Reject block versions lower than 4

Closes #1556

src/gtest/test_checkblock.cpp

@@ -0,0 +1,31 @@
+#include <gtest/gtest.h>
+#include <gmock/gmock.h>
+
+#include "consensus/validation.h"
+#include "main.h"
+
+class MockCValidationState : public CValidationState {
+public:
+    MOCK_METHOD5(DoS, bool(int level, bool ret,
+             unsigned char chRejectCodeIn, std::string strRejectReasonIn,
+             bool corruptionIn));
+    MOCK_METHOD3(Invalid, bool(bool ret,
+                 unsigned char _chRejectCode, std::string _strRejectReason));
+    MOCK_METHOD1(Error, bool(std::string strRejectReasonIn));
+    MOCK_CONST_METHOD0(IsValid, bool());
+    MOCK_CONST_METHOD0(IsInvalid, bool());
+    MOCK_CONST_METHOD0(IsError, bool());
+    MOCK_CONST_METHOD1(IsInvalid, bool(int &nDoSOut));
+    MOCK_CONST_METHOD0(CorruptionPossible, bool());
+    MOCK_CONST_METHOD0(GetRejectCode, unsigned char());
+    MOCK_CONST_METHOD0(GetRejectReason, std::string());
+};
+
+TEST(CheckBlock, VersionTooLow) {
+    CBlock block;
+    block.nVersion = 1;
+
+    MockCValidationState state;
+    EXPECT_CALL(state, DoS(100, false, REJECT_INVALID, "version-too-low", false)).Times(1);
+    EXPECT_FALSE(CheckBlock(block, state, false, false));
+}