versionbits: simplify state transitions

This removes the DEFINED->FAILED transition and changes the
STARTED->FAILED transition to only occur if signalling didn't pass the
threshold. This ensures that it is always possible for activation to
occur, no matter what settings are chosen, or the speed at which blocks
are found.

3 files changed, 29 insertions, 32 deletions

diff --git a/src/test/fuzz/versionbits.cpp b/src/test/fuzz/versionbits.cpp
index 4177323233..9186821836 100644
--- a/src/test/fuzz/versionbits.cpp
+++ b/src/test/fuzz/versionbits.cpp
@@ -147,19 +147,14 @@ FUZZ_TARGET_INIT(versionbits, initialize)
         // pick the timestamp to switch based on a block
         // note states will change *after* these blocks because mediantime lags
         int start_block = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, period * (max_periods - 3));
-        int end_block = fuzzed_data_provider.ConsumeIntegralInRange<int>(start_block, period * (max_periods - 3));
+        int end_block = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, period * (max_periods - 3));
 
         start_time = block_start_time + start_block * interval;
         timeout = block_start_time + end_block * interval;
 
-        assert(start_time <= timeout);
-
         // allow for times to not exactly match a block
         if (fuzzed_data_provider.ConsumeBool()) start_time += interval / 2;
         if (fuzzed_data_provider.ConsumeBool()) timeout += interval / 2;
-
-        // this may make timeout too early; if so, don't run the test
-        if (start_time > timeout) return;
     } else {
         if (fuzzed_data_provider.ConsumeBool()) {
             start_time = Consensus::BIP9Deployment::ALWAYS_ACTIVE;
@@ -297,13 +292,12 @@ FUZZ_TARGET_INIT(versionbits, initialize)
         assert(since == 0);
         assert(exp_state == ThresholdState::DEFINED);
         assert(current_block->GetMedianTimePast() < checker.m_begin);
-        assert(current_block->GetMedianTimePast() < checker.m_end);
         break;
     case ThresholdState::STARTED:
         assert(current_block->GetMedianTimePast() >= checker.m_begin);
-        assert(current_block->GetMedianTimePast() < checker.m_end);
         if (exp_state == ThresholdState::STARTED) {
             assert(blocks_sig < threshold);
+            assert(current_block->GetMedianTimePast() < checker.m_end);
         } else {
             assert(exp_state == ThresholdState::DEFINED);
         }
@@ -313,7 +307,6 @@ FUZZ_TARGET_INIT(versionbits, initialize)
             assert(current_block->nHeight + 1 < min_activation);
         } else {
             assert(exp_state == ThresholdState::STARTED);
-            assert(current_block->GetMedianTimePast() < checker.m_end);
             assert(blocks_sig >= threshold);
         }
         break;
@@ -323,7 +316,11 @@ FUZZ_TARGET_INIT(versionbits, initialize)
         break;
     case ThresholdState::FAILED:
         assert(never_active_test || current_block->GetMedianTimePast() >= checker.m_end);
-        assert(exp_state != ThresholdState::LOCKED_IN && exp_state != ThresholdState::ACTIVE);
+        if (exp_state == ThresholdState::STARTED) {
+            assert(blocks_sig < threshold);
+        } else {
+            assert(exp_state == ThresholdState::FAILED);
+        }
         break;
     default:
         assert(false);
diff --git a/src/test/versionbits_tests.cpp b/src/test/versionbits_tests.cpp
index 80072eff9c..05838ec19a 100644
--- a/src/test/versionbits_tests.cpp
+++ b/src/test/versionbits_tests.cpp
@@ -190,18 +190,20 @@ BOOST_FIXTURE_TEST_SUITE(versionbits_tests, TestingSetup)
 BOOST_AUTO_TEST_CASE(versionbits_test)
 {
     for (int i = 0; i < 64; i++) {
-        // DEFINED -> FAILED
+        // DEFINED -> STARTED after timeout reached -> FAILED
         VersionBitsTester().TestDefined().TestStateSinceHeight(0)
                            .Mine(1, TestTime(1), 0x100).TestDefined().TestStateSinceHeight(0)
                            .Mine(11, TestTime(11), 0x100).TestDefined().TestStateSinceHeight(0)
                            .Mine(989, TestTime(989), 0x100).TestDefined().TestStateSinceHeight(0)
