[txpackages] IsChildWithParentsTree()

Many edge cases exist when parents in a child-with-parents package can
spend each other. However, this pattern should also be uncommon in
normal use cases.

src/policy/packages.cpp

@@ -88,3 +88,18 @@ bool IsChildWithParents(const Package& package)
     return std::all_of(package.cbegin(), package.cend() - 1,
                        [&input_txids](const auto& ptx) { return input_txids.count(ptx->GetHash()) > 0; });
 }
+
+bool IsChildWithParentsTree(const Package& package)
+{
+    if (!IsChildWithParents(package)) return false;
+    std::unordered_set<uint256, SaltedTxidHasher> parent_txids;
+    std::transform(package.cbegin(), package.cend() - 1, std::inserter(parent_txids, parent_txids.end()),
+                   [](const auto& ptx) { return ptx->GetHash(); });
+    // Each parent must not have an input who is one of the other parents.
+    return std::all_of(package.cbegin(), package.cend() - 1, [&](const auto& ptx) {
+        for (const auto& input : ptx->vin) {
+            if (parent_txids.count(input.prevout.hash) > 0) return false;
+        }
+        return true;
+    });
+}

src/policy/packages.h

@@ -63,4 +63,8 @@ bool CheckPackage(const Package& txns, PackageValidationState& state);
  */
 bool IsChildWithParents(const Package& package);
 
+/** Context-free check that a package IsChildWithParents() and none of the parents depend on each
+ * other (the package is a "tree").
+ */
+bool IsChildWithParentsTree(const Package& package);
 #endif // BITCOIN_POLICY_PACKAGES_H

src/test/txpackage_tests.cpp

@@ -162,6 +162,7 @@ BOOST_FIXTURE_TEST_CASE(noncontextual_package_tests, TestChain100Setup)
         BOOST_CHECK_EQUAL(state.GetResult(), PackageValidationResult::PCKG_POLICY);
         BOOST_CHECK_EQUAL(state.GetRejectReason(), "package-not-sorted");
         BOOST_CHECK(IsChildWithParents({tx_parent, tx_child}));
+        BOOST_CHECK(IsChildWithParentsTree({tx_parent, tx_child}));
     }
 
     // 24 Parents and 1 Child
@@ -187,6 +188,7 @@ BOOST_FIXTURE_TEST_CASE(noncontextual_package_tests, TestChain100Setup)
         PackageValidationState state;
         BOOST_CHECK(CheckPackage(package, state));
         BOOST_CHECK(IsChildWithParents(package));
+        BOOST_CHECK(IsChildWithParentsTree(package));
 
         package.erase(package.begin());
         BOOST_CHECK(IsChildWithParents(package));
@@ -219,6 +221,7 @@ BOOST_FIXTURE_TEST_CASE(noncontextual_package_tests, TestChain100Setup)
         BOOST_CHECK(IsChildWithParents({tx_parent, tx_parent_also_child}));
         BOOST_CHECK(IsChildWithParents({tx_parent, tx_child}));
         BOOST_CHECK(IsChildWithParents({tx_parent, tx_parent_also_child, tx_child}));
+        BOOST_CHECK(!IsChildWithParentsTree({tx_parent, tx_parent_also_child, tx_child}));
         // IsChildWithParents does not detect unsorted parents.
         BOOST_CHECK(IsChildWithParents({tx_parent_also_child, tx_parent, tx_child}));
         BOOST_CHECK(CheckPackage({tx_parent, tx_parent_also_child, tx_child}, state));