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
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
//! Collectors collect and record trace data.
use crate::{span, Event, LevelFilter, Metadata};

use core::any::{Any, TypeId};

/// Trait representing the functions required to collect trace data.
///
/// Crates that provide implementations of methods for collecting or recording
/// trace data should implement the `Collect` interface. This trait is
/// intended to represent fundamental primitives for collecting trace events and
/// spans — other libraries may offer utility functions and types to make
/// collector implementations more modular or improve the ergonomics of writing
/// collectors.
///
/// A collector is responsible for the following:
/// - Registering new spans as they are created, and providing them with span
///   IDs. Implicitly, this means the collector may determine the strategy for
///   determining span equality.
/// - Recording the attachment of field values and follows-from annotations to
///   spans.
/// - Filtering spans and events, and determining when those filters must be
///   invalidated.
/// - Observing spans as they are entered, exited, and closed, and events as
///   they occur.
///
/// When a span is entered or exited, the collector is provided only with the
/// [ID] with which it tagged that span when it was created. This means
/// that it is up to the collector to determine whether and how span _data_ —
/// the fields and metadata describing the span — should be stored. The
/// [`new_span`] function is called when a new span is created, and at that
/// point, the collector _may_ choose to store the associated data if it will
/// be referenced again. However, if the data has already been recorded and will
/// not be needed by the implementations of `enter` and `exit`, the collector
/// may freely discard that data without allocating space to store it.
///
/// ## Overriding default impls
///
/// Some trait methods on `Collect` have default implementations, either in
/// order to reduce the surface area of implementing `Collect`, or for
/// backward-compatibility reasons. However, many collectors will likely want
/// to override these default implementations.
///
/// The following methods are likely of interest:
///
/// - [`register_callsite`] is called once for each callsite from which a span
///   event may originate, and returns an [`Interest`] value describing whether or
///   not the collector wishes to see events or spans from that callsite. By
///   default, it calls [`enabled`], and returns `Interest::always()` if
///   `enabled` returns true, or `Interest::never()` if enabled returns false.
///   However, if the collector's interest can change dynamically at runtime,
///   it may want to override this function to return `Interest::sometimes()`.
///   Additionally, collectors which wish to perform a behaviour once for each
///   callsite, such as allocating storage for data related to that callsite,
///   can perform it in `register_callsite`.
/// - [`clone_span`] is called every time a span ID is cloned, and [`try_close`]
///   is called when a span ID is dropped. By default, these functions do
///   nothing. However, they can be used to implement reference counting for
///   spans, allowing collectors to free storage for span data and to determine
///   when a span has _closed_ permanently (rather than being exited).
///   Collectors which store per-span data or which need to track span closures
///   should override these functions together.
///
/// [ID]: super::span::Id
/// [`new_span`]: Collect::new_span
/// [`register_callsite`]: Collect::register_callsite
/// [`enabled`]: Collect::enabled
/// [`clone_span`]: Collect::clone_span
/// [`try_close`]: Collect::try_close
pub trait Collect: 'static {
    // === Span registry methods ==============================================

