Merge #21701: [0.21] Speedy trial activation for Taproot

cbd64c3a28a7466f421477daadc6e6e6b69b898a Add mainnet and testnet taproot activation params (Andrew Chow)
ec7824396bdd2e93b429ddce9fea6bb29695454a chainparams: drop versionbits threshold to 90% for mainnnet and signet (Anthony Towns)
600357306e2e182a457174862ea2e41c7ba39c64 versionbits: simplify state transitions (Anthony Towns)
3acf0379e0979ea4bdd03976f4987aa6711eb92f versionbits: Add explicit NEVER_ACTIVE deployments (Anthony Towns)
b529222ad18f7facbaff394455875b4aa65d653e fuzz: test versionbits delayed activation (Anthony Towns)
71917e01ebf48790b9df48421d8e97986f92e2e4 tests: test versionbits delayed activation (Anthony Towns)
4cab84cfdfc98cd10462681b5eb0fbbc08afd2a7 versionbits: Add support for delayed activation (Anthony Towns)
f9517e6014ccfe91d5a77e2bacca928bdce7c285 tests: clean up versionbits test (Anthony Towns)
1c0164544c66b691f93b3b1114eee97cbabd99b2 tests: test ComputeBlockVersion for all deployments (Anthony Towns)
2e9e7f4329fc313adf9ba2394edbaf2a69b59bc1 tests: pull ComputeBlockVersion test into its own function (Anthony Towns)

Pull request description:

  Backport of #21377 and #21686

ACKs for top commit:
  instagibbs:
    cherry-pick ACK https://github.com/bitcoin/bitcoin/pull/21701/commits/cbd64c3a28a7466f421477daadc6e6e6b69b898a
  jnewbery:
    ACK cbd64c3a28a7466f421477daadc6e6e6b69b898a
  Sjors:
    tACK cbd64c3
  MarcoFalke:
    cherry-pick-only ACK cbd64c3a28a7466f421477daadc6e6e6b69b898a 🌾

Tree-SHA512: e9efb0ca9986d685161bcba5ed43efdc5f1dca88322cf65faccf17009b567c2d930c2aba4d1541539fc65347574ed4faa3d4558b907c779d1c128b3d2c681f31

9 files changed, 318 insertions, 165 deletions

diff --git a/src/chainparams.cpp b/src/chainparams.cpp
index 88cf5ef0a8..0a45b782b9 100644
--- a/src/chainparams.cpp
+++ b/src/chainparams.cpp
@@ -80,16 +80,18 @@ public:
         consensus.nPowTargetSpacing = 10 * 60;
         consensus.fPowAllowMinDifficultyBlocks = false;
         consensus.fPowNoRetargeting = false;
-        consensus.nRuleChangeActivationThreshold = 1916; // 95% of 2016
+        consensus.nRuleChangeActivationThreshold = 1815; // 90% of 2016
         consensus.nMinerConfirmationWindow = 2016; // nPowTargetTimespan / nPowTargetSpacing
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].bit = 28;
-        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = 1199145601; // January 1, 2008
-        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = 1230767999; // December 31, 2008
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = Consensus::BIP9Deployment::NEVER_ACTIVE;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].min_activation_height = 0; // No activation delay
 
         // Deployment of Taproot (BIPs 340-342)
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].bit = 2;
-        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nStartTime = 1199145601; // January 1, 2008
-        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nTimeout = 1230767999; // December 31, 2008
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nStartTime = 1619222400; // April 24th, 2021
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nTimeout = 1628640000; // August 11th, 2021
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].min_activation_height = 709632; // Approximately November 12th, 2021
 
         consensus.nMinimumChainWork = uint256S("0x00000000000000000000000000000000000000001533efd8d716a517fe2c5008");
         consensus.defaultAssumeValid = uint256S("0x0000000000000000000b9d2ec5a352ecba0592946514a92f14319dc2b367fc72"); // 654683
@@ -196,13 +198,15 @@ public:
         consensus.nRuleChangeActivationThreshold = 1512; // 75% for testchains
         consensus.nMinerConfirmationWindow = 2016; // nPowTargetTimespan / nPowTargetSpacing
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].bit = 28;
-        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = 1199145601; // January 1, 2008
-        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = 1230767999; // December 31, 2008
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = Consensus::BIP9Deployment::NEVER_ACTIVE;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].min_activation_height = 0; // No activation delay
 
         // Deployment of Taproot (BIPs 340-342)
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].bit = 2;
-        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nStartTime = 1199145601; // January 1, 2008
-        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nTimeout = 1230767999; // December 31, 2008
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nStartTime = 1619222400; // April 24th, 2021
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nTimeout = 1628640000; // August 11th, 2021
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].min_activation_height = 0; // No activation delay
 
         consensus.nMinimumChainWork = uint256S("0x0000000000000000000000000000000000000000000001db6ec4ac88cf2272c6");
         consensus.defaultAssumeValid = uint256S("0x000000000000006433d1efec504c53ca332b64963c425395515b01977bd7b3b0"); // 1864000
@@ -322,18 +326,20 @@ public:
         consensus.nPowTargetSpacing = 10 * 60;
         consensus.fPowAllowMinDifficultyBlocks = false;
         consensus.fPowNoRetargeting = false;
-        consensus.nRuleChangeActivationThreshold = 1916; // 95% of 2016
+        consensus.nRuleChangeActivationThreshold = 1815; // 90% of 2016
         consensus.nMinerConfirmationWindow = 2016; // nPowTargetTimespan / nPowTargetSpacing
         consensus.MinBIP9WarningHeight = 0;
         consensus.powLimit = uint256S("00000377ae000000000000000000000000000000000000000000000000000000");
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].bit = 28;
-        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = 1199145601; // January 1, 2008
-        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = 1230767999; // December 31, 2008
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = Consensus::BIP9Deployment::NEVER_ACTIVE;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].min_activation_height = 0; // No activation delay
 
         // Activation of Taproot (BIPs 340-342)
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].bit = 2;
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nStartTime = Consensus::BIP9Deployment::ALWAYS_ACTIVE;
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].min_activation_height = 0; // No activation delay
 
