Add function to validate difficulty changes

The rule against difficulty adjustments changing by more than a factor of 4 can
be helpful for anti-DoS measures in contexts where we lack a full headers
chain, so expose this functionality separately and in the narrow case where we
only know the height, new value, and old value.

Includes fuzz test by Martin Zumsande.

1 files changed, 23 insertions, 4 deletions

diff --git a/src/test/pow_tests.cpp b/src/test/pow_tests.cpp
index 2f43ae52f7..3695ea9d16 100644
--- a/src/test/pow_tests.cpp
+++ b/src/test/pow_tests.cpp
@@ -20,7 +20,14 @@ BOOST_AUTO_TEST_CASE(get_next_work)
     pindexLast.nHeight = 32255;
     pindexLast.nTime = 1262152739;  // Block #32255
     pindexLast.nBits = 0x1d00ffff;
-    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), 0x1d00d86aU);
+
+    // Here (and below): expected_nbits is calculated in
+    // CalculateNextWorkRequired(); redoing the calculation here would be just
+    // reimplementing the same code that is written in pow.cpp. Rather than
+    // copy that code, we just hardcode the expected result.
+    unsigned int expected_nbits = 0x1d00d86aU;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), expected_nbits);
+    BOOST_CHECK(PermittedDifficultyTransition(chainParams->GetConsensus(), pindexLast.nHeight+1, pindexLast.nBits, expected_nbits));
 }
 
 /* Test the constraint on the upper bound for next work */
@@ -32,7 +39,9 @@ BOOST_AUTO_TEST_CASE(get_next_work_pow_limit)
     pindexLast.nHeight = 2015;
     pindexLast.nTime = 1233061996;  // Block #2015
     pindexLast.nBits = 0x1d00ffff;
-    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), 0x1d00ffffU);
+    unsigned int expected_nbits = 0x1d00ffffU;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), expected_nbits);
+    BOOST_CHECK(PermittedDifficultyTransition(chainParams->GetConsensus(), pindexLast.nHeight+1, pindexLast.nBits, expected_nbits));
 }
 
 /* Test the constraint on the lower bound for actual time taken */
@@ -44,7 +53,12 @@ BOOST_AUTO_TEST_CASE(get_next_work_lower_limit_actual)
     pindexLast.nHeight = 68543;
     pindexLast.nTime = 1279297671;  // Block #68543
     pindexLast.nBits = 0x1c05a3f4;
-    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), 0x1c0168fdU);
+    unsigned int expected_nbits = 0x1c0168fdU;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), expected_nbits);
+    BOOST_CHECK(PermittedDifficultyTransition(chainParams->GetConsensus(), pindexLast.nHeight+1, pindexLast.nBits, expected_nbits));
+    // Test that reducing nbits further would not be a PermittedDifficultyTransition.
+    unsigned int invalid_nbits = expected_nbits-1;
+    BOOST_CHECK(!PermittedDifficultyTransition(chainParams->GetConsensus(), pindexLast.nHeight+1, pindexLast.nBits, invalid_nbits));
 }
 
 /* Test the constraint on the upper bound for actual time taken */
@@ -56,7 +70,12 @@ BOOST_AUTO_TEST_CASE(get_next_work_upper_limit_actual)
     pindexLast.nHeight = 46367;
     pindexLast.nTime = 1269211443;  // Block #46367
     pindexLast.nBits = 0x1c387f6f;
-    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), 0x1d00e1fdU);
+    unsigned int expected_nbits = 0x1d00e1fdU;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, chainParams->GetConsensus()), expected_nbits);
+    BOOST_CHECK(PermittedDifficultyTransition(chainParams->GetConsensus(), pindexLast.nHeight+1, pindexLast.nBits, expected_nbits));
+    // Test that increasing nbits further would not be a PermittedDifficultyTransition.
+    unsigned int invalid_nbits = expected_nbits+1;
+    BOOST_CHECK(!PermittedDifficultyTransition(chainParams->GetConsensus(), pindexLast.nHeight+1, pindexLast.nBits, invalid_nbits));
 }
 
 BOOST_AUTO_TEST_CASE(CheckProofOfWork_test_negative_target)