madsim_real_tokio/task/blocking.rs
1use crate::task::JoinHandle;
2
3cfg_rt_multi_thread! {
4 /// Runs the provided blocking function on the current thread without
5 /// blocking the executor.
6 ///
7 /// In general, issuing a blocking call or performing a lot of compute in a
8 /// future without yielding is problematic, as it may prevent the executor
9 /// from driving other tasks forward. Calling this function informs the
10 /// executor that the currently executing task is about to block the thread,
11 /// so the executor is able to hand off any other tasks it has to a new
12 /// worker thread before that happens. See the [CPU-bound tasks and blocking
13 /// code][blocking] section for more information.
14 ///
15 /// Be aware that although this function avoids starving other independently
16 /// spawned tasks, any other code running concurrently in the same task will
17 /// be suspended during the call to `block_in_place`. This can happen e.g.
18 /// when using the [`join!`] macro. To avoid this issue, use
19 /// [`spawn_blocking`] instead of `block_in_place`.
20 ///
21 /// Note that this function cannot be used within a [`current_thread`] runtime
22 /// because in this case there are no other worker threads to hand off tasks
23 /// to. On the other hand, calling the function outside a runtime is
24 /// allowed. In this case, `block_in_place` just calls the provided closure
25 /// normally.
26 ///
27 /// Code running behind `block_in_place` cannot be cancelled. When you shut
28 /// down the executor, it will wait indefinitely for all blocking operations
29 /// to finish. You can use [`shutdown_timeout`] to stop waiting for them
30 /// after a certain timeout. Be aware that this will still not cancel the
31 /// tasks — they are simply allowed to keep running after the method
32 /// returns.
33 ///
34 /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
35 /// [`spawn_blocking`]: fn@crate::task::spawn_blocking
36 /// [`join!`]: macro@join
37 /// [`thread::spawn`]: fn@std::thread::spawn
38 /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
39 ///
40 /// # Examples
41 ///
42 /// ```
43 /// use tokio::task;
44 ///
45 /// # async fn docs() {
46 /// task::block_in_place(move || {
47 /// // do some compute-heavy work or call synchronous code
48 /// });
49 /// # }
50 /// ```
51 ///
52 /// Code running inside `block_in_place` may use `block_on` to reenter the
53 /// async context.
54 ///
55 /// ```
56 /// use tokio::task;
57 /// use tokio::runtime::Handle;
58 ///
59 /// # async fn docs() {
60 /// task::block_in_place(move || {
61 /// Handle::current().block_on(async move {
62 /// // do something async
63 /// });
64 /// });
65 /// # }
66 /// ```
67 ///
68 /// # Panics
69 ///
70 /// This function panics if called from a [`current_thread`] runtime.
71 ///
72 /// [`current_thread`]: fn@crate::runtime::Builder::new_current_thread
73 #[track_caller]
74 pub fn block_in_place<F, R>(f: F) -> R
75 where
76 F: FnOnce() -> R,
77 {
78 crate::runtime::scheduler::block_in_place(f)
79 }
80}
81
82cfg_rt! {
83 /// Runs the provided closure on a thread where blocking is acceptable.
84 ///
85 /// In general, issuing a blocking call or performing a lot of compute in a
86 /// future without yielding is problematic, as it may prevent the executor from
87 /// driving other futures forward. This function runs the provided closure on a
88 /// thread dedicated to blocking operations. See the [CPU-bound tasks and
89 /// blocking code][blocking] section for more information.
90 ///
91 /// Tokio will spawn more blocking threads when they are requested through this
92 /// function until the upper limit configured on the [`Builder`] is reached.
93 /// After reaching the upper limit, the tasks are put in a queue.
94 /// The thread limit is very large by default, because `spawn_blocking` is often
95 /// used for various kinds of IO operations that cannot be performed
96 /// asynchronously. When you run CPU-bound code using `spawn_blocking`, you
97 /// should keep this large upper limit in mind. When running many CPU-bound
98 /// computations, a semaphore or some other synchronization primitive should be
99 /// used to limit the number of computation executed in parallel. Specialized
100 /// CPU-bound executors, such as [rayon], may also be a good fit.
101 ///
102 /// This function is intended for non-async operations that eventually finish on
103 /// their own. If you want to spawn an ordinary thread, you should use
104 /// [`thread::spawn`] instead.
