Limit the impact of reorganisations on the database

Sometimes a new block arrives in a new chain that was already the
best valid one, but wasn't marked that way. This happens for example
when network rules change to recover after a fork.

In this case, it is not necessary to do the entire reorganisation
inside a single db commit. These can become huge, and exceed the
objects/lockers limits in bdb. This patch limits the blocks the
actual reorganisation is applied to, and adds the next blocks
afterwards in separate db transactions.

src/main.cpp

@@ -1457,26 +1457,18 @@ runCommand(std::string strCommand)
         printf("runCommand error: system(%s) returned %d\n", strCommand.c_str(), nErr);
 }
 
-bool CBlock::SetBestChain(CTxDB& txdb, CBlockIndex* pindexNew)
+bool CBlock::SetBestChainInner(CTxDB& txdb, CBlockIndex *pindexNew)
 {
+    assert(pindexNew->pprev == pindexBest);
+
     uint256 hash = GetHash();
 
-    txdb.TxnBegin();
-    if (pindexGenesisBlock == NULL && hash == hashGenesisBlock)
-    {
-        txdb.WriteHashBestChain(hash);
-        if (!txdb.TxnCommit())
-            return error("SetBestChain() : TxnCommit failed");
-        pindexGenesisBlock = pindexNew;
-    }
-    else if (hashPrevBlock == hashBestChain)
-    {
     // Adding to current best branch
     if (!ConnectBlock(txdb, pindexNew) || !txdb.WriteHashBestChain(hash))
     {
         txdb.TxnAbort();
         InvalidChainFound(pindexNew);
-            return error("SetBestChain() : ConnectBlock failed");
+        return false;
     }
     if (!txdb.TxnCommit())
         return error("SetBestChain() : TxnCommit failed");
@@ -1487,16 +1479,68 @@ bool CBlock::SetBestChain(CTxDB& txdb, CBlockIndex* pindexNew)
     // Delete redundant memory transactions
     BOOST_FOREACH(CTransaction& tx, vtx)
         tx.RemoveFromMemoryPool();
+
+    return true;
+}
+
+bool CBlock::SetBestChain(CTxDB& txdb, CBlockIndex* pindexNew)
+{
+    uint256 hash = GetHash();
+
+    txdb.TxnBegin();
+    if (pindexGenesisBlock == NULL && hash == hashGenesisBlock)
+    {
+        txdb.WriteHashBestChain(hash);
+        if (!txdb.TxnCommit())
+            return error("SetBestChain() : TxnCommit failed");
+        pindexGenesisBlock = pindexNew;
+    }
+    else if (hashPrevBlock == hashBestChain)
+    {
+        if (!SetBestChainInner(txdb, pindexNew))
+            return error("SetBestChain() : SetBestChainInner failed");
     }
     else
     {
-        // New best branch
-        if (!Reorganize(txdb, pindexNew))
+        // the first block in the new chain that will cause it to become the new best chain
+        CBlockIndex *pindexIntermediate = pindexNew;
+
+        // list of blocks that need to be connected afterwards
+        std::vector<CBlockIndex*> vpindexSecondary;
+
+        // Reorganize is costly in terms of db load, as it works in a single db transaction.
+        // Try to limit how much needs to be done inside
+        while (pindexIntermediate->pprev && pindexIntermediate->pprev->bnChainWork > pindexBest->bnChainWork)
+        {
+            vpindexSecondary.push_back(pindexIntermediate);
+            pindexIntermediate = pindexIntermediate->pprev;
+        }
+
+        if (!vpindexSecondary.empty())
+            printf("Postponing %i reconnects\n", vpindexSecondary.size());
+
+        // Switch to new best branch
+        if (!Reorganize(txdb, pindexIntermediate))
         {
             txdb.TxnAbort();
             InvalidChainFound(pindexNew);
             return error("SetBestChain() : Reorganize failed");
         }
+
+        // Connect futher blocks
+        BOOST_REVERSE_FOREACH(CBlockIndex *pindex, vpindexSecondary)
+        {
+            CBlock block;
+            if (!block.ReadFromDisk(pindex))
+            {
+                printf("SetBestChain() : ReadFromDisk failed\n");
+                break;
+            }
+            txdb.TxnBegin();
+            // errors now are not fatal, we still did a reorganisation to a new chain in a valid way
+            if (!block.SetBestChainInner(txdb, pindex))
+                break;
+        }
     }
 
     // Update best block in wallet (so we can detect restored wallets)

src/main.h

@@ -1030,6 +1030,9 @@ public:
     bool AddToBlockIndex(unsigned int nFile, unsigned int nBlockPos);
     bool CheckBlock() const;
     bool AcceptBlock();
+
+private:
+    bool SetBestChainInner(CTxDB& txdb, CBlockIndex *pindexNew);
 };