Log json exception details on chain config file load error

CHANGELOG.md

@@ -12,5 +12,6 @@
 - Fixed: [#5452](https://github.com/ethereum/aleth/pull/5452) Correctly handle Discovery messages when the peer changes public key.
 - Fixed: [#5519](https://github.com/ethereum/aleth/pull/5519) Correctly handle Discovery messages with known public key but unknown endpoint.
 - Fixed: [#5523](https://github.com/ethereum/aleth/pull/5523) [#5533](https://github.com/ethereum/aleth/pull/5533) Fix syncing terminating prematurely because of race condition.
+- Added: [#5526](https://github.com/ethereum/aleth/pull/5526) Improved logging when loading chain config json containing syntax error.
 
 [1.6.0]: https://github.com/ethereum/aleth/compare/v1.6.0-alpha.1...master

aleth/main.cpp

@@ -687,9 +687,10 @@ int main(int argc, char** argv)
         }
         catch (...)
         {
-            cerr << "provided configuration is not well formatted\n";
-            cerr << "sample: \n" << genesisInfo(eth::Network::MainNetworkTest) << "\n";
-            return AlethErrors::Success;
+            cerr << "provided configuration is not well-formatted\n";
+            cerr << "well-formatted sample: \n"
+                 << genesisInfo(eth::Network::MainNetworkTest) << "\n";
+            return AlethErrors::ConfigFileInvalid;
         }
     }
 

libdevcore/Exceptions.h

@@ -70,6 +70,7 @@ DEV_SIMPLE_EXCEPTION(FailedInvariant);
 DEV_SIMPLE_EXCEPTION(ValueTooLarge);
 DEV_SIMPLE_EXCEPTION(UnknownField);
 DEV_SIMPLE_EXCEPTION(MissingField);
+DEV_SIMPLE_EXCEPTION(SyntaxError);
 DEV_SIMPLE_EXCEPTION(WrongFieldType);
 DEV_SIMPLE_EXCEPTION(InterfaceNotSupported);
 DEV_SIMPLE_EXCEPTION(ExternalFunctionFailure);

libethcore/Common.h

@@ -226,6 +226,7 @@ enum AlethErrors
     UnknownArgument,
     UnknownMiningOption,
     ConfigFileEmptyOrNotFound,
+    ConfigFileInvalid,
     UnknownNetworkType,
     BadNetworkIdOption,
     BadConfigOption,

libethereum/ChainParams.cpp

@@ -59,7 +59,20 @@ ChainParams ChainParams::loadConfig(
 {
     ChainParams cp(*this);
     js::mValue val;
-    js::read_string_or_throw(_json, val);
+
+    try
+    {
+        js::read_string_or_throw(_json, val);
+    }
+    catch (js::Error_position const& error)
+    {
+        std::string const comment = "json parsing error detected on line " +
+                                    std::to_string(error.line_) + " in column " +
+                                    std::to_string(error.column_) + ": " + error.reason_;
+        std::cerr << comment << "\n";
+        BOOST_THROW_EXCEPTION(SyntaxError() << errinfo_comment(comment));
+    }
+
     js::mObject obj = val.get_obj();
 
     validateConfigJson(obj);

test/unittests/libethereum/BlockChain.cpp

@@ -408,7 +408,7 @@ BOOST_AUTO_TEST_CASE(invalidJsonThrows)
 {
     h256 emptyStateRoot;
     /* Below, a comma is missing between fields. */
-    BOOST_CHECK_THROW(ChainParams("{ \"sealEngine\" : \"unknown\" \"accountStartNonce\" : \"3\" }", emptyStateRoot), json_spirit::Error_position);
+    BOOST_CHECK_THROW(ChainParams("{ \"sealEngine\" : \"unknown\" \"accountStartNonce\" : \"3\" }", emptyStateRoot), SyntaxError);
 }
 
 BOOST_AUTO_TEST_CASE(unknownFieldThrows)