105 ///
106 /// Closures spawned using `spawn_blocking` cannot be cancelled abruptly; there
107 /// is no standard low level API to cause a thread to stop running. However,
108 /// a useful pattern is to pass some form of "cancellation token" into
109 /// the thread. This could be an [`AtomicBool`] that the task checks periodically.
110 /// Another approach is to have the thread primarily read or write from a channel,
111 /// and to exit when the channel closes; assuming the other side of the channel is dropped
112 /// when cancellation occurs, this will cause the blocking task thread to exit
113 /// soon after as well.
114 ///
115 /// When you shut down the executor, it will wait indefinitely for all blocking operations to
116 /// finish. You can use [`shutdown_timeout`] to stop waiting for them after a
117 /// certain timeout. Be aware that this will still not cancel the tasks — they
118 /// are simply allowed to keep running after the method returns. It is possible
119 /// for a blocking task to be cancelled if it has not yet started running, but this
120 /// is not guaranteed.
121 ///
122 /// Note that if you are using the single threaded runtime, this function will
123 /// still spawn additional threads for blocking operations. The current-thread
124 /// scheduler's single thread is only used for asynchronous code.
125 ///
126 /// # Related APIs and patterns for bridging asynchronous and blocking code
127 ///
128 /// In simple cases, it is sufficient to have the closure accept input
129 /// parameters at creation time and return a single value (or struct/tuple, etc.).
130 ///
131 /// For more complex situations in which it is desirable to stream data to or from
132 /// the synchronous context, the [`mpsc channel`] has `blocking_send` and
133 /// `blocking_recv` methods for use in non-async code such as the thread created
134 /// by `spawn_blocking`.
135 ///
136 /// Another option is [`SyncIoBridge`] for cases where the synchronous context
137 /// is operating on byte streams. For example, you might use an asynchronous
138 /// HTTP client such as [hyper] to fetch data, but perform complex parsing
139 /// of the payload body using a library written for synchronous I/O.
140 ///
141 /// Finally, see also [Bridging with sync code][bridgesync] for discussions
142 /// around the opposite case of using Tokio as part of a larger synchronous
143 /// codebase.
144 ///
145 /// [`Builder`]: struct@crate::runtime::Builder
146 /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
147 /// [rayon]: https://docs.rs/rayon
148 /// [`mpsc channel`]: crate::sync::mpsc
149 /// [`SyncIoBridge`]: https://docs.rs/tokio-util/latest/tokio_util/io/struct.SyncIoBridge.html
150 /// [hyper]: https://docs.rs/hyper
151 /// [`thread::spawn`]: fn@std::thread::spawn
152 /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
153 /// [bridgesync]: https://tokio.rs/tokio/topics/bridging
154 /// [`AtomicBool`]: struct@std::sync::atomic::AtomicBool
155 ///
156 /// # Examples
157 ///
158 /// Pass an input value and receive result of computation:
159 ///
160 /// ```
161 /// use tokio::task;
162 ///
163 /// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
164 /// // Initial input
165 /// let mut v = "Hello, ".to_string();
166 /// let res = task::spawn_blocking(move || {
167 /// // Stand-in for compute-heavy work or using synchronous APIs
168 /// v.push_str("world");
169 /// // Pass ownership of the value back to the asynchronous context
170 /// v
171 /// }).await?;
172 ///
173 /// // `res` is the value returned from the thread
174 /// assert_eq!(res.as_str(), "Hello, world");
175 /// # Ok(())
176 /// # }
177 /// ```
178 ///
179 /// Use a channel:
180 ///
181 /// ```
182 /// use tokio::task;
183 /// use tokio::sync::mpsc;
184 ///
185 /// # async fn docs() {
186 /// let (tx, mut rx) = mpsc::channel(2);
187 /// let start = 5;
188 /// let worker = task::spawn_blocking(move || {
189 /// for x in 0..10 {
190 /// // Stand in for complex computation
191 /// tx.blocking_send(start + x).unwrap();
192 /// }
193 /// });
194 ///
195 /// let mut acc = 0;
196 /// while let Some(v) = rx.recv().await {
197 /// acc += v;
198 /// }
199 /// assert_eq!(acc, 95);
200 /// worker.await.unwrap();
201 /// # }
202 /// ```
203 #[track_caller]
204 pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
205 where
206 F: FnOnce() -> R + Send + 'static,
207 R: Send + 'static,
208 {
209 crate::runtime::spawn_blocking(f)
210 }
211}