Package validation: accept packages of size 1

Users may want to entirely switch to submitpackage
even for loose transactions. We should let them.

src/policy/packages.cpp

@@ -118,8 +118,9 @@ bool IsWellFormedPackage(const Package& txns, PackageValidationState& state, boo
 
 bool IsChildWithParents(const Package& package)
 {
+    if (package.empty()) return false;
     assert(std::all_of(package.cbegin(), package.cend(), [](const auto& tx){return tx != nullptr;}));
-    if (package.size() < 2) return false;
+    if (package.size() < 2) return true;
 
     // The package is expected to be sorted, so the last transaction is the child.
     const auto& child = package.back();

src/rpc/mempool.cpp

@@ -971,9 +971,9 @@ static RPCHelpMan submitpackage()
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
         {
             const UniValue raw_transactions = request.params[0].get_array();
-            if (raw_transactions.size() < 2 || raw_transactions.size() > MAX_PACKAGE_COUNT) {
+            if (raw_transactions.empty() || raw_transactions.size() > MAX_PACKAGE_COUNT) {
                 throw JSONRPCError(RPC_INVALID_PARAMETER,
-                                   "Array must contain between 2 and " + ToString(MAX_PACKAGE_COUNT) + " transactions.");
+                                   "Array must contain between 1 and " + ToString(MAX_PACKAGE_COUNT) + " transactions.");
             }
 
             // Fee check needs to be run with chainstate and package context

src/test/txpackage_tests.cpp

@@ -283,6 +283,14 @@ BOOST_AUTO_TEST_CASE(noncontextual_package_tests)
         BOOST_CHECK(GetPackageHash({tx_parent}) != GetPackageHash({tx_child}));
         BOOST_CHECK(GetPackageHash({tx_child, tx_child}) != GetPackageHash({tx_child}));
         BOOST_CHECK(GetPackageHash({tx_child, tx_parent}) != GetPackageHash({tx_child, tx_child}));
+
+        // IsChildWithParents* can also be a singleton
+        BOOST_CHECK(IsChildWithParents({tx_parent}));
+        BOOST_CHECK(IsChildWithParentsTree({tx_parent}));
+
+        // But must not be empty
+        BOOST_CHECK(!IsChildWithParents({}));
+        BOOST_CHECK(!IsChildWithParentsTree({}));
     }
 
     // 24 Parents and 1 Child

src/validation.cpp

@@ -1695,8 +1695,9 @@ PackageMempoolAcceptResult MemPoolAccept::AcceptPackage(const Package& package,
         return PackageMempoolAcceptResult(package_state_quit_early, {});
     }
 
-    // IsChildWithParents() guarantees the package is > 1 transactions.
-    assert(package.size() > 1);
+    // IsChildWithParents() guarantees the package is not empty.
+    assert(!package.empty());
+
     // The package must be 1 child with all of its unconfirmed parents. The package is expected to
     // be sorted, so the last transaction is the child.
     const auto& child = package.back();

test/functional/rpc_packages.py

@@ -381,13 +381,15 @@ def test_submitpackage(self):
         assert_raises_rpc_error(-25, "package topology disallowed", node.submitpackage, chain_hex)
         assert_equal(legacy_pool, node.getrawmempool())
 
-        assert_raises_rpc_error(-8, f"Array must contain between 2 and {MAX_PACKAGE_COUNT} transactions.", node.submitpackage, [])
-        assert_raises_rpc_error(-8, f"Array must contain between 2 and {MAX_PACKAGE_COUNT} transactions.", node.submitpackage, [chain_hex[0]] * 1)
+        assert_raises_rpc_error(-8, f"Array must contain between 1 and {MAX_PACKAGE_COUNT} transactions.", node.submitpackage, [])
         assert_raises_rpc_error(
-            -8, f"Array must contain between 2 and {MAX_PACKAGE_COUNT} transactions.",
+            -8, f"Array must contain between 1 and {MAX_PACKAGE_COUNT} transactions.",
             node.submitpackage, [chain_hex[0]] * (MAX_PACKAGE_COUNT + 1)
         )
 
+        # Singleton is ok
+        node.submitpackage([chain_hex[0]] * 1)
+
         # Create a transaction chain such as only the parent gets accepted (by making the child's
         # version non-standard). Make sure the parent does get broadcast.
         self.log.info("If a package is partially submitted, transactions included in mempool get broadcast")