Merge #15681: [mempool] Allow one extra single-ancestor transaction per package

50cede3f5a4d4fbfbb7c420b94e661a6a159bced [mempool] Allow one extra single-ancestor transaction per package (Matt Corallo)

Pull request description:

  This implements the proposed policy change from [1], which allows
  certain classes of contract protocols involving revocation
  punishments to use CPFP. Note that some such use-cases may still
  want some form of one-deep package relay, though even this alone
  may greatly simplify some lightning fee negotiation.

  [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016518.html

ACKs for top commit:
  ajtowns:
    ACK 50cede3f5a4d4fbfbb7c420b94e661a6a159bced -- looked over code again, compared with previous commit, compiles, etc.
  sdaftuar:
    ACK 50cede3f5a4d4fbfbb7c420b94e661a6a159bced
  ryanofsky:
    utACK 50cede3f5a4d4fbfbb7c420b94e661a6a159bced. Changes since last review: adding EXTRA_DESCENDANT_TX_SIZE_LIMIT constant, changing max ancestor size from 1,000,000 to nLimitAncestorSize constant (101,000), fixing test comment and getting rid of unused test node.

Tree-SHA512: b052c2a0f384855572b4579310131897b612201214b5abbb225167224e4f550049e300b471dbf320928652571e92ca2d650050b7cf39ac92b3bc1d2bcd386c1c

2 files changed, 21 insertions, 1 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index 0bc6167bad..f585bc7d2d 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -616,7 +616,21 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
         size_t nLimitDescendantSize = gArgs.GetArg("-limitdescendantsize", DEFAULT_DESCENDANT_SIZE_LIMIT)*1000;
         std::string errString;
         if (!pool.CalculateMemPoolAncestors(entry, setAncestors, nLimitAncestors, nLimitAncestorSize, nLimitDescendants, nLimitDescendantSize, errString)) {
-            return state.Invalid(ValidationInvalidReason::TX_MEMPOOL_POLICY, false, REJECT_NONSTANDARD, "too-long-mempool-chain", errString);
+            setAncestors.clear();
+            // If the new transaction is relatively small (up to 40k weight)
+            // and has at most one ancestor (ie ancestor limit of 2, including
+            // the new transaction), allow it if its parent has exactly the
+            // descendant limit descendants.
+            //
+            // This allows protocols which rely on distrusting counterparties
+            // being able to broadcast descendants of an unconfirmed transaction
+            // to be secure by simply only having two immediately-spendable
+            // outputs - one for each counterparty. For more info on the uses for
+            // this, see https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016518.html
+            if (nSize >  EXTRA_DESCENDANT_TX_SIZE_LIMIT ||
+                    !pool.CalculateMemPoolAncestors(entry, setAncestors, 2, nLimitAncestorSize, nLimitDescendants + 1, nLimitDescendantSize + EXTRA_DESCENDANT_TX_SIZE_LIMIT, errString)) {
+                return state.Invalid(ValidationInvalidReason::TX_MEMPOOL_POLICY, false, REJECT_NONSTANDARD, "too-long-mempool-chain", errString);
+            }
         }
 
         // A transaction that spends outputs that would be replaced by it is invalid. Now
diff --git a/src/validation.h b/src/validation.h
index 7739d3de4a..d747fdbf27 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -64,6 +64,12 @@ static const unsigned int DEFAULT_ANCESTOR_SIZE_LIMIT = 101;
 static const unsigned int DEFAULT_DESCENDANT_LIMIT = 25;
 /** Default for -limitdescendantsize, maximum kilobytes of in-mempool descendants */
 static const unsigned int DEFAULT_DESCENDANT_SIZE_LIMIT = 101;
+/**
+ * An extra transaction can be added to a package, as long as it only has one
+ * ancestor and is no larger than this. Not really any reason to make this
+ * configurable as it doesn't materially change DoS parameters.
+ */
+static const unsigned int EXTRA_DESCENDANT_TX_SIZE_LIMIT = 10000;
 /** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
 static const unsigned int DEFAULT_MEMPOOL_EXPIRY = 336;
 /** Maximum kilobytes for transactions to store for processing during reorg */