Send trait and thread safety question

[inikolaev]

When compiling this code

#[pyclass]
struct CelProgram {
    program: Program
}

I'm running into the following error:

error[E0277]: `Rc<std::string::String>` cannot be sent between threads safely
   --> src/lib.rs:24:1
    |
24  | #[pyclass]
    | ^^^^^^^^^^ `Rc<std::string::String>` cannot be sent between threads safely
    |
    = help: within `CelProgram`, the trait `Send` is not implemented for `Rc<std::string::String>`
note: required because it appears within the type `Expression`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:32:10
    |
32  | pub enum Expression {
    |          ^^^^^^^^^^
note: required because it appears within the type `Program`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-interpreter-0.4.1/src/lib.rs:105:12
    |
105 | pub struct Program {
    |            ^^^^^^^
note: required because it appears within the type `CelProgram`
   --> src/lib.rs:25:8
    |
25  | struct CelProgram {
    |        ^^^^^^^^^^
note: required by a bound in `ThreadCheckerStub`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.19.2/src/impl_/pyclass.rs:894:33
    |
894 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
    |                                 ^^^^ required by this bound in `ThreadCheckerStub`
    = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `Rc<Vec<u8>>` cannot be sent between threads safely
   --> src/lib.rs:24:1
    |
24  | #[pyclass]
    | ^^^^^^^^^^ `Rc<Vec<u8>>` cannot be sent between threads safely
    |
    = help: within `CelProgram`, the trait `Send` is not implemented for `Rc<Vec<u8>>`
note: required because it appears within the type `Atom`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:59:10
    |
59  | pub enum Atom {
    |          ^^^^
note: required because it appears within the type `Expression`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:32:10
    |
32  | pub enum Expression {
    |          ^^^^^^^^^^
note: required because it appears within the type `Program`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-interpreter-0.4.1/src/lib.rs:105:12
    |
105 | pub struct Program {
    |            ^^^^^^^
note: required because it appears within the type `CelProgram`
   --> src/lib.rs:25:8
    |
25  | struct CelProgram {
    |        ^^^^^^^^^^
note: required by a bound in `ThreadCheckerStub`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.19.2/src/impl_/pyclass.rs:894:33
    |
894 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
    |                                 ^^^^ required by this bound in `ThreadCheckerStub`
    = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.

I had a temporary solution for it, which worked

unsafe impl Send for CelProgram {}

but it feels like a hack to me, at least without proper understanding, and I would like to finally figure out how to do this.

In this case I don't really need this program attribute to be accessed from a different thread. I need it to be constructed when I instantiate an instance of CelProgram from Python and then use it when evaluate method is called and I don't need to hold GIL for that.

Here's an example Python code of how it's going to be used:

cel_program = CelProgram("1 == 1")
cel_program.evaluate({})

I feel like using unsafe as I do now is not the right way to do this, so would appreciate any pointers.

[messense]

https://pyo3.rs/v0.19.2/class.html?highlight=unsendable#customizing-the-class

[mejrs]

This is your type, right? You can use Arc instead of Rc.

I feel like using unsafe as I do now is not the right way to do this

Yeah, your approach is completely wrong. Do not do this.

[inikolaev]

This is what I've tried, but I'm still getting a similar error:

#[pyclass]
struct CelProgram {
    program: Arc<Program>
}


#[pymethods]
impl CelProgram {
    #[new]
    fn new(expr: String) -> Self {
        let program = Arc::new(Program::compile(&expr).unwrap());
        CelProgram { program }
    }
}

