Merge #20611: Move TX_MAX_STANDARD_VERSION to policy

fade6195b1c230edd561443637a7bde81c2594a4 Move TX_MAX_STANDARD_VERSION to policy (MarcoFalke)

Pull request description:

  `primitives` should only be used for the raw datastructures (parsing and format). It is not the right place to document relay policy.

ACKs for top commit:
  laanwj:
    Code review ACK fade6195b1c230edd561443637a7bde81c2594a4
  lontivero:
    Concept ACK https://github.com/bitcoin/bitcoin/pull/20611/commits/fade6195b1c230edd561443637a7bde81c2594a4

Tree-SHA512: f809c4aecd14d7e9feaa7b50b9c0697232991eef36190cd960bcfb0ad6e20c71a4f6aab48c7747cf8a681eb14feda60c55b09a37f128673d519567224f29cd97

2 files changed, 25 insertions, 18 deletions

diff --git a/src/policy/policy.cpp b/src/policy/policy.cpp
index 4e33fd6cb5..8e367d31d0 100644
--- a/src/policy/policy.cpp
+++ b/src/policy/policy.cpp
@@ -75,7 +75,7 @@ bool IsStandard(const CScript& scriptPubKey, TxoutType& whichType)
 
 bool IsStandardTx(const CTransaction& tx, bool permit_bare_multisig, const CFeeRate& dust_relay_fee, std::string& reason)
 {
-    if (tx.nVersion > CTransaction::MAX_STANDARD_VERSION || tx.nVersion < 1) {
+    if (tx.nVersion > TX_MAX_STANDARD_VERSION || tx.nVersion < 1) {
         reason = "version";
         return false;
     }
diff --git a/src/policy/policy.h b/src/policy/policy.h
index 726a14a27e..fc163e958b 100644
--- a/src/policy/policy.h
+++ b/src/policy/policy.h
@@ -90,25 +90,32 @@ CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFee);
 bool IsDust(const CTxOut& txout, const CFeeRate& dustRelayFee);
 
 bool IsStandard(const CScript& scriptPubKey, TxoutType& whichType);
-    /**
-     * Check for standard transaction types
-     * @return True if all outputs (scriptPubKeys) use only standard transaction forms
-     */
+
+
+// Changing the default transaction version requires a two step process: first
+// adapting relay policy by bumping TX_MAX_STANDARD_VERSION, and then later
+// allowing the new transaction version in the wallet/RPC.
+static constexpr decltype(CTransaction::nVersion) TX_MAX_STANDARD_VERSION{2};
+
+/**
+* Check for standard transaction types
+* @return True if all outputs (scriptPubKeys) use only standard transaction forms
+*/
 bool IsStandardTx(const CTransaction& tx, bool permit_bare_multisig, const CFeeRate& dust_relay_fee, std::string& reason);
-    /**
-     * Check for standard transaction types
-     * @param[in] mapInputs       Map of previous transactions that have outputs we're spending
-     * @param[in] taproot_active  Whether or taproot consensus rules are active (used to decide whether spends of them are permitted)
-     * @return True if all inputs (scriptSigs) use only standard transaction forms
-     */
+/**
+* Check for standard transaction types
+* @param[in] mapInputs       Map of previous transactions that have outputs we're spending
+* @param[in] taproot_active  Whether or taproot consensus rules are active (used to decide whether spends of them are permitted)
+* @return True if all inputs (scriptSigs) use only standard transaction forms
+*/
 bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs, bool taproot_active);
-    /**
-     * Check if the transaction is over standard P2WSH resources limit:
-     * 3600bytes witnessScript size, 80bytes per witness stack element, 100 witness stack elements
-     * These limits are adequate for multisignatures up to n-of-100 using OP_CHECKSIG, OP_ADD, and OP_EQUAL.
-     *
-     * Also enforce a maximum stack item size limit and no annexes for tapscript spends.
-     */
+/**
+* Check if the transaction is over standard P2WSH resources limit:
+* 3600bytes witnessScript size, 80bytes per witness stack element, 100 witness stack elements
+* These limits are adequate for multisignatures up to n-of-100 using OP_CHECKSIG, OP_ADD, and OP_EQUAL.
+*
+* Also enforce a maximum stack item size limit and no annexes for tapscript spends.
+*/
 bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs);
 
 /** Compute the virtual transaction size (weight reinterpreted as bytes). */