         // message start is defined as the first 4 bytes of the sha256d of the block script
         CHashWriter h(SER_DISK, 0);
@@ -391,12 +397,16 @@ public:
         consensus.fPowNoRetargeting = true;
         consensus.nRuleChangeActivationThreshold = 108; // 75% for testchains
         consensus.nMinerConfirmationWindow = 144; // Faster than normal for regtest (144 instead of 2016)
+
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].bit = 28;
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = 0;
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].min_activation_height = 0; // No activation delay
+
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].bit = 2;
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nStartTime = Consensus::BIP9Deployment::ALWAYS_ACTIVE;
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].min_activation_height = 0; // No activation delay
 
         consensus.nMinimumChainWork = uint256{};
         consensus.defaultAssumeValid = uint256{};
@@ -449,10 +459,11 @@ public:
     /**
      * Allows modifying the Version Bits regtest parameters.
      */
-    void UpdateVersionBitsParameters(Consensus::DeploymentPos d, int64_t nStartTime, int64_t nTimeout)
+    void UpdateVersionBitsParameters(Consensus::DeploymentPos d, int64_t nStartTime, int64_t nTimeout, int min_activation_height)
     {
         consensus.vDeployments[d].nStartTime = nStartTime;
         consensus.vDeployments[d].nTimeout = nTimeout;
+        consensus.vDeployments[d].min_activation_height = min_activation_height;
     }
     void UpdateActivationParametersFromArgs(const ArgsManager& args);
 };
@@ -475,22 +486,26 @@ void CRegTestParams::UpdateActivationParametersFromArgs(const ArgsManager& args)
     for (const std::string& strDeployment : args.GetArgs("-vbparams")) {
         std::vector<std::string> vDeploymentParams;
         boost::split(vDeploymentParams, strDeployment, boost::is_any_of(":"));
-        if (vDeploymentParams.size() != 3) {
-            throw std::runtime_error("Version bits parameters malformed, expecting deployment:start:end");
+        if (vDeploymentParams.size() < 3 || 4 < vDeploymentParams.size()) {
+            throw std::runtime_error("Version bits parameters malformed, expecting deployment:start:end[:min_activation_height]");
         }
         int64_t nStartTime, nTimeout;
+        int min_activation_height = 0;
         if (!ParseInt64(vDeploymentParams[1], &nStartTime)) {
             throw std::runtime_error(strprintf("Invalid nStartTime (%s)", vDeploymentParams[1]));
         }
         if (!ParseInt64(vDeploymentParams[2], &nTimeout)) {
             throw std::runtime_error(strprintf("Invalid nTimeout (%s)", vDeploymentParams[2]));
         }
+        if (vDeploymentParams.size() >= 4 && !ParseInt32(vDeploymentParams[3], &min_activation_height)) {
+            throw std::runtime_error(strprintf("Invalid min_activation_height (%s)", vDeploymentParams[3]));
+        }
         bool found = false;
         for (int j=0; j < (int)Consensus::MAX_VERSION_BITS_DEPLOYMENTS; ++j) {
             if (vDeploymentParams[0] == VersionBitsDeploymentInfo[j].name) {
-                UpdateVersionBitsParameters(Consensus::DeploymentPos(j), nStartTime, nTimeout);
+                UpdateVersionBitsParameters(Consensus::DeploymentPos(j), nStartTime, nTimeout, min_activation_height);
                 found = true;
-                LogPrintf("Setting version bits activation parameters for %s to start=%ld, timeout=%ld\n", vDeploymentParams[0], nStartTime, nTimeout);
+                LogPrintf("Setting version bits activation parameters for %s to start=%ld, timeout=%ld, min_activation_height=%d\n", vDeploymentParams[0], nStartTime, nTimeout, min_activation_height);
                 break;
             }
         }
diff --git a/src/chainparamsbase.cpp b/src/chainparamsbase.cpp
index 603969aaea..be33bb94c8 100644
--- a/src/chainparamsbase.cpp
+++ b/src/chainparamsbase.cpp
@@ -23,7 +23,7 @@ void SetupChainParamsBaseOptions(ArgsManager& argsman)
                  "This is intended for regression testing tools and app development. Equivalent to -chain=regtest.", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-segwitheight=<n>", "Set the activation height of segwit. -1 to disable. (regtest-only)", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
     argsman.AddArg("-testnet", "Use the test chain. Equivalent to -chain=test.", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);
-    argsman.AddArg("-vbparams=deployment:start:end", "Use given start/end times for specified version bits deployment (regtest-only)", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::CHAINPARAMS);
+    argsman.AddArg("-vbparams=deployment:start:end[:min_activation_height]", "Use given start/end times and min_activation_height for specified version bits deployment (regtest-only)", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-signet", "Use the signet chain. Equivalent to -chain=signet. Note that the network is defined by the -signetchallenge parameter", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-signetchallenge", "Blocks must satisfy the given script to be considered valid (only for signet networks; defaults to the global default signet test network challenge)", ArgsManager::ALLOW_STRING, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-signetseednode", "Specify a seed node for the signet network, in the hostname[:port] format, e.g. sig.net:1234 (may be used multiple times to specify multiple seed nodes; defaults to the global default signet test network seed node(s))", ArgsManager::ALLOW_STRING, OptionsCategory::CHAINPARAMS);
diff --git a/src/consensus/params.h b/src/consensus/params.h
index 0983595c6a..1b1c7f24b6 100644
--- a/src/consensus/params.h
+++ b/src/consensus/params.h
@@ -29,6 +29,11 @@ struct BIP9Deployment {
     int64_t nStartTime;
     /** Timeout/expiry MedianTime for the deployment attempt. */
     int64_t nTimeout;
+    /** If lock in occurs, delay activation until at least this block
+     *  height.  Note that activation will only occur on a retarget
+     *  boundary.
+     */
+    int min_activation_height{0};
 
     /** Constant for nTimeout very far in the future. */
     static constexpr int64_t NO_TIMEOUT = std::numeric_limits<int64_t>::max();
@@ -38,6 +43,11 @@ struct BIP9Deployment {
      *  process (which takes at least 3 BIP9 intervals). Only tests that specifically test the
      *  behaviour during activation cannot use this. */
     static constexpr int64_t ALWAYS_ACTIVE = -1;
+
+    /** Special value for nStartTime indicating that the deployment is never active.
+     *  This is useful for integrating the code changes for a new feature
+     *  prior to deploying it on some or all networks. */
+    static constexpr int64_t NEVER_ACTIVE = -2;
 };
 
