Alert if it is very likely we are getting a bad chain

Create a monitoring task that counts how many blocks have been found in the last four hours.

If very few or too many have been found, an alert is triggered.

"Very few" and "too many" are set based on a false positive rate of once every fifty years of constant running with constant hashing power, which works out to getting 5 or fewer or 48 or more blocks in four hours (instead of the average of 24).

Only one alert per day is triggered, so if you get disconnected from the network (or are being Sybil'ed) -alertnotify will be triggered after 3.5 hours but you won't get another -alertnotify for 24 hours.

Tested with a new unit test and by running on the main network with -debug=partitioncheck

Run test/test_bitcoin --log_level=message to see the alert messages:
    WARNING: check your network connection, 3 blocks received in the last 4 hours (24 expected)
    WARNING: abnormally high number of blocks generated, 60 blocks received in the last 4 hours (24 expected)

The -debug=partitioncheck debug.log messages look like:
    ThreadPartitionCheck : Found 22 blocks in the last 4 hours
    ThreadPartitionCheck : likelihood: 0.0777702

1 files changed, 64 insertions, 0 deletions

diff --git a/src/test/alert_tests.cpp b/src/test/alert_tests.cpp
index 6b6df5199a..cd5b416bd0 100644
--- a/src/test/alert_tests.cpp
+++ b/src/test/alert_tests.cpp
@@ -7,10 +7,13 @@
 //
 
 #include "alert.h"
+#include "chain.h"
+#include "chainparams.h"
 #include "clientversion.h"
 #include "data/alertTests.raw.h"
 
 #include "chainparams.h"
+#include "main.h"
 #include "serialize.h"
 #include "streams.h"
 #include "util.h"
@@ -193,4 +196,65 @@ BOOST_AUTO_TEST_CASE(AlertNotify)
     SetMockTime(0);
 }
 
+static bool falseFunc() { return false; }
+
+BOOST_AUTO_TEST_CASE(PartitionAlert)
+{
+    // Test PartitionCheck
+    CCriticalSection csDummy;
+    CChain chainDummy;
+    CBlockIndex indexDummy[100];
+    CChainParams& params = Params(CBaseChainParams::MAIN);
+    int64_t nPowTargetSpacing = params.GetConsensus().nPowTargetSpacing;
+
+    // Generate fake blockchain timestamps relative to
+    // an arbitrary time:
+    int64_t now = 1427379054;
+    SetMockTime(now);
+    for (int i = 0; i < 100; i++)
+    {
+        indexDummy[i].phashBlock = NULL;
+        if (i == 0) indexDummy[i].pprev = NULL;
+        else indexDummy[i].pprev = &indexDummy[i-1];
+        indexDummy[i].nHeight = i;
+        indexDummy[i].nTime = now - (100-i)*nPowTargetSpacing;
+        // Other members don't matter, the partition check code doesn't
+        // use them
+    }
+    chainDummy.SetTip(&indexDummy[99]);
+
+    // Test 1: chain with blocks every nPowTargetSpacing seconds,
+    // as normal, no worries:
+    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    BOOST_CHECK(strMiscWarning.empty());
+
+    // Test 2: go 3.5 hours without a block, expect a warning:
+    now += 3*60*60+30*60;
+    SetMockTime(now);
+    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    BOOST_CHECK(!strMiscWarning.empty());
+    BOOST_TEST_MESSAGE(std::string("Got alert text: ")+strMiscWarning);
+    strMiscWarning = "";
+
+    // Test 3: test the "partition alerts only go off once per day"
+    // code:
+    now += 60*10;
+    SetMockTime(now);
+    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    BOOST_CHECK(strMiscWarning.empty());
+
+    // Test 4: get 2.5 times as many blocks as expected:
+    now += 60*60*24; // Pretend it is a day later
+    SetMockTime(now);
+    int64_t quickSpacing = nPowTargetSpacing*2/5;
+    for (int i = 0; i < 100; i++) // Tweak chain timestamps:
+        indexDummy[i].nTime = now - (100-i)*quickSpacing;
+    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    BOOST_CHECK(!strMiscWarning.empty());
+    BOOST_TEST_MESSAGE(std::string("Got alert text: ")+strMiscWarning);
+    strMiscWarning = "";
+
+    SetMockTime(0);
+}
+
 BOOST_AUTO_TEST_SUITE_END()