Auto merge of #1797 - ebfull:improve-joinsplit-diagnostics, r=bitcartel

Improve joinsplit diagnostics

I don't advocate merging this for the hotfix release (to fix #1779) but this PR can be used to diagnose the real issue and should be merged ASAP afterward.

~I still need to add tests for `last()` and `element()` though.~ Done.

src/gtest/test_joinsplit.cpp

@@ -204,6 +204,7 @@ void invokeAPIFailure(
 {
     try {
         invokeAPI(js, inputs, outputs, vpub_old, vpub_new, rt);
+        FAIL() << "It worked, when it shouldn't have!";
     } catch(std::invalid_argument const & err) {
         EXPECT_EQ(err.what(), reason);
     } catch(...) {
@@ -368,7 +369,7 @@ TEST(joinsplit, full_api_test)
         tree.root(),
         "nonsensical vpub_new value");
 
-        // input is not in tree
+        // input witness for the wrong element
         invokeAPIFailure(js,
         {
             JSInput(witnesses[0], note1, sk),
@@ -381,6 +382,22 @@ TEST(joinsplit, full_api_test)
         0,
         100,
         tree.root(),
+        "witness of wrong element for joinsplit input");
+
+        // input witness doesn't match up with
+        // real root
+        invokeAPIFailure(js,
+        {
+            JSInput(witnesses[1], note1, sk),
+            JSInput()
+        },
+        {
+            JSOutput(),
+            JSOutput()
+        },
+        0,
+        100,
+        uint256(),
         "joinsplit not anchored to the correct root");
 
         // input is in the tree now! this should work

src/gtest/test_merkletree.cpp

@@ -5,6 +5,7 @@
 #include "test/data/merkle_serialization.json.h"
 #include "test/data/merkle_witness_serialization.json.h"
 #include "test/data/merkle_path.json.h"
+#include "test/data/merkle_commitments.json.h"
 
 #include <iostream>
 
@@ -55,32 +56,46 @@ void expect_ser_test_vector(B& b, const C& c, const A& tree) {
 }
 
 template<typename Tree, typename Witness>
-void test_tree(Array root_tests, Array ser_tests, Array witness_ser_tests, Array path_tests) {
+void test_tree(
+    Array commitment_tests,
+    Array root_tests,
+    Array ser_tests,
+    Array witness_ser_tests,
+    Array path_tests
+)
+{
+    Array::iterator commitment_iterator = commitment_tests.begin();
     Array::iterator root_iterator = root_tests.begin();
     Array::iterator ser_iterator = ser_tests.begin();
     Array::iterator witness_ser_iterator = witness_ser_tests.begin();
     Array::iterator path_iterator = path_tests.begin();
 
-    uint256 test_commitment = uint256S("e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855");
-
     Tree tree;
 
     // The root of the tree at this point is expected to be the root of the
     // empty tree.
     ASSERT_TRUE(tree.root() == Tree::empty_root());
 
+    // The tree doesn't have a 'last' element added since it's blank.
+    ASSERT_THROW(tree.last(), std::runtime_error);
+
     // We need to witness at every single point in the tree, so
     // that the consistency of the tree and the merkle paths can
     // be checked.
     vector<Witness> witnesses;
 
     for (size_t i = 0; i < 16; i++) {
+        uint256 test_commitment = uint256S((commitment_iterator++)->get_str());
+
         // Witness here
         witnesses.push_back(tree.witness());
 
         // Now append a commitment to the tree
         tree.append(test_commitment);
 
+        // Last element added to the tree was `test_commitment`
+        ASSERT_TRUE(tree.last() == test_commitment);
+
         // Check tree root consistency
         expect_test_vector(root_iterator, tree.root());
 
@@ -95,6 +110,7 @@ void test_tree(Array root_tests, Array ser_tests, Array witness_ser_tests, Array
 
             if (first) {
                 ASSERT_THROW(wit.path(), std::runtime_error);
+                ASSERT_THROW(wit.element(), std::runtime_error);
             } else {
                 auto path = wit.path();
 
@@ -119,7 +135,8 @@ void test_tree(Array root_tests, Array ser_tests, Array witness_ser_tests, Array
 
                     std::vector<bool> commitment_bv;
                     {
-                        std::vector<unsigned char> commitment_v(test_commitment.begin(), test_commitment.end());
+                        uint256 witnessed_commitment = wit.element();
+                        std::vector<unsigned char> commitment_v(witnessed_commitment.begin(), witnessed_commitment.end());
                         commitment_bv = convertBytesVectorToVector(commitment_v);
                     }
 
@@ -174,8 +191,9 @@ TEST(merkletree, vectors) {
     Array ser_tests = read_json(std::string(json_tests::merkle_serialization, json_tests::merkle_serialization + sizeof(json_tests::merkle_serialization)));
     Array witness_ser_tests = read_json(std::string(json_tests::merkle_witness_serialization, json_tests::merkle_witness_serialization + sizeof(json_tests::merkle_witness_serialization)));
     Array path_tests = read_json(std::string(json_tests::merkle_path, json_tests::merkle_path + sizeof(json_tests::merkle_path)));
+    Array commitment_tests = read_json(std::string(json_tests::merkle_commitments, json_tests::merkle_commitments + sizeof(json_tests::merkle_commitments)));
 
-    test_tree<ZCTestingIncrementalMerkleTree, ZCTestingIncrementalWitness>(root_tests, ser_tests, witness_ser_tests, path_tests);
+    test_tree<ZCTestingIncrementalMerkleTree, ZCTestingIncrementalWitness>(commitment_tests, root_tests, ser_tests, witness_ser_tests, path_tests);
 }
 
 TEST(merkletree, emptyroots) {