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
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
use crate::span::Span;
use core::{
    future::Future,
    marker::Sized,
    mem::ManuallyDrop,
    pin::Pin,
    task::{Context, Poll},
};
use pin_project_lite::pin_project;

#[cfg(feature = "std")]
use crate::dispatch::{self, Dispatch};

/// Attaches spans to a [`std::future::Future`].
///
/// Extension trait allowing futures to be
/// instrumented with a `tracing` [span].
///
/// [span]: super::Span
pub trait Instrument: Sized {
    /// Instruments this type with the provided [`Span`], returning an
    /// `Instrumented` wrapper.
    ///
    /// The attached [`Span`] will be [entered] every time the instrumented
    /// [`Future`] is polled or [`Drop`]ped.
    ///
    /// # Examples
    ///
    /// Instrumenting a future:
    ///
    /// ```rust
    /// use tracing::Instrument;
    ///
    /// # async fn doc() {
    /// let my_future = async {
    ///     // ...
    /// };
    ///
    /// my_future
    ///     .instrument(tracing::info_span!("my_future"))
    ///     .await
    /// # }
    /// ```
    ///
    /// The [`Span::or_current`] combinator can be used in combination with
    /// `instrument` to ensure that the [current span] is attached to the
    /// future if the span passed to `instrument` is [disabled]:
    ///
    /// ```
    /// use tracing::Instrument;
    /// # mod tokio {
    /// #     pub(super) fn spawn(_: impl std::future::Future) {}
    /// # }
    ///
    /// let my_future = async {
    ///     // ...
    /// };
    ///
    /// let outer_span = tracing::info_span!("outer").entered();
    ///
    /// // If the "my_future" span is enabled, then the spawned task will
    /// // be within both "my_future" *and* "outer", since "outer" is
    /// // "my_future"'s parent. However, if "my_future" is disabled,
    /// // the spawned task will *not* be in any span.
    /// tokio::spawn(
    ///     my_future
    ///         .instrument(tracing::debug_span!("my_future"))
    /// );
    ///
    /// // Using `Span::or_current` ensures the spawned task is instrumented
    /// // with the current span, if the new span passed to `instrument` is
    /// // not enabled. This means that if the "my_future"  span is disabled,
    /// // the spawned task will still be instrumented with the "outer" span:
    /// # let my_future = async {};
    /// tokio::spawn(
    ///    my_future
    ///         .instrument(tracing::debug_span!("my_future").or_current())
    /// );
    /// ```
    ///
    /// [entered]: super::Span::enter()
    /// [`Span::or_current`]: super::Span::or_current()
    /// [current span]: super::Span::current()
    /// [disabled]: super::Span::is_disabled()
    /// [`Future`]: std::future::Future
    fn instrument(self, span: Span) -> Instrumented<Self> {
        Instrumented {
            inner: ManuallyDrop::new(self),
            span,
        }
    }

    /// Instruments this type with the [current] [`Span`], returning an
    /// `Instrumented` wrapper.
    ///
    /// The attached [`Span`] will be [entered] every time the instrumented
    /// [`Future`] is polled or [`Drop`]ped.
    ///
    /// This can be used to propagate the current span when spawning a new future.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use tracing::Instrument;
    ///
    /// # mod tokio {
    /// #     pub(super) fn spawn(_: impl std::future::Future) {}
    /// # }
    /// # async fn doc() {
    /// let span = tracing::info_span!("my_span");
    /// let _enter = span.enter();
    ///
    /// // ...
    ///
    /// let future = async {
    ///     tracing::debug!("this event will occur inside `my_span`");
    ///     // ...
    /// };
    /// tokio::spawn(future.in_current_span());
    /// # }
    /// ```
    ///
    /// [current]: super::Span::current()
    /// [entered]: super::Span::enter()
    /// [`Span`]: crate::Span
    /// [`Future`]: std::future::Future
    #[inline]
    fn in_current_span(self) -> Instrumented<Self> {
        self.instrument(Span::current())
    }
}

/// Extension trait allowing futures to be instrumented with
/// a `tracing` collector.
///

