Warn if blockchain majority doesn't match CBlock::CURRENT_VERSION

This adds a warning "this version is obsolete, upgrade required" if
more than 50 of the previous 100 blocks in the blockchain are
a new version.

src/main.cpp

@@ -1623,6 +1623,24 @@ bool CBlock::SetBestChain(CTxDB& txdb, CBlockIndex* pindexNew)
       hashBestChain.ToString().substr(0,20).c_str(), nBestHeight, bnBestChainWork.ToString().c_str(),
       DateTimeStrFormat("%x %H:%M:%S", pindexBest->GetBlockTime()).c_str());
 
+    // Check the version of the last 100 blocks to see if we need to upgrade:
+    if (!fIsInitialDownload)
+    {
+        int nUpgraded = 0;
+        const CBlockIndex* pindex = pindexBest;
+        for (int i = 0; i < 100 && pindex != NULL; i++)
+        {
+            if (pindex->nVersion > CBlock::CURRENT_VERSION)
+                ++nUpgraded;
+            pindex = pindex->pprev;
+        }
+        if (nUpgraded > 0)
+            printf("SetBestChain: %d of last 100 blocks above version %d\n", nUpgraded, CBlock::CURRENT_VERSION);
+        if (nUpgraded > 100/2)
+            // strMiscWarning is read by GetWarnings(), called by Qt and the JSON-RPC code to warn the user:
+            strMiscWarning = _("Warning: this version is obsolete, upgrade required");
+    }
+
     std::string strCmd = GetArg("-blocknotify", "");
 
     if (!fIsInitialDownload && !strCmd.empty())