Custom error validator (#540)

Fixes #541

src/rpc/common/Validators.h

@@ -377,6 +377,36 @@ private:
         validator_;
 };
 
+/**
+ * @brief A meta-validator that wrapp other validator to send the customized
+ * error
+ */
+template <typename Requirement>
+class WithCustomError final
+{
+    Requirement requirement;
+    RPC::Status error;
+
+public:
+    /**
+     * @brief Constructs a validator that calls the given validator "req" and
+     * return customized error "err"
+     */
+    WithCustomError(Requirement req, RPC::Status err)
+        : requirement{std::move(req)}, error{err}
+    {
+    }
+
+    [[nodiscard]] MaybeError
+    verify(boost::json::value const& value, std::string_view key) const
+    {
+        if (auto const res = requirement.verify(value, key); not res)
+            return Error{error};
+
+        return {};
+    }
+};
+
 /**
  * @brief A meta-validator that allows to specify a custom validation function
  */

unittests/rpc/BaseTests.cpp

@@ -258,6 +258,36 @@ TEST_F(RPCBaseTest, IfTypeValidator)
     ASSERT_FALSE(spec.validate(failingInput));
 }
 
+TEST_F(RPCBaseTest, WithCustomError)
+{
+    auto const spec = RpcSpec{
+        {"transaction",
+         WithCustomError{
+             Uint256HexStringValidator,
+             RPC::Status{ripple::rpcBAD_FEATURE, "MyCustomError"}}},
+        {"other",
+         WithCustomError{
+             Type<std::string>{},
+             RPC::Status{ripple::rpcALREADY_MULTISIG, "MyCustomError2"}}}};
+
+    auto const passingInput = json::parse(
+        R"({ "transaction": "1B8590C01B0006EDFA9ED60296DD052DC5E90F99659B25014D08E1BC983515BC", "other": "1"})");
+    ASSERT_TRUE(spec.validate(passingInput));
+
+    auto failingInput = json::parse(
+        R"({ "transaction": "1B8590C01B0006EDFA9ED60296DD052DC5E90F99659B25014D08E1BC983515B"})");
+    auto err = spec.validate(failingInput);
+    ASSERT_FALSE(err);
+    ASSERT_EQ(err.error().message, "MyCustomError");
+    ASSERT_EQ(err.error(), ripple::rpcBAD_FEATURE);
+
+    failingInput = json::parse(R"({ "other": 1})");
+    err = spec.validate(failingInput);
+    ASSERT_FALSE(err);
+    ASSERT_EQ(err.error().message, "MyCustomError2");
+    ASSERT_EQ(err.error(), ripple::rpcALREADY_MULTISIG);
+}
+
 TEST_F(RPCBaseTest, CustomValidator)
 {
     // clang-format off