From f9e37f33ce2d8b463a0bcbe7189c9bc5b36530b7 Mon Sep 17 00:00:00 2001 From: Yuval Kogman Date: Fri, 7 Feb 2020 19:10:31 +0000 Subject: doc: IsFinalTx comment about nSequence & OP_CLTV It's somewhat surprising that a transaction's nLockTime field is ignored when all nSequence fields are final, so this change aims to clarify this behavior and cross reference relevant details of OP_CHECKLOCKTIMEVERIFY. --- src/consensus/tx_verify.cpp | 9 +++++++++ src/script/interpreter.cpp | 6 +++--- 2 files changed, 12 insertions(+), 3 deletions(-) diff --git a/src/consensus/tx_verify.cpp b/src/consensus/tx_verify.cpp index f595f16eab..88d8da6ed5 100644 --- a/src/consensus/tx_verify.cpp +++ b/src/consensus/tx_verify.cpp @@ -20,6 +20,15 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime) return true; if ((int64_t)tx.nLockTime < ((int64_t)tx.nLockTime < LOCKTIME_THRESHOLD ? (int64_t)nBlockHeight : nBlockTime)) return true; + + // Even if tx.nLockTime isn't satisfied by nBlockHeight/nBlockTime, a + // transaction is still considered final if all inputs' nSequence == + // SEQUENCE_FINAL (0xffffffff), in which case nLockTime is ignored. + // + // Because of this behavior OP_CHECKLOCKTIMEVERIFY/CheckLockTime() will + // also check that the spending input's nSequence != SEQUENCE_FINAL, + // ensuring that an unsatisfied nLockTime value will actually cause + // IsFinalTx() to return false here: for (const auto& txin : tx.vin) { if (!(txin.nSequence == CTxIn::SEQUENCE_FINAL)) return false; diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp index abc0625bb1..6682d405c8 100644 --- a/src/script/interpreter.cpp +++ b/src/script/interpreter.cpp @@ -1741,9 +1741,9 @@ bool GenericTransactionSignatureChecker::CheckLockTime(const CScriptNum& nLoc if (nLockTime > (int64_t)txTo->nLockTime) return false; - // Finally the nLockTime feature can be disabled and thus - // CHECKLOCKTIMEVERIFY bypassed if every txin has been - // finalized by setting nSequence to maxint. The + // Finally the nLockTime feature can be disabled in IsFinalTx() + // and thus CHECKLOCKTIMEVERIFY bypassed if every txin has + // been finalized by setting nSequence to maxint. The // transaction would be allowed into the blockchain, making // the opcode ineffective. // -- cgit v1.2.3