 /**
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 392073d047..90cf8d6d14 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -1214,10 +1214,8 @@ static void BuriedForkDescPushBack(UniValue& softforks, const std::string &name,
 static void BIP9SoftForkDescPushBack(UniValue& softforks, const std::string &name, const Consensus::Params& consensusParams, Consensus::DeploymentPos id) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     // For BIP9 deployments.
-    // Deployments (e.g. testdummy) with timeout value before Jan 1, 2009 are hidden.
-    // A timeout value of 0 guarantees a softfork will never be activated.
-    // This is used when merging logic to implement a proposed softfork without a specified deployment schedule.
-    if (consensusParams.vDeployments[id].nTimeout <= 1230768000) return;
+    // Deployments that are never active are hidden.
+    if (consensusParams.vDeployments[id].nStartTime == Consensus::BIP9Deployment::NEVER_ACTIVE) return;
 
     UniValue bip9(UniValue::VOBJ);
     const ThresholdState thresholdState = VersionBitsTipState(consensusParams, id);
@@ -1247,6 +1245,7 @@ static void BIP9SoftForkDescPushBack(UniValue& softforks, const std::string &nam
         statsUV.pushKV("possible", statsStruct.possible);
         bip9.pushKV("statistics", statsUV);
     }
+    bip9.pushKV("min_activation_height", consensusParams.vDeployments[id].min_activation_height);
 
     UniValue rv(UniValue::VOBJ);
     rv.pushKV("type", "bip9");
@@ -1293,6 +1292,7 @@ RPCHelpMan getblockchaininfo()
                                     {RPCResult::Type::NUM_TIME, "start_time", "the minimum median time past of a block at which the bit gains its meaning"},
                                     {RPCResult::Type::NUM_TIME, "timeout", "the median time past of a block at which the deployment is considered failed if not yet locked in"},
                                     {RPCResult::Type::NUM, "since", "height of the first block to which the status applies"},
+                                    {RPCResult::Type::NUM, "min_activation_height", "minimum height of blocks for which the rules may be enforced"},
                                     {RPCResult::Type::OBJ, "statistics", "numeric statistics about BIP9 signalling for a softfork (only for \"started\" status)",
                                     {
                                         {RPCResult::Type::NUM, "period", "the length in blocks of the BIP9 signalling period"},
diff --git a/src/test/fuzz/versionbits.cpp b/src/test/fuzz/versionbits.cpp
index 992a5c1321..645c0d23c0 100644
--- a/src/test/fuzz/versionbits.cpp
+++ b/src/test/fuzz/versionbits.cpp
@@ -25,18 +25,20 @@ private:
     const Consensus::Params dummy_params{};
 
 public:
-    const int64_t m_begin = 0;
-    const int64_t m_end = 0;
-    const int m_period = 0;
-    const int m_threshold = 0;
-    const int m_bit = 0;
-
-    TestConditionChecker(int64_t begin, int64_t end, int period, int threshold, int bit)
-        : m_begin{begin}, m_end{end}, m_period{period}, m_threshold{threshold}, m_bit{bit}
+    const int64_t m_begin;
+    const int64_t m_end;
+    const int m_period;
+    const int m_threshold;
+    const int m_min_activation_height;
+    const int m_bit;
+
+    TestConditionChecker(int64_t begin, int64_t end, int period, int threshold, int min_activation_height, int bit)
+        : m_begin{begin}, m_end{end}, m_period{period}, m_threshold{threshold}, m_min_activation_height{min_activation_height}, m_bit{bit}
     {
         assert(m_period > 0);
         assert(0 <= m_threshold && m_threshold <= m_period);
-        assert(0 <= m_bit && m_bit <= 32 && m_bit < VERSIONBITS_NUM_BITS);
+        assert(0 <= m_bit && m_bit < 32 && m_bit < VERSIONBITS_NUM_BITS);
+        assert(0 <= m_min_activation_height);
     }
 
     bool Condition(const CBlockIndex* pindex, const Consensus::Params& params) const override { return Condition(pindex->nVersion); }
@@ -44,6 +46,7 @@ public:
     int64_t EndTime(const Consensus::Params& params) const override { return m_end; }
     int Period(const Consensus::Params& params) const override { return m_period; }
     int Threshold(const Consensus::Params& params) const override { return m_threshold; }
+    int MinActivationHeight(const Consensus::Params& params) const override { return m_min_activation_height; }
 
     ThresholdState GetStateFor(const CBlockIndex* pindexPrev) const { return AbstractThresholdConditionChecker::GetStateFor(pindexPrev, dummy_params, m_cache); }
     int GetStateSinceHeightFor(const CBlockIndex* pindexPrev) const { return AbstractThresholdConditionChecker::GetStateSinceHeightFor(pindexPrev, dummy_params, m_cache); }
@@ -141,32 +144,27 @@ void test_one_input(const std::vector<uint8_t>& buffer)
         // 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;
-            timeout = Consensus::BIP9Deployment::NO_TIMEOUT;
             always_active_test = true;
         } else {
-            start_time = 1199145601; // January 1, 2008
-            timeout = 1230767999;    // December 31, 2008
+            start_time = Consensus::BIP9Deployment::NEVER_ACTIVE;
             never_active_test = true;
         }
+        timeout = fuzzed_data_provider.ConsumeBool() ? Consensus::BIP9Deployment::NO_TIMEOUT : fuzzed_data_provider.ConsumeIntegral<int64_t>();
     }
+    int min_activation = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, period * max_periods);
 
-    TestConditionChecker checker(start_time, timeout, period, threshold, bit);
+    TestConditionChecker checker(start_time, timeout, period, threshold, min_activation, bit);
 
     // Early exit if the versions don't signal sensibly for the deployment
     if (!checker.Condition(ver_signal)) return;
@@ -289,28 +287,35 @@ void test_one_input(const std::vector<uint8_t>& buffer)
         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);
         }
         break;
     case ThresholdState::LOCKED_IN:
-        assert(exp_state == ThresholdState::STARTED);
-        assert(current_block->GetMedianTimePast() < checker.m_end);
-        assert(blocks_sig >= threshold);
+        if (exp_state == ThresholdState::LOCKED_IN) {
+            assert(current_block->nHeight + 1 < min_activation);
+        } else {
+            assert(exp_state == ThresholdState::STARTED);
+            assert(blocks_sig >= threshold);
+        }
         break;
     case ThresholdState::ACTIVE:
+        assert(always_active_test || min_activation <= current_block->nHeight + 1);
         assert(exp_state == ThresholdState::ACTIVE || exp_state == ThresholdState::LOCKED_IN);
         break;
     case ThresholdState::FAILED:
-        assert(current_block->GetMedianTimePast() >= checker.m_end);
-        assert(exp_state != ThresholdState::LOCKED_IN && exp_state != ThresholdState::ACTIVE);
+        assert(never_active_test || current_block->GetMedianTimePast() >= checker.m_end);
+        if (exp_state == ThresholdState::STARTED) {
+            assert(blocks_sig < threshold);
+        } else {
+            assert(exp_state == ThresholdState::FAILED);
+        }
         break;
     default:
         assert(false);
@@ -321,25 +326,19 @@ void test_one_input(const std::vector<uint8_t>& buffer)
         assert(state == ThresholdState::ACTIVE || state == ThresholdState::FAILED);
     }
 
