use of parking_lot send_guard feature prevents use of deadlock_detection in overall program

[wez]

Not sure what you think of this, but I'm trying to track down an issue that manifests like a deadlock, and I'd like to use parking_lot's deadlock_detection feature to to see if it can help with this.

The use of send_guard here in mlua when send is enabled is not compatible with that feature and prevents the program from compiling with a compile error.

If I build against mlua from git with this quick and dirty patch:

diff --git a/Cargo.toml b/Cargo.toml
index 91f373b..82c071b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -38,7 +38,8 @@ luau-vector4 = ["luau", "ffi/luau-vector4"]
 vendored = ["ffi/vendored"]
 module = ["dep:mlua_derive", "ffi/module"]
 async = ["dep:futures-util"]
-send = ["parking_lot/send_guard", "error-send"]
+send = ["error-send"]
 error-send = []
 serialize = ["dep:serde", "dep:erased-serde", "dep:serde-value", "bstr/serde"]
 macros = ["mlua_derive/macros"]

then my overall program continues to compile and will allow me to enable deadlock_detection.

Can we find a way to allow the embedding program to opt-in or opt-out of using send_guard?

AFAICT from a quick look, it was added in #147 to facilitate external usage, so it doesn't appear as though mlua requires that feature at all.

Thanks!

[khvzak]

The parking_lot/send_guard is needed only for userdata-wrappers feature (if remove parking_lot/send_guard and add userdata-wrappers then mlua would not compile).

This is because (when userdata-wrappers is enabled), the UserDataRef<T> can hold MutexGuard, which is Send-able only when GuardMarker: Send

Unfortunately there is not way to say in Cargo.toml that when both send and userdata-wrappers are enabled, add parking_lot/send_guard requirement.

I probably can move send_guard to userdata-wrappers = ["parking_lot/send_guard"] which makes more sense?