fc exception handling

[heifner]

fc::exception has support for dynamic_rethrow_exception which is documented as:

   /**
    * Rethrows the exception restoring the proper type based upon
    * the error code.  This is used to propagate exception types
    * across conversions to/from JSON
    */

We use this in a few places:

spring/plugins/http_plugin/include/eosio/http_plugin/macros.hpp

Line 16 in 8169da9

std::get<fc::exception_ptr>(result)->dynamic_rethrow_exception(); \

spring/libraries/chain/snapshot_scheduler.cpp

Line 136 in 8169da9

std::get<fc::exception_ptr>(result)->dynamic_rethrow_exception();

This requires:

spring/libraries/libfc/src/exception.cpp

Lines 10 to 28 in 8169da9

	FC_REGISTER_EXCEPTIONS( (timeout_exception)
	(file_not_found_exception)
	(parse_error_exception)
	(invalid_arg_exception)
	(invalid_operation_exception)
	(key_not_found_exception)
	(bad_cast_exception)
	(out_of_range_exception)
	(canceled_exception)
	(assert_exception)
	(eof_exception)
	(unknown_host_exception)
	(null_optional)
	(udt_exception)
	(aes_exception)
	(overflow_exception)
	(underflow_exception)
	(divide_by_zero_exception)
	)

Which we do NOT do for any of the chain exceptions in:

spring/libraries/chain/include/eosio/chain/exceptions.hpp

Line 185 in 8169da9

FC_DECLARE_DERIVED_EXCEPTION( chain_type_exception, chain_exception,

Note we add an error_code to chain_exception which all chain exceptions derive from. This is used here:

spring/libraries/chain/controller.cpp

Lines 5858 to 5866 in 8169da9

	std::optional<uint64_t> controller::convert_exception_to_error_code( const fc::exception& e ) {
	const chain_exception* e_ptr = dynamic_cast<const chain_exception*>( &e );
	if( e_ptr == nullptr ) return {};
	if( !e_ptr->error_code ) return static_cast<uint64_t>(system_error_code::generic_system_error);
	return e_ptr->error_code;
	}

But even if we did register them, they would slice off the error_code on rethrow:

spring/libraries/libfc/include/fc/exception/exception.hpp

Lines 221 to 224 in 8169da9

	virtual NO_RETURN void rethrow( const exception& e )const override
	{
	throw T( e );
	}

When exceptions are placed action_trace and transaction_trace they are sliced to a fc::exception losing this error_code:

spring/libraries/chain/include/eosio/chain/trace.hpp

Line 45 in 8169da9

std::optional<fc::exception> except;

When an exception is placed in the action_trace or transaction_trace and is rethrown, it is always rethrown as a fc::exception. As commented here:

spring/libraries/testing/tester.cpp

Lines 485 to 486 in 8169da9

	// this always throws an fc::exception, since the original exception is copied into an fc::exception
	trace->except->dynamic_rethrow_exception();

Changing action_trace and transaction_trace to use an fc::exception_ptr is an API breaking change.
Changing fc::exception to have an error_code is also an API breaking change. Since this is an addition, maybe this would cause no harm. Not sure if that affects consensus.

[bhazzard]

Plan for 1.1.0 is to avoid breaking change in API, and remove dynamic-rethrow-exception