-    // "always active" has additional restrictions
     if (always_active_test) {
+        // "always active" has additional restrictions
         assert(state == ThresholdState::ACTIVE);
         assert(exp_state == ThresholdState::ACTIVE);
         assert(since == 0);
+    } else if (never_active_test) {
+        // "never active" does too
+        assert(state == ThresholdState::FAILED);
+        assert(exp_state == ThresholdState::FAILED);
+        assert(since == 0);
     } else {
-        // except for always active, the initial state is always DEFINED
+        // for signalled deployments, the initial state is always DEFINED
         assert(since > 0 || state == ThresholdState::DEFINED);
         assert(exp_since > 0 || exp_state == ThresholdState::DEFINED);
     }
-
-    // "never active" does too
-    if (never_active_test) {
-        assert(state == ThresholdState::FAILED);
-        assert(since == period);
-        if (exp_since == 0) {
-            assert(exp_state == ThresholdState::DEFINED);
-        } else {
-            assert(exp_state == ThresholdState::FAILED);
-        }
-    }
 }
diff --git a/src/test/versionbits_tests.cpp b/src/test/versionbits_tests.cpp
index 8841a540f2..774c5670cb 100644
--- a/src/test/versionbits_tests.cpp
+++ b/src/test/versionbits_tests.cpp
@@ -44,6 +44,12 @@ public:
     int GetStateSinceHeightFor(const CBlockIndex* pindexPrev) const { return AbstractThresholdConditionChecker::GetStateSinceHeightFor(pindexPrev, paramsDummy, cache); }
 };
 
+class TestDelayedActivationConditionChecker : public TestConditionChecker
+{
+public:
+    int MinActivationHeight(const Consensus::Params& params) const override { return 15000; }
+};
+
 class TestAlwaysActiveConditionChecker : public TestConditionChecker
 {
 public:
@@ -53,8 +59,7 @@ public:
 class TestNeverActiveConditionChecker : public TestConditionChecker
 {
 public:
-    int64_t BeginTime(const Consensus::Params& params) const override { return 0; }
-    int64_t EndTime(const Consensus::Params& params) const override { return 1230768000; }
+    int64_t BeginTime(const Consensus::Params& params) const override { return Consensus::BIP9Deployment::NEVER_ACTIVE; }
 };
 
 #define CHECKERS 6
@@ -68,6 +73,8 @@ class VersionBitsTester
     // The first one performs all checks, the second only 50%, the third only 25%, etc...
     // This is to test whether lack of cached information leads to the same results.
     TestConditionChecker checker[CHECKERS];
+    // Another 6 that assume delayed activation
+    TestDelayedActivationConditionChecker checker_delayed[CHECKERS];
     // Another 6 that assume always active activation
     TestAlwaysActiveConditionChecker checker_always[CHECKERS];
     // Another 6 that assume never active activation
@@ -77,14 +84,18 @@ class VersionBitsTester
     int num;
 
 public:
-    VersionBitsTester() : num(0) {}
+    VersionBitsTester() : num(1000) {}
 
     VersionBitsTester& Reset() {
+        // Have each group of tests be counted by the 1000s part, starting at 1000
+        num = num - (num % 1000) + 1000;
+
         for (unsigned int i = 0; i < vpblock.size(); i++) {
             delete vpblock[i];
         }
         for (unsigned int  i = 0; i < CHECKERS; i++) {
             checker[i] = TestConditionChecker();
+            checker_delayed[i] = TestDelayedActivationConditionChecker();
             checker_always[i] = TestAlwaysActiveConditionChecker();
             checker_never[i] = TestNeverActiveConditionChecker();
         }
@@ -100,7 +111,7 @@ public:
         while (vpblock.size() < height) {
             CBlockIndex* pindex = new CBlockIndex();
             pindex->nHeight = vpblock.size();
-            pindex->pprev = vpblock.size() > 0 ? vpblock.back() : nullptr;
+            pindex->pprev = Tip();
             pindex->nTime = nTime;
             pindex->nVersion = nVersion;
             pindex->BuildSkip();
@@ -109,34 +120,53 @@ public:
         return *this;
     }
 
-    VersionBitsTester& TestStateSinceHeight(int height) {
+    VersionBitsTester& TestStateSinceHeight(int height)
+    {
+        return TestStateSinceHeight(height, height);
+    }
+
+    VersionBitsTester& TestStateSinceHeight(int height, int height_delayed)
+    {
+        const CBlockIndex* tip = Tip();
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                BOOST_CHECK_MESSAGE(checker[i].GetStateSinceHeightFor(vpblock.empty() ? nullptr : vpblock.back()) == height, strprintf("Test %i for StateSinceHeight", num));
-                BOOST_CHECK_MESSAGE(checker_always[i].GetStateSinceHeightFor(vpblock.empty() ? nullptr : vpblock.back()) == 0, strprintf("Test %i for StateSinceHeight (always active)", num));
-
-                // never active may go from DEFINED -> FAILED at the first period
-                const auto never_height = checker_never[i].GetStateSinceHeightFor(vpblock.empty() ? nullptr : vpblock.back());
-                BOOST_CHECK_MESSAGE(never_height == 0 || never_height == checker_never[i].Period(paramsDummy), strprintf("Test %i for StateSinceHeight (never active)", num));
+                BOOST_CHECK_MESSAGE(checker[i].GetStateSinceHeightFor(tip) == height, strprintf("Test %i for StateSinceHeight", num));
+                BOOST_CHECK_MESSAGE(checker_delayed[i].GetStateSinceHeightFor(tip) == height_delayed, strprintf("Test %i for StateSinceHeight (delayed)", num));
+                BOOST_CHECK_MESSAGE(checker_always[i].GetStateSinceHeightFor(tip) == 0, strprintf("Test %i for StateSinceHeight (always active)", num));
+                BOOST_CHECK_MESSAGE(checker_never[i].GetStateSinceHeightFor(tip) == 0, strprintf("Test %i for StateSinceHeight (never active)", num));
             }
         }
         num++;
         return *this;
     }
 
