EVM runtime: handle REVERT

[raulk]

REVERT is equivalent to an FVM abort syscall. It interrupts the callee but returns control to the caller. However, REVERT is capable of returning a payload, which Filecoin disallows in combination with a non-zero exit code (originating in an abort). Thus, supporting this as an abort may require a protocol extension. There are alternative solutions that @Stebalien will document shortly.

[vyzo]

Possibly the simplest way to handle this is to return a struct from InvokeContract in the EvmContractActor, which includes a boolean on whether it was a revert, together with the return value.

[Stebalien]

Proposal: Replace abort with an exit syscall.

fn exit(code: u32,       // Exit code.
        value: u32,      // Block ID (return value).
        msg: *const u8,  // Optional message.
        msg_len: u32);

• If code is non-zero, revert.
• Always return the value where 0 identifies the "empty" value.
• The message is optional and only for tracing (it doesn't affect consensus).

[maciejwitowski]

REVERT implementation filecoin-project/builtin-actors#633
As per notes:

REVERT doesn't propagate return data yet; this will be fixed in a subsequent pr, as it requires fvm support for aborting with data.