lock cs_main for chainActive

ActivateBestChain uses chainActive after releasing the lock; reorder operations
to move all access to synchronized object into existing LOCK(cs_main) block.

src/main.cpp

@@ -2897,14 +2897,15 @@ static bool ActivateBestChainStep(CValidationState& state, const CChainParams& c
  */
 bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams, const CBlock *pblock) {
     CBlockIndex *pindexMostWork = NULL;
+    CBlockIndex *pindexNewTip = NULL;
     do {
         boost::this_thread::interruption_point();
         if (ShutdownRequested())
             break;
 
-        CBlockIndex *pindexNewTip = NULL;
         const CBlockIndex *pindexFork;
         bool fInitialDownload;
+        int nNewHeight;
         {
             LOCK(cs_main);
             CBlockIndex *pindexOldTip = chainActive.Tip();
@@ -2920,6 +2921,7 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
             pindexNewTip = chainActive.Tip();
             pindexFork = chainActive.FindFork(pindexOldTip);
             fInitialDownload = IsInitialBlockDownload();
+            nNewHeight = chainActive.Height();
         }
         // When we reach this point, we switched to a new tip (stored in pindexNewTip).
 
@@ -2948,7 +2950,7 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
                 {
                     LOCK(cs_vNodes);
                     BOOST_FOREACH(CNode* pnode, vNodes) {
-                        if (chainActive.Height() > (pnode->nStartingHeight != -1 ? pnode->nStartingHeight - 2000 : nBlockEstimate)) {
+                        if (nNewHeight > (pnode->nStartingHeight != -1 ? pnode->nStartingHeight - 2000 : nBlockEstimate)) {
                             BOOST_REVERSE_FOREACH(const uint256& hash, vHashes) {
                                 pnode->PushBlockHash(hash);
                             }
@@ -2961,7 +2963,7 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
                 }
             }
         }
-    } while(pindexMostWork != chainActive.Tip());
+    } while (pindexNewTip != pindexMostWork);
     CheckBlockIndex(chainparams.GetConsensus());
 
     // Write changes periodically to disk, after relay.