-    VersionBitsTester& TestState(ThresholdState exp) {
+    VersionBitsTester& TestState(ThresholdState exp)
+    {
+        return TestState(exp, exp);
+    }
+
+    VersionBitsTester& TestState(ThresholdState exp, ThresholdState exp_delayed)
+    {
+        if (exp != exp_delayed) {
+            // only expected differences are that delayed stays in locked_in longer
+            BOOST_CHECK_EQUAL(exp, ThresholdState::ACTIVE);
+            BOOST_CHECK_EQUAL(exp_delayed, ThresholdState::LOCKED_IN);
+        }
+
+        const CBlockIndex* pindex = Tip();
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                const CBlockIndex* pindex = vpblock.empty() ? nullptr : vpblock.back();
                 ThresholdState got = checker[i].GetStateFor(pindex);
+                ThresholdState got_delayed = checker_delayed[i].GetStateFor(pindex);
                 ThresholdState got_always = checker_always[i].GetStateFor(pindex);
                 ThresholdState got_never = checker_never[i].GetStateFor(pindex);
                 // nHeight of the next block. If vpblock is empty, the next (ie first)
                 // block should be the genesis block with nHeight == 0.
                 int height = pindex == nullptr ? 0 : pindex->nHeight + 1;
                 BOOST_CHECK_MESSAGE(got == exp, strprintf("Test %i for %s height %d (got %s)", num, StateName(exp), height, StateName(got)));
+                BOOST_CHECK_MESSAGE(got_delayed == exp_delayed, strprintf("Test %i for %s height %d (got %s; delayed case)", num, StateName(exp_delayed), height, StateName(got_delayed)));
                 BOOST_CHECK_MESSAGE(got_always == ThresholdState::ACTIVE, strprintf("Test %i for ACTIVE height %d (got %s; always active case)", num, height, StateName(got_always)));
-                BOOST_CHECK_MESSAGE(got_never == ThresholdState::DEFINED|| got_never == ThresholdState::FAILED, strprintf("Test %i for DEFINED/FAILED height %d (got %s; never active case)", num, height, StateName(got_never)));
+                BOOST_CHECK_MESSAGE(got_never == ThresholdState::FAILED, strprintf("Test %i for FAILED height %d (got %s; never active case)", num, height, StateName(got_never)));
             }
         }
         num++;
@@ -149,7 +179,10 @@ public:
     VersionBitsTester& TestActive() { return TestState(ThresholdState::ACTIVE); }
     VersionBitsTester& TestFailed() { return TestState(ThresholdState::FAILED); }
 
-    CBlockIndex * Tip() { return vpblock.size() ? vpblock.back() : nullptr; }
+    // non-delayed should be active; delayed should still be locked in
+    VersionBitsTester& TestActiveDelayed() { return TestState(ThresholdState::ACTIVE, ThresholdState::LOCKED_IN); }
+
+    CBlockIndex* Tip() { return vpblock.empty() ? nullptr : vpblock.back(); }
 };
 
 BOOST_FIXTURE_TEST_SUITE(versionbits_tests, TestingSetup)
@@ -157,18 +190,19 @@ 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)
@@ -180,19 +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
@@ -202,9 +236,10 @@ BOOST_AUTO_TEST_CASE(versionbits_test)
                            .Mine(2999, TestTime(19999), 0x200).TestStarted().TestStateSinceHeight(2000) // 49 old blocks
                            .Mine(3000, TestTime(29999), 0x200).TestLockedIn().TestStateSinceHeight(3000) // 1 old block (so 900 out of the past 1000)
                            .Mine(3999, TestTime(30001), 0).TestLockedIn().TestStateSinceHeight(3000)
-                           .Mine(4000, TestTime(30002), 0).TestActive().TestStateSinceHeight(4000)
-                           .Mine(14333, TestTime(30003), 0).TestActive().TestStateSinceHeight(4000)
-                           .Mine(24000, TestTime(40000), 0).TestActive().TestStateSinceHeight(4000)
+                           .Mine(4000, TestTime(30002), 0).TestActiveDelayed().TestStateSinceHeight(4000, 3000) // delayed will not become active until height=15000
+                           .Mine(14333, TestTime(30003), 0).TestActiveDelayed().TestStateSinceHeight(4000, 3000)
+                           .Mine(15000, TestTime(40000), 0).TestActive().TestStateSinceHeight(4000, 15000)
+                           .Mine(24000, TestTime(40000), 0).TestActive().TestStateSinceHeight(4000, 15000)
 
         // DEFINED multiple periods -> STARTED multiple periods -> FAILED
                            .Reset().TestDefined().TestStateSinceHeight(0)
@@ -214,10 +249,16 @@ 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(7000, TestTime(20000), 0x100).TestFailed().TestStateSinceHeight(6000)
+                           .Mine(24000, TestTime(20000), 0x100).TestFailed().TestStateSinceHeight(6000) // stay in FAILED no matter how much we signal
+        ;
     }
+}
 
+BOOST_AUTO_TEST_CASE(versionbits_sanity)
+{
     // Sanity checks of version bit deployments
     const auto chainParams = CreateChainParams(*m_node.args, CBaseChainParams::MAIN);
     const Consensus::Params &mainnetParams = chainParams->GetConsensus();
@@ -226,6 +267,13 @@ BOOST_AUTO_TEST_CASE(versionbits_test)
         // Make sure that no deployment tries to set an invalid bit.
         BOOST_CHECK_EQUAL(bitmask & ~(uint32_t)VERSIONBITS_TOP_MASK, bitmask);
 