#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub trait WithCollector: Sized {
    /// Attaches the provided [collector] to this type, returning a
    /// [`WithDispatch`] wrapper.
    ///
    /// The attached [collector] will be set as the [default] when the returned
    /// [`Future`] is polled.
    ///
    /// # Examples
    ///
    /// ```
    /// # pub struct MyCollector;
    /// # impl tracing::Collect for MyCollector {
    /// #   fn new_span(&self, _: &tracing::span::Attributes) -> tracing::span::Id {
    /// #       tracing::span::Id::from_u64(0)
    /// #   }
    /// #   fn record(&self, _: &tracing::span::Id, _: &tracing::span::Record) {}
    /// #   fn event(&self, _: &tracing::Event<'_>) {}
    /// #   fn record_follows_from(&self, _: &tracing::span::Id, _: &tracing::span::Id) {}
    /// #   fn enabled(&self, _: &tracing::Metadata) -> bool { false }
    /// #   fn enter(&self, _: &tracing::span::Id) {}
    /// #   fn exit(&self, _: &tracing::span::Id) {}
    /// #   fn current_span(&self) -> tracing_core::span::Current {
    /// #       tracing_core::span::Current::unknown()
    /// #    }
    /// # }
    /// # impl MyCollector { fn new() -> Self { Self } }
    /// # async fn docs() {
    /// use tracing::instrument::WithCollector;
    ///
    /// // Set the default collector
    /// let _default = tracing::collect::set_default(MyCollector::new());
    ///
    /// tracing::info!("this event will be recorded by the default collector");
    ///
    /// // Create a different collector and attach it to a future.
    /// let other_collector = MyCollector::new();
    /// let future = async {
    ///     tracing::info!("this event will be recorded by the other collector");
    ///     // ...
    /// };
    ///
    /// future
    ///     // Attach the other collector to the future before awaiting it
    ///     .with_collector(other_collector)
    ///     .await;
    ///
    /// // Once the future has completed, we return to the default collector.
    /// tracing::info!("this event will be recorded by the default collector");
    /// # }
    /// ```
    ///
    /// [collector]: super::Collect
    /// [default]: crate::dispatch#setting-the-default-collector
    /// [`Future`]: std::future::Future
    fn with_collector<C>(self, collector: C) -> WithDispatch<Self>
    where
        C: Into<Dispatch>,
    {
        WithDispatch {
            inner: self,
            dispatch: collector.into(),
        }
    }

    /// Attaches the current [default] [collector] to this type, returning a
    /// [`WithDispatch`] wrapper.
    ///
    /// The attached collector will be set as the [default] when the returned
    /// [`Future`] is polled.
    ///
    /// This can be used to propagate the current dispatcher context when
    /// spawning a new future that may run on a different thread.
    ///
    /// # Examples
    ///
    /// ```
    /// # mod tokio {
    /// #     pub(super) fn spawn(_: impl std::future::Future) {}
    /// # }
    /// # pub struct MyCollector;
    /// # impl tracing::Collect for MyCollector {
    /// #   fn new_span(&self, _: &tracing::span::Attributes) -> tracing::span::Id {
    /// #       tracing::span::Id::from_u64(0)
    /// #   }
    /// #   fn record(&self, _: &tracing::span::Id, _: &tracing::span::Record) {}
    /// #   fn event(&self, _: &tracing::Event<'_>) {}
    /// #   fn record_follows_from(&self, _: &tracing::span::Id, _: &tracing::span::Id) {}
    /// #   fn enabled(&self, _: &tracing::Metadata) -> bool { false }
    /// #   fn enter(&self, _: &tracing::span::Id) {}
    /// #   fn exit(&self, _: &tracing::span::Id) {}
    /// #   fn current_span(&self) -> tracing_core::span::Current {
    /// #       tracing_core::span::Current::unknown()
    /// #    }
    /// # }
    /// # impl MyCollector { fn new() -> Self { Self } }
    /// # async fn docs() {
    /// use tracing::instrument::WithCollector;
    ///
    /// // Using `set_default` (rather than `set_global_default`) sets the
    /// // default collector for *this* thread only.
    /// let _default = tracing::collect::set_default(MyCollector::new());
    ///
    /// let future = async {
    ///     // ...
    /// };
    ///
    /// // If a multi-threaded async runtime is in use, this spawned task may
    /// // run on a different thread, in a different default collector's context.
    /// tokio::spawn(future);
    ///
    /// // However, calling `with_current_collector` on the future before
    /// // spawning it, ensures that the current thread's default collector is
    /// // propagated to the spawned task, regardless of where it executes:
    /// # let future = async { };
    /// tokio::spawn(future.with_current_collector());
    /// # }
    /// ```
    /// [collector]: super::Collect
    /// [default]: crate::dispatch#setting-the-default-collector
    /// [`Future`]: std::future::Future
    #[inline]
    fn with_current_collector(self) -> WithDispatch<Self> {
        WithDispatch {
            inner: self,
            dispatch: dispatch::get_default(|default| default.clone()),
        }
    }
}

#[cfg(feature = "std")]
pin_project! {
    /// A [`Future`] that has been instrumented with a `tracing` [collector].
    ///
    /// This type is returned by the [`WithCollector`] extension trait. See that
    /// trait's documentation for details.
    ///
    /// [`Future`]: std::future::Future
    /// [collector]: crate::Collector
    #[derive(Clone, Debug)]
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    pub struct WithDispatch<T> {
        #[pin]
        inner: T,
        dispatch: Dispatch,
    }
}

pin_project! {
    /// A [`Future`] that has been instrumented with a `tracing` [`Span`].
    ///
    /// This type is returned by the [`Instrument`] extension trait. See that
    /// trait's documentation for details.
    ///
    /// [`Future`]: std::future::Future
    /// [`Span`]: crate::Span
    #[project = InstrumentedProj]
    #[project_ref = InstrumentedProjRef]
    #[derive(Debug, Clone)]
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    pub struct Instrumented<T> {
        // `ManuallyDrop` is used here to to enter instrument `Drop` by entering
        // `Span` and executing `ManuallyDrop::drop`.
        #[pin]
        inner: ManuallyDrop<T>,
        span: Span,
    }

