Merge bitcoin/bitcoin#24136: Extract CTxIn::MAX_SEQUENCE_NONFINAL constant, rework BIP 65/68/112 docs

fa4339e4c1bb60e0d9263d4f0fe65d03aad52f88 Extract CTxIn::MAX_SEQUENCE_NONFINAL constant (MarcoFalke)

Pull request description:

  Extracting the constant makes it possible to attach documentation to it.

  Also, rework the docs for the other "sequence constants".

ACKs for top commit:
  w0xlt:
    reACK fa4339e for specifying the transaction version.
  darosior:
    re-ACK fa4339e4c1bb60e0d9263d4f0fe65d03aad52f88
  luke-jr:
    crACK fa4339e4c1bb60e0d9263d4f0fe65d03aad52f88

Tree-SHA512: 8d8f3dd5afb33eb5b72aa558e1e03de874c5ed02aa1084888e92ed86f3aaa5c725db45ded02e14cdfa67a92ac6774e97185b697f20a8ab63abbfcaa2fcd1fc6a

5 files changed, 32 insertions, 12 deletions

diff --git a/src/primitives/transaction.h b/src/primitives/transaction.h
index 67ea4a3747..1fcbc45c72 100644
--- a/src/primitives/transaction.h
+++ b/src/primitives/transaction.h
@@ -70,25 +70,45 @@ public:
     uint32_t nSequence;
     CScriptWitness scriptWitness; //!< Only serialized through CTransaction
 
-    /* Setting nSequence to this value for every input in a transaction
-     * disables nLockTime. */
+    /**
+     * Setting nSequence to this value for every input in a transaction
+     * disables nLockTime/IsFinalTx().
+     * It fails OP_CHECKLOCKTIMEVERIFY/CheckLockTime() for any input that has
+     * it set (BIP 65).
+     * It has SEQUENCE_LOCKTIME_DISABLE_FLAG set (BIP 68/112).
+     */
     static const uint32_t SEQUENCE_FINAL = 0xffffffff;
+    /**
+     * This is the maximum sequence number that enables both nLockTime and
+     * OP_CHECKLOCKTIMEVERIFY (BIP 65).
+     * It has SEQUENCE_LOCKTIME_DISABLE_FLAG set (BIP 68/112).
+     */
+    static const uint32_t MAX_SEQUENCE_NONFINAL{SEQUENCE_FINAL - 1};
 
-    /* Below flags apply in the context of BIP 68*/
-    /* If this flag set, CTxIn::nSequence is NOT interpreted as a
-     * relative lock-time. */
+    // Below flags apply in the context of BIP 68. BIP 68 requires the tx
+    // version to be set to 2, or higher.
+    /**
+     * If this flag is set, CTxIn::nSequence is NOT interpreted as a
+     * relative lock-time.
+     * It skips SequenceLocks() for any input that has it set (BIP 68).
+     * It fails OP_CHECKSEQUENCEVERIFY/CheckSequence() for any input that has
+     * it set (BIP 112).
+     */
     static const uint32_t SEQUENCE_LOCKTIME_DISABLE_FLAG = (1U << 31);
 
-    /* If CTxIn::nSequence encodes a relative lock-time and this flag
+    /**
+     * If CTxIn::nSequence encodes a relative lock-time and this flag
      * is set, the relative lock-time has units of 512 seconds,
      * otherwise it specifies blocks with a granularity of 1. */
     static const uint32_t SEQUENCE_LOCKTIME_TYPE_FLAG = (1 << 22);
 
-    /* If CTxIn::nSequence encodes a relative lock-time, this mask is
+    /**
+     * If CTxIn::nSequence encodes a relative lock-time, this mask is
      * applied to extract that lock-time from the sequence field. */
     static const uint32_t SEQUENCE_LOCKTIME_MASK = 0x0000ffff;
 
-    /* In order to use the same number of bits to encode roughly the
+    /**
+     * In order to use the same number of bits to encode roughly the
      * same wall-clock duration, and because blocks are naturally
      * limited to occur every 600s on average, the minimum granularity
      * for time-based relative lock-time is fixed at 512 seconds.
diff --git a/src/rpc/rawtransaction_util.cpp b/src/rpc/rawtransaction_util.cpp
index 3459897fe5..e23fe34480 100644
--- a/src/rpc/rawtransaction_util.cpp
+++ b/src/rpc/rawtransaction_util.cpp
@@ -63,7 +63,7 @@ CMutableTransaction ConstructTransaction(const UniValue& inputs_in, const UniVal
         if (rbf) {
             nSequence = MAX_BIP125_RBF_SEQUENCE; /* CTxIn::SEQUENCE_FINAL - 2 */
         } else if (rawTx.nLockTime) {
-            nSequence = CTxIn::SEQUENCE_FINAL - 1;
+            nSequence = CTxIn::MAX_SEQUENCE_NONFINAL; /* CTxIn::SEQUENCE_FINAL - 1 */
         } else {
             nSequence = CTxIn::SEQUENCE_FINAL;
         }
diff --git a/src/test/fuzz/util.cpp b/src/test/fuzz/util.cpp
index 47c2be3faa..2514636d6e 100644
--- a/src/test/fuzz/util.cpp
+++ b/src/test/fuzz/util.cpp
@@ -408,7 +408,7 @@ uint32_t ConsumeSequence(FuzzedDataProvider& fuzzed_data_provider) noexcept
     return fuzzed_data_provider.ConsumeBool() ?
                fuzzed_data_provider.PickValueInArray({
                    CTxIn::SEQUENCE_FINAL,
-                   CTxIn::SEQUENCE_FINAL - 1,
+                   CTxIn::MAX_SEQUENCE_NONFINAL,
                    MAX_BIP125_RBF_SEQUENCE,
                }) :
                fuzzed_data_provider.ConsumeIntegral<uint32_t>();
diff --git a/src/test/miner_tests.cpp b/src/test/miner_tests.cpp
index e07eb95856..c453dae701 100644
--- a/src/test/miner_tests.cpp
+++ b/src/test/miner_tests.cpp
@@ -459,7 +459,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
 
     // absolute height locked
     tx.vin[0].prevout.hash = txFirst[2]->GetHash();
-    tx.vin[0].nSequence = CTxIn::SEQUENCE_FINAL - 1;
+    tx.vin[0].nSequence = CTxIn::MAX_SEQUENCE_NONFINAL;
     prevheights[0] = baseheight + 3;
     tx.nLockTime = m_node.chainman->ActiveChain().Tip()->nHeight + 1;
     hash = tx.GetHash();
diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index a42df8262c..3d8ae2da69 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -796,7 +796,7 @@ static bool CreateTransactionInternal(
     // to avoid conflicting with other possible uses of nSequence,
     // and in the spirit of "smallest possible change from prior
     // behavior."
-    const uint32_t nSequence = coin_control.m_signal_bip125_rbf.value_or(wallet.m_signal_rbf) ? MAX_BIP125_RBF_SEQUENCE : (CTxIn::SEQUENCE_FINAL - 1);
+    const uint32_t nSequence{coin_control.m_signal_bip125_rbf.value_or(wallet.m_signal_rbf) ? MAX_BIP125_RBF_SEQUENCE : CTxIn::MAX_SEQUENCE_NONFINAL};
     for (const auto& coin : selected_coins) {
         txNew.vin.push_back(CTxIn(coin.outpoint, CScript(), nSequence));
     }