When signing a block that updates the signing witness's signing key, use correct signing key

[vikramrajkumar]

From @theoreticalbts on November 2, 2015 13:27

If block applies transactions then validates witness signature, we should sign with the new key.

If block validates witness signature then applies transactions, we should sign with the old key.

Copied from original issue: cryptonomex/graphene#430

[abitmore]

Just realized this is not a consensus changing issue, but a bug about generating / signing blocks.

Block signing key is checked here before generating the block:

bitshares-core/libraries/chain/db_block.cpp

Lines 334 to 337 in 60e9314

	const auto& witness_obj = witness_id(*this);
	if( !(skip & skip_witness_signature) )
	FC_ASSERT( witness_obj.signing_key == block_signing_private_key.get_public_key() );

At a glance the check is done before generating the block, however, actually the check is done with pending transactions, which means the state is dirty -- if there is a witness_update_operation pending, witness_obj.signing_key retrieved by the code would actually be the new key, consequently, the block will be signed with the new key.

Then, when pushing the block, pending transactions will be temporarily removed first, so the code will check whether the block is signed with the old key:

bitshares-core/libraries/chain/db_block.cpp

Lines 120 to 124 in 60e9314

	detail::without_pending_transactions( *this, std::move(_pending_tx),
	[&]()
	{
	result = _push_block(new_block);
	});

bitshares-core/libraries/chain/db_block.cpp

Lines 664 to 667 in 60e9314

	const witness_object& witness = next_block.witness(*this);
	if( !(skip&skip_witness_signature) )
	FC_ASSERT( next_block.validate_signee( witness.signing_key ) );

I'll try to fix this. After it's fixed, switching keys (at any time) will no longer cause missing of blocks.

[abitmore]

This code checks signing key before trying to generate a block, which is done with pending transactions / dirty state as well:

bitshares-core/libraries/plugins/witness/witness.cpp

Lines 253 to 260 in 60e9314

	graphene::chain::public_key_type scheduled_key = scheduled_witness( db ).signing_key;
	auto private_key_itr = _private_keys.find( scheduled_key );
	if( private_key_itr == _private_keys.end() )
	{
	capture("scheduled_key", scheduled_key);
	return block_production_condition::no_private_key;
	}

Need to fix.

[abitmore]

See PR #1203.

[pmconrad]

Good catch!

[abitmore]

Fixed by #1203.

[abitmore]

Interestingly, Steem is moving the whole block generation code to witness plugin: steemit/steem#2861 steemit/steem#2717