1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
use crate::task::JoinHandle;

cfg_rt_multi_thread! {
    /// Runs the provided blocking function on the current thread without
    /// blocking the executor.
    ///
    /// In general, issuing a blocking call or performing a lot of compute in a
    /// future without yielding is problematic, as it may prevent the executor
    /// from driving other tasks forward. Calling this function informs the
    /// executor that the currently executing task is about to block the thread,
    /// so the executor is able to hand off any other tasks it has to a new
    /// worker thread before that happens. See the [CPU-bound tasks and blocking
    /// code][blocking] section for more information.
    ///
    /// Be aware that although this function avoids starving other independently
    /// spawned tasks, any other code running concurrently in the same task will
    /// be suspended during the call to `block_in_place`. This can happen e.g.
    /// when using the [`join!`] macro. To avoid this issue, use
    /// [`spawn_blocking`] instead of `block_in_place`.
    ///
    /// Note that this function cannot be used within a [`current_thread`] runtime
    /// because in this case there are no other worker threads to hand off tasks
    /// to. On the other hand, calling the function outside a runtime is
    /// allowed. In this case, `block_in_place` just calls the provided closure
    /// normally.
    ///
    /// Code running behind `block_in_place` cannot be cancelled. When you shut
    /// down the executor, it will wait indefinitely for all blocking operations
    /// to finish. You can use [`shutdown_timeout`] to stop waiting for them
    /// after a certain timeout. Be aware that this will still not cancel the
    /// tasks — they are simply allowed to keep running after the method
    /// returns.
    ///
    /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
    /// [`spawn_blocking`]: fn@crate::task::spawn_blocking
    /// [`join!`]: macro@join
    /// [`thread::spawn`]: fn@std::thread::spawn
    /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::task;
    ///
    /// # async fn docs() {
    /// task::block_in_place(move || {
    ///     // do some compute-heavy work or call synchronous code
    /// });
    /// # }
    /// ```
    ///
    /// Code running inside `block_in_place` may use `block_on` to reenter the
    /// async context.
    ///
    /// ```
    /// use tokio::task;
    /// use tokio::runtime::Handle;
    ///
    /// # async fn docs() {
    /// task::block_in_place(move || {
    ///     Handle::current().block_on(async move {
    ///         // do something async
    ///     });
    /// });
    /// # }
    /// ```
    ///
    /// # Panics
    ///
    /// This function panics if called from a [`current_thread`] runtime.
    ///
    /// [`current_thread`]: fn@crate::runtime::Builder::new_current_thread
    pub fn block_in_place<F, R>(f: F) -> R
    where
        F: FnOnce() -> R,
    {
        crate::runtime::scheduler::multi_thread::block_in_place(f)
    }
}

cfg_rt! {
    /// Runs the provided closure on a thread where blocking is acceptable.
    ///
    /// In general, issuing a blocking call or performing a lot of compute in a
    /// future without yielding is problematic, as it may prevent the executor from
    /// driving other futures forward. This function runs the provided closure on a
    /// thread dedicated to blocking operations. See the [CPU-bound tasks and
    /// blocking code][blocking] section for more information.
    ///
    /// Tokio will spawn more blocking threads when they are requested through this
    /// function until the upper limit configured on the [`Builder`] is reached.
    /// After reaching the upper limit, the tasks are put in a queue.
    /// The thread limit is very large by default, because `spawn_blocking` is often
    /// used for various kinds of IO operations that cannot be performed
    /// asynchronously.  When you run CPU-bound code using `spawn_blocking`, you
    /// should keep this large upper limit in mind. When running many CPU-bound
    /// computations, a semaphore or some other synchronization primitive should be
    /// used to limit the number of computation executed in parallel. Specialized
    /// CPU-bound executors, such as [rayon], may also be a good fit.
    ///
    /// This function is intended for non-async operations that eventually finish on
    /// their own. If you want to spawn an ordinary thread, you should use
    /// [`thread::spawn`] instead.
