Merge bitcoin/bitcoin#21946: Document and test lack of inherited signaling in RBF policy

2eb0eeda39 validation: document lack of inherited signaling in RBF policy (Antoine Riard)
906b6d9da6 test: Extend feature_rbf.py with no inherited signaling (Antoine Riard)

Pull request description:

  Contrary to BIP125 or other full-node implementation (e.g btcd), Bitcoin Core's mempool policy doesn't implement inherited signaling.

  This PR documents our mempool behavior on this and add a test demonstrating the case.

ACKs for top commit:
  jonatack:
    ACK 2eb0eeda39
  benthecarman:
    ACK 2eb0eeda39

Tree-SHA512: d41453d3b49bae3c1eb532a968f43bc047084913bd285929d4d9cba142777ff2be38163d912e28dfc635f4ecf446de68effad799c6e71be52f81e83410c712fb

src/validation.cpp

@@ -629,10 +629,13 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
                 // is for the sake of multi-party protocols, where we don't
                 // want a single party to be able to disable replacement.
                 //
-                // The opt-out ignores descendants as anyone relying on
-                // first-seen mempool behavior should be checking all
-                // unconfirmed ancestors anyway; doing otherwise is hopelessly
-                // insecure.
+                // Transactions that don't explicitly signal replaceability are
+                // *not* replaceable with the current logic, even if one of their
+                // unconfirmed ancestors signals replaceability. This diverges
+                // from BIP125's inherited signaling description (see CVE-2021-31876).
+                // Applications relying on first-seen mempool behavior should
+                // check all unconfirmed ancestors; otherwise an opt-in ancestor
+                // might be replaced, causing removal of this descendant.
                 bool fReplacementOptOut = true;
                 for (const CTxIn &_txin : ptxConflicting->vin)
                 {