State management and VM instance creation

How the AppLayer ecosystem manages the blockchain state alongside an EVM.

The VM itself is owned and instantiated by the State class, which reflects a crucial design decision: centralizing virtual machine resource management like this ensures that each contract execution context is cleanly managed and isolated. Whenever a new transaction or contract call needs to be executed, regardless of its nature (be it an EVM/C++ contract execution or a simple native transfer), the State class is responsible for instantiating a new ContractHost object with the relevant parameters required for execution:

ContractHost(
  evmc_vm* vm,
  EventManager& eventManager,
  const Storage& storage,
  const evmc_tx_context& currentTxContext,
  std::unordered_map<Address, NonNullUniquePtr<Account>, SafeHash>& accounts,
  std::unordered_map<StorageKey, Hash, SafeHash>& vmStorage,
  const Hash& txHash,
  const uint64_t txIndex,
  const Hash& blockHash,
  int64_t& txGasLimit
);

Once an instance of ContractHost is created, it offers methods like execute() to run the contract, simulate() for simulating the transaction (useful for gas estimation), and ethCallView() for making calls to other contracts within a non-state-changing context.

ContractHost also extends the functionalities of evmc::Host by overriding several key functions that interface directly with the Ethereum Virtual Machine, which are obligatory for the VM to interact with the State:

bool account_exists(const evmc::address& addr) const noexcept final;
evmc::bytes32 get_storage(const evmc::address& addr, const evmc::bytes32& key) const noexcept final;
evmc_storage_status set_storage(const evmc::address& addr, const evmc::bytes32& key, const evmc::bytes32& value) noexcept final;
evmc::uint256be get_balance(const evmc::address& addr) const noexcept final;
size_t get_code_size(const evmc::address& addr) const noexcept final;
evmc::bytes32 get_code_hash(const evmc::address& addr) const noexcept final;
size_t copy_code(const evmc::address& addr, size_t code_offset, uint8_t* buffer_data, size_t buffer_size) const noexcept final;
bool selfdestruct(const evmc::address& addr, const evmc::address& beneficiary) noexcept final;
evmc::Result call(const evmc_message& msg) noexcept final;
evmc_tx_context get_tx_context() const noexcept final;
evmc::bytes32 get_block_hash(int64_t number) const noexcept final;
void emit_log(const evmc::address& addr, const uint8_t* data, size_t data_size, const evmc::bytes32 topics[], size_t topics_count) noexcept final;
evmc_access_status access_account(const evmc::address& addr) noexcept final;
evmc_access_status access_storage(const evmc::address& addr, const evmc::bytes32& key) noexcept final;
evmc::bytes32 get_transient_storage(const evmc::address &addr, const evmc::bytes32 &key) const noexcept final;
void set_transient_storage(const evmc::address &addr, const evmc::bytes32 &key, const evmc::bytes32 &value) noexcept final;

These methods manage everything from account validation to logging, providing access to the state and storage, and handling calls between contracts. The ContractHost class encapsulates these functions, ensuring that each contract execution is isolated and secure.