And the error I'm getting is

error[E0277]: `Rc<std::string::String>` cannot be shared between threads safely
   --> src/lib.rs:25:1
    |
25  | #[pyclass]
    | ^^^^^^^^^^ `Rc<std::string::String>` cannot be shared between threads safely
    |
    = help: within `Program`, the trait `Sync` is not implemented for `Rc<std::string::String>`
note: required because it appears within the type `Expression`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:32:10
    |
32  | pub enum Expression {
    |          ^^^^^^^^^^
note: required because it appears within the type `Program`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-interpreter-0.4.1/src/lib.rs:105:12
    |
105 | pub struct Program {
    |            ^^^^^^^
    = note: required for `Arc<Program>` to implement `Send`
note: required because it appears within the type `CelProgram`
   --> src/lib.rs:26:8
    |
26  | struct CelProgram {
    |        ^^^^^^^^^^
note: required by a bound in `ThreadCheckerStub`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.19.2/src/impl_/pyclass.rs:894:33
    |
894 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
    |                                 ^^^^ required by this bound in `ThreadCheckerStub`
    = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `Rc<std::string::String>` cannot be sent between threads safely
   --> src/lib.rs:25:1
    |
25  | #[pyclass]
    | ^^^^^^^^^^ `Rc<std::string::String>` cannot be sent between threads safely
    |
    = help: within `Program`, the trait `Send` is not implemented for `Rc<std::string::String>`
note: required because it appears within the type `Expression`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:32:10
    |
32  | pub enum Expression {
    |          ^^^^^^^^^^
note: required because it appears within the type `Program`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-interpreter-0.4.1/src/lib.rs:105:12
    |
105 | pub struct Program {
    |            ^^^^^^^
    = note: required for `Arc<Program>` to implement `Send`
note: required because it appears within the type `CelProgram`
   --> src/lib.rs:26:8
    |
26  | struct CelProgram {
    |        ^^^^^^^^^^
note: required by a bound in `ThreadCheckerStub`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.19.2/src/impl_/pyclass.rs:894:33
    |
894 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
    |                                 ^^^^ required by this bound in `ThreadCheckerStub`
    = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `Rc<Vec<u8>>` cannot be shared between threads safely
   --> src/lib.rs:25:1
    |
25  | #[pyclass]
    | ^^^^^^^^^^ `Rc<Vec<u8>>` cannot be shared between threads safely
    |
    = help: within `Program`, the trait `Sync` is not implemented for `Rc<Vec<u8>>`
note: required because it appears within the type `Atom`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:59:10
    |
59  | pub enum Atom {
    |          ^^^^
note: required because it appears within the type `Expression`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:32:10
    |
32  | pub enum Expression {
    |          ^^^^^^^^^^
note: required because it appears within the type `Program`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-interpreter-0.4.1/src/lib.rs:105:12
    |
105 | pub struct Program {
    |            ^^^^^^^
    = note: required for `Arc<Program>` to implement `Send`
note: required because it appears within the type `CelProgram`
   --> src/lib.rs:26:8
    |
26  | struct CelProgram {
    |        ^^^^^^^^^^
note: required by a bound in `ThreadCheckerStub`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.19.2/src/impl_/pyclass.rs:894:33
    |
894 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
    |                                 ^^^^ required by this bound in `ThreadCheckerStub`
    = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: `Rc<Vec<u8>>` cannot be sent between threads safely
   --> src/lib.rs:25:1
    |
25  | #[pyclass]
    | ^^^^^^^^^^ `Rc<Vec<u8>>` cannot be sent between threads safely
    |
    = help: within `Program`, the trait `Send` is not implemented for `Rc<Vec<u8>>`
note: required because it appears within the type `Atom`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:59:10
    |
59  | pub enum Atom {
    |          ^^^^
note: required because it appears within the type `Expression`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-parser-0.5.1/src/ast.rs:32:10
    |
32  | pub enum Expression {
    |          ^^^^^^^^^^
note: required because it appears within the type `Program`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cel-interpreter-0.4.1/src/lib.rs:105:12
    |
105 | pub struct Program {
    |            ^^^^^^^
    = note: required for `Arc<Program>` to implement `Send`
note: required because it appears within the type `CelProgram`
   --> src/lib.rs:26:8
    |
26  | struct CelProgram {
    |        ^^^^^^^^^^
note: required by a bound in `ThreadCheckerStub`
   --> /Users/inikolaev/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.19.2/src/impl_/pyclass.rs:894:33
    |
894 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
    |                                 ^^^^ required by this bound in `ThreadCheckerStub`
    = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.

Basically all the classes it mentioned - Program, Expression, Atom - are coming from a crate that I'm trying to wrap.

[mejrs]

Then you have to use the unsendable option that messense linked above.

[inikolaev]

Is there a way to make it work in multithreaded environment? I want to create a single instance of CelProgram once and then call evaluate on it from potentially multiple threads.

And overall there might be many instances of CelProgram exist - once for each distinct expression.

Or should I approach this somehow differently and keep track of all instances in some global state myself?

[mejrs]

No, this Rc that is being used is fundamentally not threadsafe. You will need a threadsafe version of that library you are using.

[inikolaev]

Thank you! This makes sense!