1 use crate::task::JoinHandle; 2 3 cfg_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 82 cfg_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 } 212