Merge bitcoin/bitcoin#31600: rpc: have getblocktemplate mintime account for timewarp

e1676b08f7 doc: release notes (Sjors Provoost)
0082f6acc1 rpc: have mintime account for timewarp rule (Sjors Provoost)
79d45b10f1 rpc: clarify BIP94 behavior for curtime (Sjors Provoost)
0713548137 refactor: add GetMinimumTime() helper (Sjors Provoost)

Pull request description:

  #30681 fixed the `curtime` field of `getblocktemplate` to take the timewarp rule into account. However I forgot to do the same for the `mintime` field, which was hardcoded to use `pindexPrev->GetMedianTimePast()+1`.

  This PR adds a helper `GetMinimumTime()` and uses it for the `mintime` field.

  #31376 changed the `curtime` field to always account for the timewarp rule. This PR maintains that behavior.

  Note that `mintime` now always applies BIP94, including on mainnet. This makes future softfork activation safer.

  It could be backported to v28.

ACKs for top commit:
  fjahr:
    tACK e1676b08f7
  achow101:
    ACK e1676b08f7
  darosior:
    utACK e1676b08f7 on the code changes
  tdb3:
    brief code review re ACK e1676b08f7
  TheCharlatan:
    ACK e1676b08f7

Tree-SHA512: 0e322d8cc3b8ff770849bce211edcb5b6f55d04e5e0dee0657805049663d758f27423b047ee6363bd8f6c6fead13f974760f48b3321ea86f514f446e1b23231c

doc/release-notes-31600.md

@@ -0,0 +1,11 @@
+Updated RPCs
+---
+- the `getblocktemplate` RPC `curtime` (BIP22) and `mintime` (BIP23) fields now
+  account for the timewarp fix proposed in BIP94 on all networks. This ensures
+  that, in the event a timewarp fix softfork activates on mainnet, un-upgraded
+  miners will not accidentally violate the timewarp rule. (#31376, #31600)
+
+As a reminder, it's important that any software which uses the `getblocktemplate`
+RPC takes these values into account (either `curtime` or `mintime` is fine).
+Relying only on a clock can lead to invalid blocks under some circumstances,
+especially once a timewarp fix is deployed.

src/node/miner.cpp

@@ -28,16 +28,25 @@
 #include <utility>
 
 namespace node {
+
+int64_t GetMinimumTime(const CBlockIndex* pindexPrev, const int64_t difficulty_adjustment_interval)
+{
+    int64_t min_time{pindexPrev->GetMedianTimePast() + 1};
+    // Height of block to be mined.
+    const int height{pindexPrev->nHeight + 1};
+    // Account for BIP94 timewarp rule on all networks. This makes future
+    // activation safer.
+    if (height % difficulty_adjustment_interval == 0) {
+        min_time = std::max<int64_t>(min_time, pindexPrev->GetBlockTime() - MAX_TIMEWARP);
+    }
+    return min_time;
+}
+
 int64_t UpdateTime(CBlockHeader* pblock, const Consensus::Params& consensusParams, const CBlockIndex* pindexPrev)
 {
     int64_t nOldTime = pblock->nTime;
-    int64_t nNewTime{std::max<int64_t>(pindexPrev->GetMedianTimePast() + 1, TicksSinceEpoch<std::chrono::seconds>(NodeClock::now()))};
-
-    // Height of block to be mined.
-    const int height{pindexPrev->nHeight + 1};
-    if (height % consensusParams.DifficultyAdjustmentInterval() == 0) {
-        nNewTime = std::max<int64_t>(nNewTime, pindexPrev->GetBlockTime() - MAX_TIMEWARP);
-    }
+    int64_t nNewTime{std::max<int64_t>(GetMinimumTime(pindexPrev, consensusParams.DifficultyAdjustmentInterval()),
+                                       TicksSinceEpoch<std::chrono::seconds>(NodeClock::now()))};
 
     if (nOldTime < nNewTime) {
         pblock->nTime = nNewTime;

src/node/miner.h

@@ -211,6 +211,13 @@ private:
     void SortForBlock(const CTxMemPool::setEntries& package, std::vector<CTxMemPool::txiter>& sortedEntries);
 };
 
