Merge #15846: [POLICY] Make sending to future native witness outputs standard

c634b1e [POLICY] Make sending to future native witness outputs standard (Pieter Wuille)

Pull request description:

  As discussed in the April 18 2019 IRC meeting.

  This makes sending to future Segwit versions via native outputs (bech32) standard for relay, mempool acceptance, and mining. The reasons are:
  * This may interfere with smooth adoption of future segwit versions, if they're defined (by the sender wallet/node).
  * It violates BIP173 ("Version 0 witness addresses are always 42 or 62 characters, but implementations MUST allow the use of any version."), though admittedly this code was written before BIP173.
  * It doesn't protect much, as P2SH-embedded segwit cannot be filtered in this way.
  * As a general policy, the sender shouldn't care what the receiver likes his outputs to be.

  Note that _spending_ such outputs (including P2SH-embedded ones) remains nonstandard, as that is actually required for softfork safety.

ACKs for commit c634b1:
  MarcoFalke:
    utACK c634b1e
  harding:
    Tested ACK c634b1e
  meshcollider:
    utACK bitcoin/bitcoin@c634b1e

Tree-SHA512: e37168a1be9f445a04d4280593f0a92bdae33eee00ecd803d5eb16acb5c9cfc0f1f0a1dfbd5a0cc73da2c9928ec11cbdac7911513a78f85b789ae0d00e1b5962

Author: meshcollider

src/policy/policy.cpp

@@ -59,7 +59,7 @@ bool IsStandard(const CScript& scriptPubKey, txnouttype& whichType)
     std::vector<std::vector<unsigned char> > vSolutions;
     whichType = Solver(scriptPubKey, vSolutions);
 
-    if (whichType == TX_NONSTANDARD || whichType == TX_WITNESS_UNKNOWN) {
+    if (whichType == TX_NONSTANDARD) {
         return false;
     } else if (whichType == TX_MULTISIG) {
         unsigned char m = vSolutions.front()[0];

test/functional/p2p_segwit.py

@@ -1360,7 +1360,8 @@ def test_tx_relay_after_segwit_activation(self):
     def test_segwit_versions(self):
         """Test validity of future segwit version transactions.
 
-        Future segwit version transactions are non-standard, but valid in blocks.
+        Future segwit versions are non-standard to spend, but valid in blocks.
+        Sending to future segwit versions is always allowed.
         Can run this before and after segwit activation."""
 
         NUM_SEGWIT_VERSIONS = 17  # will test OP_0, OP1, ..., OP_16
@@ -1400,18 +1401,17 @@ def test_segwit_versions(self):
         assert len(self.nodes[0].getrawmempool()) == 0
 
         # Finally, verify that version 0 -> version 1 transactions
-        # are non-standard
+        # are standard
         script_pubkey = CScript([CScriptOp(OP_1), witness_hash])
         tx2 = CTransaction()
         tx2.vin = [CTxIn(COutPoint(tx.sha256, 0), b"")]
         tx2.vout = [CTxOut(tx.vout[0].nValue - 1000, script_pubkey)]
         tx2.wit.vtxinwit.append(CTxInWitness())
         tx2.wit.vtxinwit[0].scriptWitness.stack = [witness_program]
         tx2.rehash()
-        # Gets accepted to test_node, because standardness of outputs isn't
-        # checked with fRequireStandard
+        # Gets accepted to both policy-enforcing nodes and others.
         test_transaction_acceptance(self.nodes[0], self.test_node, tx2, with_witness=True, accepted=True)
-        test_transaction_acceptance(self.nodes[1], self.std_node, tx2, with_witness=True, accepted=False)
+        test_transaction_acceptance(self.nodes[1], self.std_node, tx2, with_witness=True, accepted=True)
         temp_utxo.pop()  # last entry in temp_utxo was the output we just spent
         temp_utxo.append(UTXO(tx2.sha256, 0, tx2.vout[0].nValue))