-                           .Mine(999, TestTime(20000), 0x100).TestDefined().TestStateSinceHeight(0)
-                           .Mine(1000, TestTime(20000), 0x100).TestFailed().TestStateSinceHeight(1000)
-                           .Mine(1999, TestTime(30001), 0x100).TestFailed().TestStateSinceHeight(1000)
-                           .Mine(2000, TestTime(30002), 0x100).TestFailed().TestStateSinceHeight(1000)
-                           .Mine(2001, TestTime(30003), 0x100).TestFailed().TestStateSinceHeight(1000)
-                           .Mine(2999, TestTime(30004), 0x100).TestFailed().TestStateSinceHeight(1000)
-                           .Mine(3000, TestTime(30005), 0x100).TestFailed().TestStateSinceHeight(1000)
+                           .Mine(999, TestTime(20000), 0x100).TestDefined().TestStateSinceHeight(0) // Timeout and start time reached simultaneously
+                           .Mine(1000, TestTime(20000), 0).TestStarted().TestStateSinceHeight(1000) // Hit started, stop signalling
+                           .Mine(1999, TestTime(30001), 0).TestStarted().TestStateSinceHeight(1000)
+                           .Mine(2000, TestTime(30002), 0x100).TestFailed().TestStateSinceHeight(2000) // Hit failed, start signalling again
+                           .Mine(2001, TestTime(30003), 0x100).TestFailed().TestStateSinceHeight(2000)
+                           .Mine(2999, TestTime(30004), 0x100).TestFailed().TestStateSinceHeight(2000)
+                           .Mine(3000, TestTime(30005), 0x100).TestFailed().TestStateSinceHeight(2000)
+                           .Mine(4000, TestTime(30006), 0x100).TestFailed().TestStateSinceHeight(2000)
+
         // DEFINED -> STARTED -> FAILED
                            .Reset().TestDefined().TestStateSinceHeight(0)
                            .Mine(1, TestTime(1), 0).TestDefined().TestStateSinceHeight(0)
@@ -212,19 +214,19 @@ BOOST_AUTO_TEST_CASE(versionbits_test)
                            .Mine(3000, TestTime(20000), 0).TestFailed().TestStateSinceHeight(3000) // 50 old blocks (so 899 out of the past 1000)
                            .Mine(4000, TestTime(20010), 0x100).TestFailed().TestStateSinceHeight(3000)
 
-        // DEFINED -> STARTED -> FAILED while threshold reached
+        // DEFINED -> STARTED -> LOCKEDIN after timeout reached -> ACTIVE
                            .Reset().TestDefined().TestStateSinceHeight(0)
                            .Mine(1, TestTime(1), 0).TestDefined().TestStateSinceHeight(0)
                            .Mine(1000, TestTime(10000) - 1, 0x101).TestDefined().TestStateSinceHeight(0) // One second more and it would be defined
                            .Mine(2000, TestTime(10000), 0x101).TestStarted().TestStateSinceHeight(2000) // So that's what happens the next period
                            .Mine(2999, TestTime(30000), 0x100).TestStarted().TestStateSinceHeight(2000) // 999 new blocks
-                           .Mine(3000, TestTime(30000), 0x100).TestFailed().TestStateSinceHeight(3000) // 1 new block (so 1000 out of the past 1000 are new)
-                           .Mine(3999, TestTime(30001), 0).TestFailed().TestStateSinceHeight(3000)
-                           .Mine(4000, TestTime(30002), 0).TestFailed().TestStateSinceHeight(3000)
-                           .Mine(14333, TestTime(30003), 0).TestFailed().TestStateSinceHeight(3000)
-                           .Mine(24000, TestTime(40000), 0).TestFailed().TestStateSinceHeight(3000)
+                           .Mine(3000, TestTime(30000), 0x100).TestLockedIn().TestStateSinceHeight(3000) // 1 new block (so 1000 out of the past 1000 are new)
+                           .Mine(3999, TestTime(30001), 0).TestLockedIn().TestStateSinceHeight(3000)
+                           .Mine(4000, TestTime(30002), 0).TestActiveDelayed().TestStateSinceHeight(4000, 3000)
+                           .Mine(14333, TestTime(30003), 0).TestActiveDelayed().TestStateSinceHeight(4000, 3000)
+                           .Mine(24000, TestTime(40000), 0).TestActive().TestStateSinceHeight(4000, 15000)
 