+        // Check min_activation_height is on a retarget boundary
+        BOOST_CHECK_EQUAL(mainnetParams.vDeployments[i].min_activation_height % mainnetParams.nMinerConfirmationWindow, 0U);
+        // Check min_activation_height is 0 for ALWAYS_ACTIVE and never active deployments
+        if (mainnetParams.vDeployments[i].nStartTime == Consensus::BIP9Deployment::ALWAYS_ACTIVE || mainnetParams.vDeployments[i].nStartTime == Consensus::BIP9Deployment::NEVER_ACTIVE) {
+            BOOST_CHECK_EQUAL(mainnetParams.vDeployments[i].min_activation_height, 0);
+        }
+
         // Verify that the deployment windows of different deployment using the
         // same bit are disjoint.
         // This test may need modification at such time as a new deployment
@@ -242,81 +290,118 @@ BOOST_AUTO_TEST_CASE(versionbits_test)
     }
 }
 
-BOOST_AUTO_TEST_CASE(versionbits_computeblockversion)
+/** Check that ComputeBlockVersion will set the appropriate bit correctly */
+static void check_computeblockversion(const Consensus::Params& params, Consensus::DeploymentPos dep)
 {
-    // Check that ComputeBlockVersion will set the appropriate bit correctly
-    // on mainnet.
-    const auto chainParams = CreateChainParams(*m_node.args, CBaseChainParams::MAIN);
-    const Consensus::Params &mainnetParams = chainParams->GetConsensus();
+    // This implicitly uses versionbitscache, so clear it every time
+    versionbitscache.Clear();
+
+    int64_t bit = params.vDeployments[dep].bit;
+    int64_t nStartTime = params.vDeployments[dep].nStartTime;
+    int64_t nTimeout = params.vDeployments[dep].nTimeout;
+    int min_activation_height = params.vDeployments[dep].min_activation_height;
+
+    // should not be any signalling for first block
+    BOOST_CHECK_EQUAL(ComputeBlockVersion(nullptr, params), VERSIONBITS_TOP_BITS);
 
-    // Use the TESTDUMMY deployment for testing purposes.
-    int64_t bit = mainnetParams.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].bit;
-    int64_t nStartTime = mainnetParams.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime;
-    int64_t nTimeout = mainnetParams.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout;
+    // always/never active deployments shouldn't need to be tested further
+    if (nStartTime == Consensus::BIP9Deployment::ALWAYS_ACTIVE) return;
+    if (nStartTime == Consensus::BIP9Deployment::NEVER_ACTIVE) return;
 
-    assert(nStartTime < nTimeout);
+    BOOST_REQUIRE(nStartTime < nTimeout);
+    BOOST_REQUIRE(nStartTime >= 0);
+    BOOST_REQUIRE(nTimeout <= std::numeric_limits<uint32_t>::max() || nTimeout == Consensus::BIP9Deployment::NO_TIMEOUT);
+    BOOST_REQUIRE(0 <= bit && bit < 32);
+    BOOST_REQUIRE(((1 << bit) & VERSIONBITS_TOP_MASK) == 0);
+    BOOST_REQUIRE(min_activation_height >= 0);
+    BOOST_REQUIRE_EQUAL(min_activation_height % params.nMinerConfirmationWindow, 0U);
 
     // In the first chain, test that the bit is set by CBV until it has failed.
     // In the second chain, test the bit is set by CBV while STARTED and
     // LOCKED-IN, and then no longer set while ACTIVE.
     VersionBitsTester firstChain, secondChain;
 
-    // Start generating blocks before nStartTime
-    int64_t nTime = nStartTime - 1;
+    int64_t nTime = nStartTime;
+
+    const CBlockIndex *lastBlock = nullptr;
 
     // Before MedianTimePast of the chain has crossed nStartTime, the bit
     // should not be set.
-    CBlockIndex *lastBlock = nullptr;
-    lastBlock = firstChain.Mine(mainnetParams.nMinerConfirmationWindow, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-    BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit), 0);
-
-    // Mine more blocks (4 less than the adjustment period) at the old time, and check that CBV isn't setting the bit yet.
-    for (uint32_t i = 1; i < mainnetParams.nMinerConfirmationWindow - 4; i++) {
-        lastBlock = firstChain.Mine(mainnetParams.nMinerConfirmationWindow + i, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-        // This works because VERSIONBITS_LAST_OLD_BLOCK_VERSION happens
-        // to be 4, and the bit we're testing happens to be bit 28.
-        BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit), 0);
-    }
-    // Now mine 5 more blocks at the start time -- MTP should not have passed yet, so
-    // CBV should still not yet set the bit.
-    nTime = nStartTime;
-    for (uint32_t i = mainnetParams.nMinerConfirmationWindow - 4; i <= mainnetParams.nMinerConfirmationWindow; i++) {
-        lastBlock = firstChain.Mine(mainnetParams.nMinerConfirmationWindow + i, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-        BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit), 0);
+    if (nTime == 0) {
+        // since CBlockIndex::nTime is uint32_t we can't represent any
+        // earlier time, so will transition from DEFINED to STARTED at the
+        // end of the first period by mining blocks at nTime == 0
+        lastBlock = firstChain.Mine(params.nMinerConfirmationWindow - 1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+        BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & (1<<bit), 0);
+        lastBlock = firstChain.Mine(params.nMinerConfirmationWindow, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+        BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1<<bit)) != 0);
+        // then we'll keep mining at nStartTime...
+    } else {
+        // use a time 1s earlier than start time to check we stay DEFINED
+        --nTime;
+
+        // Start generating blocks before nStartTime
+        lastBlock = firstChain.Mine(params.nMinerConfirmationWindow, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+        BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & (1<<bit), 0);
+
+        // Mine more blocks (4 less than the adjustment period) at the old time, and check that CBV isn't setting the bit yet.
+        for (uint32_t i = 1; i < params.nMinerConfirmationWindow - 4; i++) {
+            lastBlock = firstChain.Mine(params.nMinerConfirmationWindow + i, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+            BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & (1<<bit), 0);
+        }
+        // Now mine 5 more blocks at the start time -- MTP should not have passed yet, so
+        // CBV should still not yet set the bit.
+        nTime = nStartTime;
+        for (uint32_t i = params.nMinerConfirmationWindow - 4; i <= params.nMinerConfirmationWindow; i++) {
+            lastBlock = firstChain.Mine(params.nMinerConfirmationWindow + i, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+            BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & (1<<bit), 0);
+        }
+        // Next we will advance to the next period and transition to STARTED,
     }
 
