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Merge pull request #2040 from bendk/blocking-task-queue-without-speci…
…alized-code Blocking task queue without specialized code
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| This crate is a toy build an async API client, with some parts implemented in Rust and some parts | ||
| implemented in the foreign language. Each side makes async calls across the FFI. | ||
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| The motivation is to show how to build an async-based Rust library, using a foreign async executor to drive the futures. | ||
| Note that the Rust code does not start any threads of its own, nor does it use startup an async runtime like tokio. | ||
| Instead, it awaits async calls to the foreign code and the foreign executor manages the threads. | ||
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| There are two basic ways the Rust code in this crate awaits the foreign code: | ||
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| ## API calls | ||
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| API calls are the simple case. | ||
| Rust awaits an HTTP call to the foreign side, then uses `serde` to parse the JSON into a structured response. | ||
| As long as the Rust code is "non-blocking" this system should work fine. | ||
| Note: there is not a strict definition for "non-blocking", but typically it means not performing IO and not executing a long-running CPU operation. | ||
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| ## Blocking tasks | ||
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| The more difficult case is a blocking Rust call. | ||
| The example from this crate is reading the API credentials from disk. | ||
| The `tasks.rs` module and the foreign implementations of the `TaskRunner` interface are an experiment to show how this can be accomplished using async callback methods. | ||
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| The code works, but is a bit clunky. | ||
| For example requiring that the task closure is `'static` creates some extra work for the `load_credentials` function. | ||
| It also requires an extra `Mutex` and `Arc`. | ||
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| The UniFFI team is looking for ways to simplify this process by handling it natively in UniFFI, see https://github.com/mozilla/uniffi-rs/pull/1837. | ||
| If you are writing Rust code that needs to make async blocking calls, please tell us about your use case which will help us develop the feature. |
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| /* This Source Code Form is subject to the terms of the Mozilla Public | ||
| * License, v. 2.0. If a copy of the MPL was not distributed with this | ||
| * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ | ||
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| use crate::{run_task, ApiError, Result, TaskRunner}; | ||
| use std::sync::Arc; | ||
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| #[async_trait::async_trait] | ||
| pub trait HttpClient: Send + Sync { | ||
| async fn fetch(&self, url: String, credentials: String) -> Result<String>; | ||
| } | ||
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| impl From<serde_json::Error> for ApiError { | ||
| fn from(e: serde_json::Error) -> Self { | ||
| Self::Json { | ||
| reason: e.to_string(), | ||
| } | ||
| } | ||
| } | ||
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| #[derive(Debug, serde::Deserialize)] | ||
| pub struct Issue { | ||
| pub url: String, | ||
| pub title: String, | ||
| pub state: IssueState, | ||
| } | ||
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| #[derive(Debug, serde::Deserialize)] | ||
| pub enum IssueState { | ||
| #[serde(rename = "open")] | ||
| Open, | ||
| #[serde(rename = "closed")] | ||
| Closed, | ||
| } | ||
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| pub struct ApiClient { | ||
| http_client: Arc<dyn HttpClient>, | ||
| task_runner: Arc<dyn TaskRunner>, | ||
| } | ||
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| impl ApiClient { | ||
| // Pretend this is a blocking call that needs to load the credentials from disk/network | ||
| fn load_credentials_sync(&self) -> String { | ||