-        // DEFINED -> STARTED -> LOCKEDIN at the last minute -> ACTIVE
+        // DEFINED -> STARTED -> LOCKEDIN before timeout -> ACTIVE
                            .Reset().TestDefined()
                            .Mine(1, TestTime(1), 0).TestDefined().TestStateSinceHeight(0)
                            .Mine(1000, TestTime(10000) - 1, 0x101).TestDefined().TestStateSinceHeight(0) // One second more and it would be defined
@@ -247,8 +249,10 @@ BOOST_AUTO_TEST_CASE(versionbits_test)
                            .Mine(3000, TestTime(10000), 0).TestStarted().TestStateSinceHeight(3000)
                            .Mine(4000, TestTime(10000), 0).TestStarted().TestStateSinceHeight(3000)
                            .Mine(5000, TestTime(10000), 0).TestStarted().TestStateSinceHeight(3000)
+                           .Mine(5999, TestTime(20000), 0).TestStarted().TestStateSinceHeight(3000)
                            .Mine(6000, TestTime(20000), 0).TestFailed().TestStateSinceHeight(6000)
                            .Mine(7000, TestTime(20000), 0x100).TestFailed().TestStateSinceHeight(6000)
+                           .Mine(24000, TestTime(20000), 0x100).TestFailed().TestStateSinceHeight(6000) // stay in FAILED no matter how much we signal
         ;
     }
 }
diff --git a/src/versionbits.cpp b/src/versionbits.cpp
index df666c963f..df2ec4e056 100644
--- a/src/versionbits.cpp
+++ b/src/versionbits.cpp
@@ -57,18 +57,12 @@ ThresholdState AbstractThresholdConditionChecker::GetStateFor(const CBlockIndex*
 
         switch (state) {
             case ThresholdState::DEFINED: {
-                if (pindexPrev->GetMedianTimePast() >= nTimeTimeout) {
-                    stateNext = ThresholdState::FAILED;
-                } else if (pindexPrev->GetMedianTimePast() >= nTimeStart) {
+                if (pindexPrev->GetMedianTimePast() >= nTimeStart) {
                     stateNext = ThresholdState::STARTED;
                 }
                 break;
             }
             case ThresholdState::STARTED: {
-                if (pindexPrev->GetMedianTimePast() >= nTimeTimeout) {
-                    stateNext = ThresholdState::FAILED;
-                    break;
-                }
                 // We need to count
                 const CBlockIndex* pindexCount = pindexPrev;
                 int count = 0;
@@ -80,6 +74,8 @@ ThresholdState AbstractThresholdConditionChecker::GetStateFor(const CBlockIndex*
                 }
                 if (count >= nThreshold) {
                     stateNext = ThresholdState::LOCKED_IN;
+                } else if (pindexPrev->GetMedianTimePast() >= nTimeTimeout) {
+                    stateNext = ThresholdState::FAILED;
                 }
                 break;
             }