Errors returned to lua

[soerenmeier]

When deserialization fails in a next call this error gets returned from rust:

rust-server/src/lib.rs

Lines 151 to 158 in 4a04655

	next.success(lua, &res).map_err(|err| {
	mlua::Error::ExternalError(Arc::new(Error::DeserializeResult {
	err,
	method,
	result: pretty_print_value(res, 0)
	.unwrap_or_else(|err| format!("failed to pretty print result: {}", err)),
	}))
	})?;

But in lua only the following line get's outputed: Error retrieving next command: attempt to concatenate a userdata value. This is very misleading.
I tried to debug this but I haven't found anything. Something might be wrong with the lua version of dcs.
Did this works once and is this a known issue?

mlua:
function that passes the error to lua: callback_error
function that converts the mlua error into a string: init_error_registry

[rkusa]

This is indeed very unhelpful. I've seen this error too, and as far as I remember, it was caused by the hot-reloading. Are you using the hot-reloading feature by any chance?
It would have been fixed in #150, but we decided to not merge it. If you are using the hot-reloading feature, I am afraid that the only workaround right now would be to not use hot-reloading, or to make a local change similar to: https://github.com/DCS-gRPC/rust-server/pull/150/files#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R168

[soerenmeier]

No, I compiled with cargo build --release and then copied dcs_grpc.dll and the pdb file to the mods folder.

For the moment i copied the next function and renamed it to try_next and now call that function from next and log::error the Err case.

[rkusa]

I can reproduce it now.

While trying to create a small example that reproduces the error to report the issue to mlua, I've learned that this seems to be caused by DCS' lua.dll instead. I couldn't reproduce it outside of DCS, and then dropped DCS' lua.dll into my example, and boom, error was there.
I unfortunately have no idea, why this is caused by DCS' lua.dll (but I am not surprised either). I've confirmed that DCS runs Lua 5.1.5 and that DCS-gRPC is compiled against exactly this version.

DCS:

DCS-gRPC:
https://github.com/DCS-gRPC/rust-server/blob/main/src/lua5.1/include/lua.h#L20

What is strange though, if I download the Lua 5.1.5 release, its dll has clearly a different size (308 KB) compared to the on from DCS (257 KB).

In summary. Not caused by mlua. Seems to be caused by DCS' lua.dll. For now, I have no idea how to solve this (except avoiding to return errors from Rust to Lua).

[soerenmeier]

That's weird, probably dcs compiled lua with some special flags or something.

Personally, I would probably just do something like this: soerenmeier@7386d9a the errors are always outputted to the dcs-grpc.log and if the lua version supports it also to lua.

[rkusa]

@soerenmeier That is definitely a solution I'd also have in mind 👍. Our only concern in the past was that people tend to look into the dcs.log file for errors and easily forget that there is also a DCS-gRPC.log file. I have some ideas of how to pipe them into dcs.log without having to return them as Lua errors. I'll try to create a POC for that soon.