Trigger -alertnotify if network is upgrading without you

This adds a -regetest-only undocumented (for regression testing only)
command-line option -blockversion=N to set block.nVersion.

Adds to the "has the rest of the network upgraded to a
block.nVersion we don't understand" code so it calls
-alertnotify when 51 of the last 100 blocks are up-version.
But it only alerts once, not with every subsequent new, upversion
block.

And adds a forknotify.py regression test to make sure it works.

Tested using forknotify.py:

Before adding CAlert::Notify, get:
Assertion failed: -alertnotify did not warn of up-version blocks

Before adding code to only alert once:
Assertion failed: -alertnotify excessive warning of up-version blocks

After final code in this pull:
Tests successful

4 files changed, 77 insertions, 1 deletions

diff --git a/qa/pull-tester/build-tests.sh.in b/qa/pull-tester/build-tests.sh.in
index ebf377a489..1ef47d77f4 100755
--- a/qa/pull-tester/build-tests.sh.in
+++ b/qa/pull-tester/build-tests.sh.in
@@ -75,6 +75,7 @@ make check
 # Run RPC integration test on Linux:
 @abs_top_srcdir@/qa/rpc-tests/wallet.sh @abs_top_srcdir@/linux-build/src
 @abs_top_srcdir@/qa/rpc-tests/listtransactions.py --srcdir @abs_top_srcdir@/linux-build/src
+@abs_top_srcdir@/qa/rpc-tests/forknotify.py --srcdir @abs_top_srcdir@/linux-build/src
 # Clean up cache/ directory that the python regression tests create
 rm -rf cache
 
diff --git a/qa/rpc-tests/forknotify.py b/qa/rpc-tests/forknotify.py
new file mode 100755
index 0000000000..a482f7cc5a
--- /dev/null
+++ b/qa/rpc-tests/forknotify.py
@@ -0,0 +1,65 @@
+#!/usr/bin/env python
+# Copyright (c) 2014 The Bitcoin Core developers
+# Distributed under the MIT/X11 software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#
+# Test -alertnotify 
+#
+
+from test_framework import BitcoinTestFramework
+from bitcoinrpc.authproxy import AuthServiceProxy, JSONRPCException
+from util import *
+import os
+import shutil
+
+class ForkNotifyTest(BitcoinTestFramework):
+
+    alert_filename = None  # Set by setup_network
+
+    def setup_network(self, test_dir):
+        nodes = []
+        self.alert_filename = os.path.join(test_dir, "alert.txt")
+        with open(self.alert_filename, 'w') as f:
+            pass  # Just open then close to create zero-length file
+        nodes.append(start_node(0, test_dir,
+                            ["-blockversion=2", "-alertnotify=echo %s >> '" + self.alert_filename + "'"]))
+        # Node1 mines block.version=211 blocks
+        nodes.append(start_node(1, test_dir,
+                                ["-blockversion=211"]))
+        connect_nodes(nodes[1], 0)
+
+        sync_blocks(nodes)
+        return nodes
+        
+
+    def run_test(self, nodes):
+        # Mine 51 up-version blocks
+        nodes[1].setgenerate(True, 51)
+        sync_blocks(nodes)
+        # -alertnotify should trigger on the 51'st,
+        # but mine and sync another to give
+        # -alertnotify time to write
+        nodes[1].setgenerate(True, 1)
+        sync_blocks(nodes)
+
+        with open(self.alert_filename, 'r') as f:
+            alert_text = f.read()
+
+        if len(alert_text) == 0:
+            raise AssertionError("-alertnotify did not warn of up-version blocks")
+
+        # Mine more up-version blocks, should not get more alerts:
+        nodes[1].setgenerate(True, 1)
+        sync_blocks(nodes)
+        nodes[1].setgenerate(True, 1)
+        sync_blocks(nodes)
+
+        with open(self.alert_filename, 'r') as f:
+            alert_text2 = f.read()
+
+        if alert_text != alert_text2:
+            raise AssertionError("-alertnotify excessive warning of up-version blocks")
+
+if __name__ == '__main__':
+    ForkNotifyTest().main()
diff --git a/src/main.cpp b/src/main.cpp
index 106c336a83..43b81aaef8 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1755,7 +1755,8 @@ void static UpdateTip(CBlockIndex *pindexNew) {
     cvBlockChange.notify_all();
 
     // Check the version of the last 100 blocks to see if we need to upgrade:
-    if (!IsInitialBlockDownload())
+    static bool fWarned = false;
+    if (!IsInitialBlockDownload() && !fWarned)
     {
         int nUpgraded = 0;
         const CBlockIndex* pindex = chainActive.Tip();
@@ -1768,8 +1769,12 @@ void static UpdateTip(CBlockIndex *pindexNew) {
         if (nUpgraded > 0)
             LogPrintf("SetBestChain: %d of last 100 blocks above version %d\n", nUpgraded, (int)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!");
+            CAlert::Notify(strMiscWarning, true);
+            fWarned = true;
+        }
     }
 }
 
diff --git a/src/miner.cpp b/src/miner.cpp
index 280349e8c2..2eb028b1d3 100644
--- a/src/miner.cpp
+++ b/src/miner.cpp
@@ -83,6 +83,11 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         return NULL;
     CBlock *pblock = &pblocktemplate->block; // pointer for convenience
 
+    // -regtest only: allow overriding block.nVersion with
+    // -blockversion=N to test forking scenarios
+    if (Params().MineBlocksOnDemand())
+        pblock->nVersion = GetArg("-blockversion", pblock->nVersion);
+
     // Create coinbase tx
     CMutableTransaction txNew;
     txNew.vin.resize(1);