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#19 in Procedural macros

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MIT/Apache

84KB
1.5K SLoC

procspawn

This crate provides the ability to spawn processes with a function similar to thread::spawn.

Unlike thread::spawn data cannot be passed by the use of closures. Instead if must be explicitly passed as serializable object (specifically it must be serde serializable). The return value from the spawned closure also must be serializable and can then be retrieved from the returned join handle.

If the spawned functiom causes a panic it will also be serialized across the process boundaries.

Example

First for all of this to work you need to invoke procspawn::init at a point early in your program (somewhere at the beginning of the main function). Whatever happens before that point also happens in your spawned functions.

Subprocesses are by default invoked with the same arguments and environment variables as the parent process.

procspawn::init();

Now you can start spawning functions:

let data = vec![1, 2, 3, 4];
let handle = procspawn::spawn(data, |data| {
    println!("Received data {:?}", &data);
    data.into_iter().sum::<i64>()
});
let result = handle.join().unwrap();

Because procspawn will invoke a subprocess and there is currently no reliable way to intercept main in Rust it's necessary for you to call procspawn::init explicitly an early time in the program.

Alternatively you can use the ProcConfig builder object to initialize the process which gives you some extra abilities to customize the processes spawned. This for instance lets you disable the default panic handling.

spawn can pass arbitrary serializable data, including IPC senders and receivers from the ipc-channel crate, down to the new process.

Pools

The default way to spawn processes will start and stop processes constantly. For more uses it's a better idea to spawn a Pool which will keep processes around for reuse. Between calls the processes will stay around which also means the can keep state between calls if needed.

Panics

By default panics are captured and serialized across process boundaries. This requires that the backtrace crate is used with serialization support. If you do not need this feature you can disable the backtrace crate and disable panic handling through the ProcConfig object.

Feature Flags

The following feature flags exist:

  • safe-shared-libraries: this feature is enabled by default. When this feature is disable then no validation about shared library load status is performed around IPC calls. This is highly unsafe if shared libraries are being used and a function from a shared library is spawned.
  • backtrace: this feature is enabled by default. When in use then backtraces are captured with the backtrace-rs crate and serialized across process boundaries.
  • test-support: when this feature is enabled procspawn can be used with rusttest. See testing for more information.
  • json: enables optional JSON serialization. For more information see Bincode Limitations.

Bincode Limitations

This crate uses bincode internally for inter process communication. Bincode currently has some limitations which make some serde features incompatible with it. Most notably if you use #[serde(flatten)] data cannot be sent across the processes. To work around this you can enable the json feature and wrap affected objects in the Json wrapper to force JSON serialization.

Testing

Due to limitations of the rusttest testing system there are some restrictions to how this crate operates. First of all you need to enable the test-support feature for procspawn to work with rusttest at all. Secondly your tests need to invoke the enable_test_support! macro once top-level.

With this done the following behavior applies:

  • Tests behave as if procspawn::init was called (that means with the default arguments). Other configuration is not supported.
  • procspawn will register a dummy test (named procspawn_test_helper) which doesn't do anything when called directly, but acts as the spawning helper for all spawn calls.
  • stdout is silenced by default unless --show-output or --nocapture is passed to tests.
  • when trying to spawn with intercepted stdout be aware that there is extra noise that will be emitted by rusttest.
procspawn::enable_test_support!();

#[test]
fn test_basic() {
    let handle = procspawn::spawn((1, 2), |(a, b)| a + b);
    let value = handle.join().unwrap();
    assert_eq!(value, 3);
}

Shared Libraries

procspawn uses the findshlibs crate to determine where a function is located in memory in both processes. If a shared library is not loaded in the subprocess (because for instance it is loaded at runtime) then the call will fail. Because this adds quite some overhead over every call you can also disable the safe-shared-libraries feature (which is on by default) in which case you are not allowed to invoke functions from shared libraries and no validation is performed.

This in normal circumstances should be okay but you need to validate this. Spawning processes will be disabled if the feature is not enabled until you call the assert_spawn_is_safe function.

Macros

Alternatively the spawn! macro can be used which can make passing more than one argument easier:

let a = 42u32;
let b = 23u32;
let c = 1;
let handle = procspawn::spawn!((a => base, b, mut c) || -> Result<_, ()> {
    c += 1;
    Ok(base + b + c)
});

Platform Support

Currently this crate only supports macOS and Linux because ipc-channel itself does not support Windows yet. Additionally the findshlibs which is used for the safe-shared-libraries feature also does not yet support Windows.

More Examples

Here are some examples of procspawn in action:

  • simple.rs: a very simple example showing the basics.
  • args.rs: shows how arguments are available to the subprocess as well.
  • timeout.rs: shows how you can wait on a process with timeouts.
  • bad-serialization.rs: shows JSON based workarounds for bincode limitations.
  • macro.rs: demonstrates macro usage.

More examples can be found in the example folder: examples

License: MIT/Apache-2.0

Dependencies

~2.3–3.5MB
~70K SLoC