| String::from("username:password") | ||
| } | ||
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| async fn load_credentials(self: Arc<Self>) -> String { | ||
| let self_cloned = Arc::clone(&self); | ||
| run_task(&self.task_runner, move || { | ||
| self_cloned.load_credentials_sync() | ||
| }) | ||
| .await | ||
| } | ||
| } | ||
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| impl ApiClient { | ||
| pub fn new(http_client: Arc<dyn HttpClient>, task_runner: Arc<dyn TaskRunner>) -> Self { | ||
| Self { | ||
| http_client, | ||
| task_runner, | ||
| } | ||
| } | ||
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| pub async fn get_issue( | ||
| self: Arc<Self>, | ||
| owner: String, | ||
| repository: String, | ||
| issue_number: u32, | ||
| ) -> Result<Issue> { | ||
| let credentials = self.clone().load_credentials().await; | ||
| let url = | ||
| format!("https://api.github.com/repos/{owner}/{repository}/issues/{issue_number}"); | ||
| let body = self.http_client.fetch(url, credentials).await?; | ||
| Ok(serde_json::from_str(&body)?) | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,90 @@ | ||
| /* This Source Code Form is subject to the terms of the Mozilla Public | ||
| * License, v. 2.0. If a copy of the MPL was not distributed with this | ||
| * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ | ||
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| use std::sync::{Arc, Mutex}; | ||
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| #[async_trait::async_trait] | ||
| pub trait TaskRunner: Send + Sync { | ||
| async fn run_task(&self, task: Arc<dyn RustTask>); | ||
| } | ||
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| pub trait RustTask: Send + Sync { | ||
| fn execute(&self); | ||
| } | ||
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| pub async fn run_task<F, T>(runner: &Arc<dyn TaskRunner>, closure: F) -> T | ||
| where | ||
| F: FnOnce() -> T + Send + Sync + 'static, | ||
| T: Send + 'static, | ||
| { | ||
| let closure = Arc::new(TaskClosure::new(closure)); | ||
| runner | ||
| .run_task(Arc::clone(&closure) as Arc<dyn RustTask>) | ||
| .await; | ||
| closure.take_result() | ||
| } | ||
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| struct TaskClosure<F, T> | ||
| where | ||
| F: FnOnce() -> T + Send + Sync, | ||
| T: Send, | ||
| { | ||
| inner: Mutex<TaskClosureInner<F, T>>, | ||
| } | ||
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| enum TaskClosureInner<F, T> | ||
| where | ||
| F: FnOnce() -> T + Send + Sync, | ||
| T: Send, | ||
| { | ||
| Pending(F), | ||
| Running, | ||
| Complete(T), | ||
| Finished, | ||
| } | ||
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| impl<F, T> TaskClosure<F, T> | ||
| where | ||
| F: FnOnce() -> T + Send + Sync, | ||
| T: Send, | ||
| { | ||
| fn new(closure: F) -> Self { | ||
| Self { | ||
| inner: Mutex::new(TaskClosureInner::Pending(closure)), | ||
| } | ||
| } | ||
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| fn take_result(&self) -> T { | ||
| let mut inner = self.inner.lock().unwrap(); | ||
| match *inner { | ||
| TaskClosureInner::Pending(_) => panic!("Task never ran"), | ||
| TaskClosureInner::Running => panic!("Task still running"), | ||
| TaskClosureInner::Finished => panic!("Task already finished"), | ||
| TaskClosureInner::Complete(_) => (), | ||
| }; | ||
| match std::mem::replace(&mut *inner, TaskClosureInner::Finished) { | ||
| TaskClosureInner::Complete(v) => v, | ||
| _ => unreachable!(), | ||
| } | ||
| } | ||
| } | ||
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| impl<F, T> RustTask for TaskClosure<F, T> | ||
| where | ||
| F: FnOnce() -> T + Send + Sync, | ||
| T: Send, | ||
| { | ||
| fn execute(&self) { | ||
| let mut inner = self.inner.lock().unwrap(); | ||
| match std::mem::replace(&mut *inner, TaskClosureInner::Running) { | ||
| TaskClosureInner::Pending(f) => { | ||
| let result = f(); | ||
| *inner = TaskClosureInner::Complete(result) | ||
| } | ||
| TaskClosureInner::Running => panic!("Task already started"), | ||
| TaskClosureInner::Complete(_) => panic!("Task already executed"), | ||
| TaskClosureInner::Finished => panic!("Task already finished"), | ||
| } | ||
| } | ||
| } |
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