    ///
    /// Closures spawned using `spawn_blocking` cannot be cancelled abruptly; there
    /// is no standard low level API to cause a thread to stop running.  However,
    /// a useful pattern is to pass some form of "cancellation token" into
    /// the thread.  This could be an [`AtomicBool`] that the task checks periodically.
    /// Another approach is to have the thread primarily read or write from a channel,
    /// and to exit when the channel closes; assuming the other side of the channel is dropped
    /// when cancellation occurs, this will cause the blocking task thread to exit
    /// soon after as well.
    ///
    /// When you shut down the executor, it will wait indefinitely for all blocking operations to
    /// finish. You can use [`shutdown_timeout`] to stop waiting for them after a
    /// certain timeout. Be aware that this will still not cancel the tasks — they
    /// are simply allowed to keep running after the method returns.  It is possible
    /// for a blocking task to be cancelled if it has not yet started running, but this
    /// is not guaranteed.
    ///
    /// Note that if you are using the single threaded runtime, this function will
    /// still spawn additional threads for blocking operations. The current-thread
    /// scheduler's single thread is only used for asynchronous code.
    ///
    /// # Related APIs and patterns for bridging asynchronous and blocking code
    ///
    /// In simple cases, it is sufficient to have the closure accept input
    /// parameters at creation time and return a single value (or struct/tuple, etc.).
    ///
    /// For more complex situations in which it is desirable to stream data to or from
    /// the synchronous context, the [`mpsc channel`] has `blocking_send` and
    /// `blocking_recv` methods for use in non-async code such as the thread created
    /// by `spawn_blocking`.
    ///
    /// Another option is [`SyncIoBridge`] for cases where the synchronous context
    /// is operating on byte streams.  For example, you might use an asynchronous
    /// HTTP client such as [hyper] to fetch data, but perform complex parsing
    /// of the payload body using a library written for synchronous I/O.
    ///
    /// Finally, see also [Bridging with sync code][bridgesync] for discussions
    /// around the opposite case of using Tokio as part of a larger synchronous
    /// codebase.
    ///
    /// [`Builder`]: struct@crate::runtime::Builder
    /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
    /// [rayon]: https://docs.rs/rayon
    /// [`mpsc channel`]: crate::sync::mpsc
    /// [`SyncIoBridge`]: https://docs.rs/tokio-util/0.6/tokio_util/io/struct.SyncIoBridge.html
    /// [hyper]: https://docs.rs/hyper
    /// [`thread::spawn`]: fn@std::thread::spawn
    /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
    /// [bridgesync]: https://tokio.rs/tokio/topics/bridging
    /// [`AtomicBool`]: struct@std::sync::atomic::AtomicBool
    ///
    /// # Examples
    ///
    /// Pass an input value and receive result of computation:
    ///
    /// ```
    /// use tokio::task;
    ///
    /// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
    /// // Initial input
    /// let mut v = "Hello, ".to_string();
    /// let res = task::spawn_blocking(move || {
    ///     // Stand-in for compute-heavy work or using synchronous APIs
    ///     v.push_str("world");
    ///     // Pass ownership of the value back to the asynchronous context
    ///     v
    /// }).await?;
    ///
    /// // `res` is the value returned from the thread
    /// assert_eq!(res.as_str(), "Hello, world");
    /// # Ok(())
    /// # }
    /// ```
    ///
    /// Use a channel:
    ///
    /// ```
    /// use tokio::task;
    /// use tokio::sync::mpsc;
    ///
    /// # async fn docs() {
    /// let (tx, mut rx) = mpsc::channel(2);
    /// let start = 5;
    /// let worker = task::spawn_blocking(move || {
    ///     for x in 0..10 {
    ///         // Stand in for complex computation
    ///         tx.blocking_send(start + x).unwrap();
    ///     }
    /// });
    ///
    /// let mut acc = 0;
    /// while let Some(v) = rx.recv().await {
    ///     acc += v;
    /// }
    /// assert_eq!(acc, 95);
    /// worker.await.unwrap();
    /// # }
    /// ```
    #[track_caller]
    pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        crate::runtime::spawn_blocking(f)
    }
}