-    // Advance to the next period and transition to STARTED,
-    lastBlock = firstChain.Mine(mainnetParams.nMinerConfirmationWindow * 3, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+    lastBlock = firstChain.Mine(params.nMinerConfirmationWindow * 3, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
     // so ComputeBlockVersion should now set the bit,
-    BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
+    BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1<<bit)) != 0);
     // and should also be using the VERSIONBITS_TOP_BITS.
-    BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & VERSIONBITS_TOP_MASK, VERSIONBITS_TOP_BITS);
+    BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & VERSIONBITS_TOP_MASK, VERSIONBITS_TOP_BITS);
 
     // Check that ComputeBlockVersion will set the bit until nTimeout
     nTime += 600;
-    uint32_t blocksToMine = mainnetParams.nMinerConfirmationWindow * 2; // test blocks for up to 2 time periods
-    uint32_t nHeight = mainnetParams.nMinerConfirmationWindow * 3;
+    uint32_t blocksToMine = params.nMinerConfirmationWindow * 2; // test blocks for up to 2 time periods
+    uint32_t nHeight = params.nMinerConfirmationWindow * 3;
     // These blocks are all before nTimeout is reached.
     while (nTime < nTimeout && blocksToMine > 0) {
         lastBlock = firstChain.Mine(nHeight+1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-        BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
-        BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & VERSIONBITS_TOP_MASK, VERSIONBITS_TOP_BITS);
+        BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1<<bit)) != 0);
+        BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & VERSIONBITS_TOP_MASK, VERSIONBITS_TOP_BITS);
         blocksToMine--;
         nTime += 600;
         nHeight += 1;
     }
 
-    nTime = nTimeout;
-    // FAILED is only triggered at the end of a period, so CBV should be setting
-    // the bit until the period transition.
-    for (uint32_t i = 0; i < mainnetParams.nMinerConfirmationWindow - 1; i++) {
+    if (nTimeout != Consensus::BIP9Deployment::NO_TIMEOUT) {
+        // can reach any nTimeout other than NO_TIMEOUT due to earlier BOOST_REQUIRE
+
+        nTime = nTimeout;
+
+        // finish the last period before we start timing out
+        while (nHeight % params.nMinerConfirmationWindow != 0) {
+            lastBlock = firstChain.Mine(nHeight+1, nTime - 1, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+            BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1<<bit)) != 0);
+            nHeight += 1;
+        }
+
+        // FAILED is only triggered at the end of a period, so CBV should be setting
+        // the bit until the period transition.
+        for (uint32_t i = 0; i < params.nMinerConfirmationWindow - 1; i++) {
+            lastBlock = firstChain.Mine(nHeight+1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+            BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1<<bit)) != 0);
+            nHeight += 1;
+        }
+        // The next block should trigger no longer setting the bit.
         lastBlock = firstChain.Mine(nHeight+1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-        BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
-        nHeight += 1;
+        BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & (1<<bit), 0);
     }
-    // The next block should trigger no longer setting the bit.
-    lastBlock = firstChain.Mine(nHeight+1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-    BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit), 0);
 
     // On a new chain:
     // verify that the bit will be set after lock-in, and then stop being set
@@ -325,26 +410,62 @@ BOOST_AUTO_TEST_CASE(versionbits_computeblockversion)
 
     // Mine one period worth of blocks, and check that the bit will be on for the
     // next period.
-    lastBlock = secondChain.Mine(mainnetParams.nMinerConfirmationWindow, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-    BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
+    lastBlock = secondChain.Mine(params.nMinerConfirmationWindow, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+    BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1<<bit)) != 0);
 
     // Mine another period worth of blocks, signaling the new bit.
-    lastBlock = secondChain.Mine(mainnetParams.nMinerConfirmationWindow * 2, nTime, VERSIONBITS_TOP_BITS | (1<<bit)).Tip();
+    lastBlock = secondChain.Mine(params.nMinerConfirmationWindow * 2, nTime, VERSIONBITS_TOP_BITS | (1<<bit)).Tip();
     // After one period of setting the bit on each block, it should have locked in.
     // We keep setting the bit for one more period though, until activation.
-    BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
+    BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1<<bit)) != 0);
 
     // Now check that we keep mining the block until the end of this period, and
     // then stop at the beginning of the next period.
-    lastBlock = secondChain.Mine((mainnetParams.nMinerConfirmationWindow * 3) - 1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-    BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1 << bit)) != 0);
-    lastBlock = secondChain.Mine(mainnetParams.nMinerConfirmationWindow * 3, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
-    BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit), 0);
-
-    // Finally, verify that after a soft fork has activated, CBV no longer uses
-    // VERSIONBITS_LAST_OLD_BLOCK_VERSION.
-    //BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & VERSIONBITS_TOP_MASK, VERSIONBITS_TOP_BITS);
+    lastBlock = secondChain.Mine((params.nMinerConfirmationWindow * 3) - 1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+    BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1 << bit)) != 0);
+    lastBlock = secondChain.Mine(params.nMinerConfirmationWindow * 3, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+
+    if (lastBlock->nHeight + 1 < min_activation_height) {
+        // check signalling continues while min_activation_height is not reached
+        lastBlock = secondChain.Mine(min_activation_height - 1, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+        BOOST_CHECK((ComputeBlockVersion(lastBlock, params) & (1 << bit)) != 0);
+        // then reach min_activation_height, which was already REQUIRE'd to start a new period
+        lastBlock = secondChain.Mine(min_activation_height, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+    }
+
+    // Check that we don't signal after activation
+    BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, params) & (1<<bit), 0);
 }
 
+BOOST_AUTO_TEST_CASE(versionbits_computeblockversion)
+{
+    // check that any deployment on any chain can conceivably reach both
+    // ACTIVE and FAILED states in roughly the way we expect
+    for (const auto& chain_name : {CBaseChainParams::MAIN, CBaseChainParams::TESTNET, CBaseChainParams::SIGNET, CBaseChainParams::REGTEST}) {
+        const auto chainParams = CreateChainParams(*m_node.args, chain_name);
+        for (int i = 0; i < (int)Consensus::MAX_VERSION_BITS_DEPLOYMENTS; ++i) {
+            check_computeblockversion(chainParams->GetConsensus(), static_cast<Consensus::DeploymentPos>(i));
+        }
+    }
+
+    {
+        // Use regtest/testdummy to ensure we always exercise some
+        // deployment that's not always/never active
+        ArgsManager args;
+        args.ForceSetArg("-vbparams", "testdummy:1199145601:1230767999"); // January 1, 2008 - December 31, 2008
+        const auto chainParams = CreateChainParams(args, CBaseChainParams::REGTEST);
+        check_computeblockversion(chainParams->GetConsensus(), Consensus::DEPLOYMENT_TESTDUMMY);
+    }
+
+    {
+        // Use regtest/testdummy to ensure we always exercise the
+        // min_activation_height test, even if we're not using that in a
+        // live deployment
+        ArgsManager args;
+        args.ForceSetArg("-vbparams", "testdummy:1199145601:1230767999:403200"); // January 1, 2008 - December 31, 2008, min act height 403200
+        const auto chainParams = CreateChainParams(args, CBaseChainParams::REGTEST);
+        check_computeblockversion(chainParams->GetConsensus(), Consensus::DEPLOYMENT_TESTDUMMY);
+    }
+}
 