+/**
+ * Get the minimum time a miner should use in the next block. This always
+ * accounts for the BIP94 timewarp rule, so does not necessarily reflect the
+ * consensus limit.
+ */
+int64_t GetMinimumTime(const CBlockIndex* pindexPrev, const int64_t difficulty_adjustment_interval);
+
 int64_t UpdateTime(CBlockHeader* pblock, const Consensus::Params& consensusParams, const CBlockIndex* pindexPrev);
 
 /** Update an old GenerateCoinbaseCommitment from CreateNewBlock after the block txs have changed */

src/rpc/mining.cpp

@@ -49,6 +49,7 @@
 using interfaces::BlockTemplate;
 using interfaces::Mining;
 using node::BlockAssembler;
+using node::GetMinimumTime;
 using node::NodeContext;
 using node::RegenerateCommitments;
 using node::UpdateTime;
@@ -674,7 +675,7 @@ static RPCHelpMan getblocktemplate()
                 {RPCResult::Type::NUM, "coinbasevalue", "maximum allowable input to coinbase transaction, including the generation award and transaction fees (in satoshis)"},
                 {RPCResult::Type::STR, "longpollid", "an id to include with a request to longpoll on an update to this template"},
                 {RPCResult::Type::STR, "target", "The hash target"},
-                {RPCResult::Type::NUM_TIME, "mintime", "The minimum timestamp appropriate for the next block time, expressed in " + UNIX_EPOCH_TIME},
+                {RPCResult::Type::NUM_TIME, "mintime", "The minimum timestamp appropriate for the next block time, expressed in " + UNIX_EPOCH_TIME + ". Adjusted for the proposed BIP94 timewarp rule."},
                 {RPCResult::Type::ARR, "mutable", "list of ways the block template may be changed",
                 {
                     {RPCResult::Type::STR, "value", "A way the block template may be changed, e.g. 'time', 'transactions', 'prevblock'"},
@@ -683,7 +684,7 @@ static RPCHelpMan getblocktemplate()
                 {RPCResult::Type::NUM, "sigoplimit", "limit of sigops in blocks"},
                 {RPCResult::Type::NUM, "sizelimit", "limit of block size"},
                 {RPCResult::Type::NUM, "weightlimit", /*optional=*/true, "limit of block weight"},
-                {RPCResult::Type::NUM_TIME, "curtime", "current timestamp in " + UNIX_EPOCH_TIME},
+                {RPCResult::Type::NUM_TIME, "curtime", "current timestamp in " + UNIX_EPOCH_TIME + ". Adjusted for the proposed BIP94 timewarp rule."},
                 {RPCResult::Type::STR, "bits", "compressed target of next block"},
                 {RPCResult::Type::NUM, "height", "The height of the next block"},
                 {RPCResult::Type::STR_HEX, "signet_challenge", /*optional=*/true, "Only on signet"},
@@ -977,7 +978,7 @@ static RPCHelpMan getblocktemplate()
     result.pushKV("coinbasevalue", (int64_t)block.vtx[0]->vout[0].nValue);
     result.pushKV("longpollid", tip.GetHex() + ToString(nTransactionsUpdatedLast));
     result.pushKV("target", hashTarget.GetHex());
-    result.pushKV("mintime", (int64_t)pindexPrev->GetMedianTimePast()+1);
+    result.pushKV("mintime", GetMinimumTime(pindexPrev, consensusParams.DifficultyAdjustmentInterval()));
     result.pushKV("mutable", std::move(aMutable));
     result.pushKV("noncerange", "00000000ffffffff");
     int64_t nSigOpLimit = MAX_BLOCK_SIGOPS_COST;

test/functional/mining_basic.py

@@ -153,6 +153,8 @@ class MiningTest(BitcoinTestFramework):
         # The template will have an adjusted timestamp, which we then modify
         tmpl = node.getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)
         assert_greater_than_or_equal(tmpl['curtime'], t + MAX_FUTURE_BLOCK_TIME - MAX_TIMEWARP)
+        # mintime and curtime should match
+        assert_equal(tmpl['mintime'], tmpl['curtime'])
 
         block = CBlock()
         block.nVersion = tmpl["version"]