Call the -alertnotify script when we see a long or invalid fork.

src/init.cpp

@@ -225,7 +225,7 @@ std::string HelpMessage()
     strUsage += "  -rpcthreads=<n>        " + _("Set the number of threads to service RPC calls (default: 4)") + "\n";
     strUsage += "  -blocknotify=<cmd>     " + _("Execute command when the best block changes (%s in cmd is replaced by block hash)") + "\n";
     strUsage += "  -walletnotify=<cmd>    " + _("Execute command when a wallet transaction changes (%s in cmd is replaced by TxID)") + "\n";
-    strUsage += "  -alertnotify=<cmd>     " + _("Execute command when a relevant alert is received (%s in cmd is replaced by message)") + "\n";
+    strUsage += "  -alertnotify=<cmd>     " + _("Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)") + "\n";
     strUsage += "  -upgradewallet         " + _("Upgrade wallet to latest format") + "\n";
     strUsage += "  -keypool=<n>           " + _("Set key pool size to <n> (default: 100)") + "\n";
     strUsage += "  -rescan                " + _("Rescan the block chain for missing wallet transactions") + "\n";

src/main.cpp

@@ -1395,6 +1395,16 @@ void CheckForkWarningConditions()
 
     if (pindexBestForkTip || nBestInvalidWork > nBestChainWork + (pindexBest->GetBlockWork() * 6).getuint256())
     {
+        if (!fLargeWorkForkFound)
+        {
+            std::string strCmd = GetArg("-alertnotify", "");
+            if (!strCmd.empty())
+            {
+                std::string warning("'Warning: Large-work fork detected. You may need to upgrade, or other nodes may need to upgrade.'");
+                boost::replace_all(strCmd, "%s", warning);
+                boost::thread t(runCommand, strCmd); // thread runs free
+            }
+        }
         fLargeWorkForkFound = true;
         printf("CheckForkWarningConditions: Warning: Displayed transactions may not be correct! You may need to upgrade, or other nodes may need to upgrade.\n");
     } else