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/versionbits.cpp b/src/versionbits.cpp
index af07c67ccf..df2ec4e056 100644
--- a/src/versionbits.cpp
+++ b/src/versionbits.cpp
@@ -9,6 +9,7 @@ ThresholdState AbstractThresholdConditionChecker::GetStateFor(const CBlockIndex*
 {
     int nPeriod = Period(params);
     int nThreshold = Threshold(params);
+    int min_activation_height = MinActivationHeight(params);
     int64_t nTimeStart = BeginTime(params);
     int64_t nTimeTimeout = EndTime(params);
 
@@ -17,6 +18,11 @@ ThresholdState AbstractThresholdConditionChecker::GetStateFor(const CBlockIndex*
         return ThresholdState::ACTIVE;
     }
 
+    // Check if this deployment is never active.
+    if (nTimeStart == Consensus::BIP9Deployment::NEVER_ACTIVE) {
+        return ThresholdState::FAILED;
+    }
+
     // A block's state is always the same as that of the first of its period, so it is computed based on a pindexPrev whose height equals a multiple of nPeriod - 1.
     if (pindexPrev != nullptr) {
         pindexPrev = pindexPrev->GetAncestor(pindexPrev->nHeight - ((pindexPrev->nHeight + 1) % nPeriod));
@@ -51,18 +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;
@@ -74,12 +74,16 @@ ThresholdState AbstractThresholdConditionChecker::GetStateFor(const CBlockIndex*
                 }
                 if (count >= nThreshold) {
                     stateNext = ThresholdState::LOCKED_IN;
+                } else if (pindexPrev->GetMedianTimePast() >= nTimeTimeout) {
+                    stateNext = ThresholdState::FAILED;
                 }
                 break;
             }
             case ThresholdState::LOCKED_IN: {
-                // Always progresses into ACTIVE.
-                stateNext = ThresholdState::ACTIVE;
+                // Progresses into ACTIVE provided activation height will have been reached.
+                if (pindexPrev->nHeight + 1 >= min_activation_height) {
+                    stateNext = ThresholdState::ACTIVE;
+                }
                 break;
             }
             case ThresholdState::FAILED:
@@ -126,7 +130,7 @@ BIP9Stats AbstractThresholdConditionChecker::GetStateStatisticsFor(const CBlockI
 int AbstractThresholdConditionChecker::GetStateSinceHeightFor(const CBlockIndex* pindexPrev, const Consensus::Params& params, ThresholdConditionCache& cache) const
 {
     int64_t start_time = BeginTime(params);
-    if (start_time == Consensus::BIP9Deployment::ALWAYS_ACTIVE) {
+    if (start_time == Consensus::BIP9Deployment::ALWAYS_ACTIVE || start_time == Consensus::BIP9Deployment::NEVER_ACTIVE) {
         return 0;
     }
 
@@ -170,6 +174,7 @@ private:
 protected:
     int64_t BeginTime(const Consensus::Params& params) const override { return params.vDeployments[id].nStartTime; }
     int64_t EndTime(const Consensus::Params& params) const override { return params.vDeployments[id].nTimeout; }
+    int MinActivationHeight(const Consensus::Params& params) const override { return params.vDeployments[id].min_activation_height; }
     int Period(const Consensus::Params& params) const override { return params.nMinerConfirmationWindow; }
     int Threshold(const Consensus::Params& params) const override { return params.nRuleChangeActivationThreshold; }
 
diff --git a/src/versionbits.h b/src/versionbits.h
index b02f848b67..faf09343ea 100644
--- a/src/versionbits.h
+++ b/src/versionbits.h
@@ -25,7 +25,7 @@ static const int32_t VERSIONBITS_NUM_BITS = 29;
 enum class ThresholdState {
     DEFINED,   // First state that each softfork starts out as. The genesis block is by definition in this state for each deployment.
     STARTED,   // For blocks past the starttime.
-    LOCKED_IN, // For one retarget period after the first retarget period with STARTED blocks of which at least threshold have the associated bit set in nVersion.
+    LOCKED_IN, // For at least one retarget period after the first retarget period with STARTED blocks of which at least threshold have the associated bit set in nVersion, until min_activation_height is reached.
     ACTIVE,    // For all blocks after the LOCKED_IN retarget period (final state)
     FAILED,    // For all blocks once the first retarget period after the timeout time is hit, if LOCKED_IN wasn't already reached (final state)
 };
@@ -57,6 +57,7 @@ protected:
     virtual bool Condition(const CBlockIndex* pindex, const Consensus::Params& params) const =0;
     virtual int64_t BeginTime(const Consensus::Params& params) const =0;
     virtual int64_t EndTime(const Consensus::Params& params) const =0;
+    virtual int MinActivationHeight(const Consensus::Params& params) const { return 0; }
     virtual int Period(const Consensus::Params& params) const =0;
     virtual int Threshold(const Consensus::Params& params) const =0;
 
diff --git a/test/functional/rpc_blockchain.py b/test/functional/rpc_blockchain.py
index f965677408..79d437abd3 100755
--- a/test/functional/rpc_blockchain.py
+++ b/test/functional/rpc_blockchain.py
@@ -145,6 +145,7 @@ class BlockchainTest(BitcoinTestFramework):
                         'count': 57,
                         'possible': True,
                     },
+                    'min_activation_height': 0,
                 },
                 'active': False
             },
@@ -154,7 +155,8 @@ class BlockchainTest(BitcoinTestFramework):
                     'status': 'active',
                     'start_time': -1,
                     'timeout': 9223372036854775807,
-                    'since': 0
+                    'since': 0,
+                    'min_activation_height': 0,
                 },
                 'height': 0,
                 'active': True