Package validation: accept packages of size 1

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

doc/policy/packages.md

@@ -76,7 +76,7 @@ The following rules are only enforced for packages to be submitted to the mempoo
 enforced for test accepts):
 
 * Packages must be child-with-unconfirmed-parents packages. This also means packages must contain at
-  least 2 transactions. (#22674)
+  least 1 transaction. (#31096)
 
    - *Rationale*: This allows for fee-bumping by CPFP. Allowing multiple parents makes it possible
      to fee-bump a batch of transactions. Restricting packages to a defined topology is easier to

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

@@ -923,7 +923,7 @@ static RPCHelpMan submitpackage()
         ,
         {
             {"package", RPCArg::Type::ARR, RPCArg::Optional::NO, "An array of raw transactions.\n"
-                "The package must solely consist of a child and its parents. None of the parents may depend on each other.\n"
+                "The package must solely consist of a child transaction and all of its unconfirmed parents, if any. None of the parents may depend on each other.\n"
                 "The package must be topologically sorted, with the child being the last element in the array.",
                 {
                     {"rawtx", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, ""},
@@ -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_child}));
+        BOOST_CHECK(IsChildWithParentsTree({tx_child}));
+
+        // 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

@@ -369,8 +369,8 @@ def test_submit_child_with_parents(self, num_parents, partial_submit):
     def test_submitpackage(self):
         node = self.nodes[0]
 
-        self.log.info("Submitpackage valid packages with 1 child and some number of parents")
-        for num_parents in [1, 2, 24]:
+        self.log.info("Submitpackage valid packages with 1 child and some number of parents (or none)")
+        for num_parents in [0, 1, 2, 24]:
             self.test_submit_child_with_parents(num_parents, False)
             self.test_submit_child_with_parents(num_parents, True)
 
@@ -381,10 +381,9 @@ 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)
         )