    impl<T> PinnedDrop for Instrumented<T> {
        fn drop(this: Pin<&mut Self>) {
            let this = this.project();
            let _enter = this.span.enter();
            // SAFETY: 1. `Pin::get_unchecked_mut()` is safe, because this isn't
            //             different from wrapping `T` in `Option` and calling
            //             `Pin::set(&mut this.inner, None)`, except avoiding
            //             additional memory overhead.
            //         2. `ManuallyDrop::drop()` is safe, because
            //            `PinnedDrop::drop()` is guaranteed to be called only
            //            once.
            unsafe { ManuallyDrop::drop(this.inner.get_unchecked_mut()) }
        }
    }
}

impl<'a, T> InstrumentedProj<'a, T> {
    /// Get a mutable reference to the [`Span`] a pinned mutable reference to
    /// the wrapped type.
    fn span_and_inner_pin_mut(self) -> (&'a mut Span, Pin<&'a mut T>) {
        // SAFETY: As long as `ManuallyDrop<T>` does not move, `T` won't move
        //         and `inner` is valid, because `ManuallyDrop::drop` is called
        //         only inside `Drop` of the `Instrumented`.
        let inner = unsafe { self.inner.map_unchecked_mut(|v| &mut **v) };
        (self.span, inner)
    }
}

impl<'a, T> InstrumentedProjRef<'a, T> {
    /// Get a reference to the [`Span`] a pinned reference to the wrapped type.
    fn span_and_inner_pin_ref(self) -> (&'a Span, Pin<&'a T>) {
        // SAFETY: As long as `ManuallyDrop<T>` does not move, `T` won't move
        //         and `inner` is valid, because `ManuallyDrop::drop` is called
        //         only inside `Drop` of the `Instrumented`.
        let inner = unsafe { self.inner.map_unchecked(|v| &**v) };
        (self.span, inner)
    }
}

// === impl Instrumented ===

impl<T: Future> Future for Instrumented<T> {
    type Output = T::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let (span, inner) = self.project().span_and_inner_pin_mut();
        let _enter = span.enter();
        inner.poll(cx)
    }
}

impl<T: Sized> Instrument for T {}

impl<T> Instrumented<T> {
    /// Borrows the `Span` that this type is instrumented by.
    pub fn span(&self) -> &Span {
        &self.span
    }

    /// Mutably borrows the `Span` that this type is instrumented by.
    pub fn span_mut(&mut self) -> &mut Span {
        &mut self.span
    }

    /// Borrows the wrapped type.
    pub fn inner(&self) -> &T {
        &self.inner
    }

    /// Mutably borrows the wrapped type.
    pub fn inner_mut(&mut self) -> &mut T {
        &mut self.inner
    }

    /// Get a pinned reference to the wrapped type.
    pub fn inner_pin_ref(self: Pin<&Self>) -> Pin<&T> {
        self.project_ref().span_and_inner_pin_ref().1
    }

    /// Get a pinned mutable reference to the wrapped type.
    pub fn inner_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T> {
        self.project().span_and_inner_pin_mut().1
    }

    /// Consumes the `Instrumented`, returning the wrapped type.
    ///
    /// Note that this drops the span.
    pub fn into_inner(self) -> T {
        // To manually destructure `Instrumented` without `Drop`, we
        // move it into a ManuallyDrop and use pointers to its fields
        let this = ManuallyDrop::new(self);
        let span: *const Span = &this.span;
        let inner: *const ManuallyDrop<T> = &this.inner;
        // SAFETY: Those pointers are valid for reads, because `Drop` didn't
        //         run, and properly aligned, because `Instrumented` isn't
        //         `#[repr(packed)]`.
        let _span = unsafe { span.read() };
        let inner = unsafe { inner.read() };
        ManuallyDrop::into_inner(inner)
    }
}

// === impl WithDispatch ===

#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
impl<T: Future> Future for WithDispatch<T> {
    type Output = T::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = self.project();
        let dispatch = this.dispatch;
        let future = this.inner;
        let _default = dispatch::set_default(dispatch);
        future.poll(cx)
    }
}

#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
impl<T: Sized> WithCollector for T {}

#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
impl<T> WithDispatch<T> {
    /// Borrows the [`Dispatch`] that is entered when this type is polled.
    pub fn dispatch(&self) -> &Dispatch {
        &self.dispatch
    }

    /// Borrows the wrapped type.
    pub fn inner(&self) -> &T {
        &self.inner
    }

    /// Mutably borrows the wrapped type.
    pub fn inner_mut(&mut self) -> &mut T {
        &mut self.inner
    }

    /// Get a pinned reference to the wrapped type.
    pub fn inner_pin_ref(self: Pin<&Self>) -> Pin<&T> {
        self.project_ref().inner
    }

    /// Get a pinned mutable reference to the wrapped type.
    pub fn inner_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T> {
        self.project().inner
    }

    /// Consumes the `Instrumented`, returning the wrapped type.
    ///
    /// Note that this drops the span.
    pub fn into_inner(self) -> T {
        self.inner
    }
}