From afc32c5eeada01b141706e32f0405bdb86c00f04 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Fri, 10 Oct 2014 13:13:47 -0700
Subject: [PATCH] Fix rebuild-chainstate feature and improve its performance

Previous refactorings broke the ability to rebuild the chainstate by deleting the chainstate
directory, resulting in an incorrect "Incorrect or no genesis block found" error message. Fix
that.

Also, improve the performance of ActivateBestBlockStep by using the skiplist to only discover
a few potential blocks to connect at a time, instead of all blocks forever - as we likely bail
out after connecting a single one anyway.
---
 src/init.cpp |  2 +-
 src/main.cpp | 17 ++++++++++++++---
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/src/init.cpp b/src/init.cpp
index 8772be553..55991ac9b 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -974,7 +974,7 @@ bool AppInit2(boost::thread_group& threadGroup)
 
                 // If the loaded chain has a wrong genesis, bail out immediately
                 // (we're likely using a testnet datadir, or the other way around).
-                if (!mapBlockIndex.empty() && chainActive.Genesis() == NULL)
+                if (!mapBlockIndex.empty() && mapBlockIndex.count(Params().HashGenesisBlock()) == 0)
                     return InitError(_("Incorrect or no genesis block found. Wrong datadir for network?"));
 
                 // Initialize the block index (no-op if non-empty database was already loaded)
diff --git a/src/main.cpp b/src/main.cpp
index 908f4c95b..0612f584a 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1968,12 +1968,20 @@ static bool ActivateBestChainStep(CValidationState &state, CBlockIndex *pindexMo
 
     // Build list of new blocks to connect.
     std::vector<CBlockIndex*> vpindexToConnect;
-    vpindexToConnect.reserve(pindexMostWork->nHeight - (pindexFork ? pindexFork->nHeight : -1));
-    CBlockIndex *pindexIter = pindexMostWork;
-    while (pindexIter && pindexIter != pindexFork) {
+    bool fContinue = true;
+    int nHeight = pindexFork ? pindexFork->nHeight : -1;
+    while (fContinue && nHeight != pindexMostWork->nHeight) {
+    // Don't iterate the entire list of potential improvements toward the best tip, as we likely only need
+    // a few blocks along the way.
+    int nTargetHeight = std::min(nHeight + 32, pindexMostWork->nHeight);
+    vpindexToConnect.clear();
+    vpindexToConnect.reserve(nTargetHeight - nHeight);
+    CBlockIndex *pindexIter = pindexMostWork->GetAncestor(nTargetHeight);
+    while (pindexIter && pindexIter->nHeight != nHeight) {
         vpindexToConnect.push_back(pindexIter);
         pindexIter = pindexIter->pprev;
     }
+    nHeight = nTargetHeight;
 
     // Connect new blocks.
     BOOST_REVERSE_FOREACH(CBlockIndex *pindexConnect, vpindexToConnect) {
@@ -1984,6 +1992,7 @@ static bool ActivateBestChainStep(CValidationState &state, CBlockIndex *pindexMo
                     InvalidChainFound(vpindexToConnect.back());
                 state = CValidationState();
                 fInvalidFound = true;
+                fContinue = false;
                 break;
             } else {
                 // A system error occurred (disk space, database error, ...).
@@ -2001,10 +2010,12 @@ static bool ActivateBestChainStep(CValidationState &state, CBlockIndex *pindexMo
             assert(!setBlockIndexCandidates.empty());
             if (!pindexOldTip || chainActive.Tip()->nChainWork > pindexOldTip->nChainWork) {
                 // We're in a better position than we were. Return temporarily to release the lock.
+                fContinue = false;
                 break;
             }
         }
     }
+    }
 
     // Callbacks/notifications for a new best chain.
     if (fInvalidFound)