and automatic recovery (when possible) or automatic reindexing are
left as future work.

src/init.cpp

@@ -300,7 +300,7 @@ std::string HelpMessage()
         "  -rescan                " + _("Rescan the block chain for missing wallet transactions") + "\n" +
         "  -salvagewallet         " + _("Attempt to recover private keys from a corrupt wallet.dat") + "\n" +
         "  -checkblocks=<n>       " + _("How many blocks to check at startup (default: 2500, 0 = all)") + "\n" +
-        "  -checklevel=<n>        " + _("How thorough the block verification is (0-6, default: 1)") + "\n" +
+        "  -checklevel=<n>        " + _("How thorough the block verification is (0-4, default: 3)") + "\n" +
         "  -loadblock=<file>      " + _("Imports blocks from external blk000??.dat file") + "\n" +
         "  -reindex               " + _("Rebuild blockchain index from current blk000??.dat files") + "\n" +
 
@@ -752,7 +752,10 @@ bool AppInit2()
         pblocktree->WriteReindexing(true);
 
     if (!LoadBlockIndex())
-        return InitError(_("Error loading blkindex.dat"));
+        return InitError(_("Error loading block/coin databases"));
+
+    if (!VerifyDB())
+        return InitError(_("Corrupted database detected. Please restart the client with -reindex."));
 
     // as LoadBlockIndex can take several minutes, it's possible the user
     // requested to kill bitcoin-qt during the last operation. If so, exit.

src/main.cpp

@@ -2388,36 +2388,77 @@ bool static LoadBlockIndexDB()
         BlockHashStr(hashBestChain).c_str(), nBestHeight,
         DateTimeStrFormat("%Y-%m-%d %H:%M:%S", pindexBest->GetBlockTime()).c_str());
 
+    return true;
+}
+
+bool VerifyDB() {
+    if (pindexBest == NULL || pindexBest->pprev == NULL)
+        return true;
+
     // Verify blocks in the best chain
-    int nCheckLevel = GetArg("-checklevel", 1);
+    int nCheckLevel = GetArg("-checklevel", 3);
     int nCheckDepth = GetArg( "-checkblocks", 2500);
     if (nCheckDepth == 0)
         nCheckDepth = 1000000000; // suffices until the year 19000
     if (nCheckDepth > nBestHeight)
         nCheckDepth = nBestHeight;
+    nCheckLevel = std::max(0, std::min(4, nCheckLevel));
     printf("Verifying last %i blocks at level %i\n", nCheckDepth, nCheckLevel);
-    CBlockIndex* pindexFork = NULL;
+    CCoinsViewCache coins(*pcoinsTip, true);
+    CBlockIndex* pindexState = pindexBest;
+    CBlockIndex* pindexFailure = NULL;
+    int nGoodTransactions = 0;
     for (CBlockIndex* pindex = pindexBest; pindex && pindex->pprev; pindex = pindex->pprev)
     {
         if (fRequestShutdown || pindex->nHeight < nBestHeight-nCheckDepth)
             break;
         CBlock block;
+        // check level 0: read from disk
         if (!block.ReadFromDisk(pindex))
-            return error("LoadBlockIndex() : block.ReadFromDisk failed");
+            return error("VerifyDB() : *** block.ReadFromDisk failed at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());
         // check level 1: verify block validity
-        if (nCheckLevel>0 && !block.CheckBlock())
-        {
-            printf("LoadBlockIndex() : *** found bad block at %d, hash=%s\n", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());
-            pindexFork = pindex->pprev;
+        if (nCheckLevel >= 1 && !block.CheckBlock())
+            return error("VerifyDB() : *** found bad block at %d, hash=%s\n", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());
+        // check level 2: verify undo validity
+        if (nCheckLevel >= 2 && pindex) {
+            CBlockUndo undo;
+            CDiskBlockPos pos = pindex->GetUndoPos();
+            if (!pos.IsNull()) {
+                if (!undo.ReadFromDisk(pos, pindex->pprev->GetBlockHash()))
+                    return error("VerifyDB() : *** found bad undo data at %d, hash=%s\n", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());
+            }
+        }
+        // check level 3: check for inconsistencies during memory-only disconnect of tip blocks
+        if (nCheckLevel >= 3 && pindex == pindexState && (coins.GetCacheSize() + pcoinsTip->GetCacheSize()) <= 2*nCoinCacheSize + 32000) {
+            bool fClean = true;
+            if (!block.DisconnectBlock(pindex, coins, &fClean))
+                return error("VerifyDB() : *** irrecoverable inconsistency in block data at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());
+            pindexState = pindex->pprev;
+            if (!fClean) {
+                nGoodTransactions = 0;
+                pindexFailure = pindex;
+            } else
+                nGoodTransactions += block.vtx.size();
         }
-        // TODO: stronger verifications
     }
-    if (pindexFork && !fRequestShutdown)
-    {
-        // TODO: reorg back
-        return error("LoadBlockIndex(): chain database corrupted");
+    if (pindexFailure)
+        return error("VerifyDB() : *** coin database inconsistencies found (last %i blocks, %i good transactions before that)\n", pindexBest->nHeight - pindexFailure->nHeight + 1, nGoodTransactions);
+
+    // check level 4: try reconnecting blocks
+    if (nCheckLevel >= 4) {
+        CBlockIndex *pindex = pindexState;
+        while (pindex != pindexBest && !fRequestShutdown) {
+             pindex = pindex->pnext;
+             CBlock block;
+             if (!block.ReadFromDisk(pindex))
+                 return error("VerifyDB() : *** block.ReadFromDisk failed at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());
+             if (!block.ConnectBlock(pindex, coins))
+                 return error("VerifyDB() : *** found unconnectable block at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());
+        }
     }
 
+    printf("No coin database inconsistencies in last %i blocks (%i transactions)\n", pindexBest->nHeight - pindexState->nHeight, nGoodTransactions);
+
     return true;
 }
 

src/main.h

@@ -126,6 +126,8 @@ FILE* OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly = false);
 bool LoadExternalBlockFile(FILE* fileIn, CDiskBlockPos *dbp = NULL);
 /** Load the block tree and coins database from disk */
 bool LoadBlockIndex();
+/** Verify consistency of the block and coin databases */
+bool VerifyDB();
 /** Print the loaded block tree */
 void PrintBlockTree();
 /** Find a block by height in the currently-connected chain */