    /// Registers a new callsite with this collector, returning whether or not
    /// the collector is interested in being notified about the callsite.
    ///
    /// By default, this function assumes that the collector's [filter]
    /// represents an unchanging view of its interest in the callsite. However,
    /// if this is not the case, collectors may override this function to
    /// indicate different interests, or to implement behaviour that should run
    /// once for every callsite.
    ///
    /// This function is guaranteed to be called at least once per callsite on
    /// every active collector. The collector may store the keys to fields it
    /// cares about in order to reduce the cost of accessing fields by name,
    /// preallocate storage for that callsite, or perform any other actions it
    /// wishes to perform once for each callsite.
    ///
    /// The collector should then return an [`Interest`], indicating
    /// whether it is interested in being notified about that callsite in the
    /// future. This may be `Always` indicating that the collector always
    /// wishes to be notified about the callsite, and its filter need not be
    /// re-evaluated; `Sometimes`, indicating that the collector may sometimes
    /// care about the callsite but not always (such as when sampling), or
    /// `Never`, indicating that the collector never wishes to be notified about
    /// that callsite. If all active collectors return `Never`, a callsite will
    /// never be enabled unless a new collector expresses interest in it.
    ///
    /// `Collector`s which require their filters to be run every time an event
    /// occurs or a span is entered::exited should return `Interest::sometimes`.
    /// If a collector returns `Interest::sometimes`, then its' [`enabled`] method
    /// will be called every time an event or span is created from that callsite.
    ///
    /// For example, suppose a sampling collector is implemented by
    /// incrementing a counter every time `enabled` is called and only returning
    /// `true` when the counter is divisible by a specified sampling rate. If
    /// that collector returns `Interest::always` from `register_callsite`, then
    /// the filter will not be re-evaluated once it has been applied to a given
    /// set of metadata. Thus, the counter will not be incremented, and the span
    /// or event that corresponds to the metadata will never be `enabled`.
    ///
    /// `Collector`s that need to change their filters occasionally should call
    /// [`rebuild_interest_cache`] to re-evaluate `register_callsite` for all
    /// callsites.
    ///
    /// Similarly, if a `Collector` has a filtering strategy that can be
    /// changed dynamically at runtime, it would need to re-evaluate that filter
    /// if the cached results have changed.
    ///
    /// A collector which manages fanout to multiple other collectors
    /// should proxy this decision to all of its child collectors,
    /// returning `Interest::never` only if _all_ such children return
    /// `Interest::never`. If the set of collectors to which spans are
    /// broadcast may change dynamically, the collector should also never
    /// return `Interest::Never`, as a new collector may be added that _is_
    /// interested.
    ///
    /// # Notes
    /// This function may be called again when a new collector is created or
    /// when the registry is invalidated.
    ///
    /// If a collector returns `Interest::never` for a particular callsite, it
    /// _may_ still see spans and events originating from that callsite, if
    /// another collector expressed interest in it.
    ///
    /// [filter]: Self::enabled
    /// [metadata]: super::metadata::Metadata
    /// [`enabled`]: Self::enabled
    /// [`rebuild_interest_cache`]: super::callsite::fn.rebuild_interest_cache.html
    fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest {
        if self.enabled(metadata) {
            Interest::always()
        } else {
            Interest::never()
        }
    }

    /// Returns true if a span or event with the specified [metadata] would be
    /// recorded.
    ///
    /// By default, it is assumed that this filter needs only be evaluated once
    /// for each callsite, so it is called by [`register_callsite`] when each
    /// callsite is registered. The result is used to determine if the collector
    /// is always [interested] or never interested in that callsite. This is intended
    /// primarily as an optimization, so that expensive filters (such as those
    /// involving string search, et cetera) need not be re-evaluated.
    ///
    /// However, if the collector's interest in a particular span or event may
    /// change, or depends on contexts only determined dynamically at runtime,
    /// then the `register_callsite` method should be overridden to return
    /// [`Interest::sometimes`]. In that case, this function will be called every
    /// time that span or event occurs.
    ///
    /// [metadata]: super::metadata::Metadata
    /// [interested]: Interest
    /// [`register_callsite`]: Self::register_callsite
    fn enabled(&self, metadata: &Metadata<'_>) -> bool;

    /// Returns the highest [verbosity level][level] that this `Collector` will
    /// enable, or `None`, if the collector does not implement level-based
    /// filtering or chooses not to implement this method.
    ///
    /// If this method returns a [`Level`][level], it will be used as a hint to
    /// determine the most verbose level that will be enabled. This will allow
    /// spans and events which are more verbose than that level to be skipped
    /// more efficiently. collectors which perform filtering are strongly
    /// encouraged to provide an implementation of this method.
    ///
    /// If the maximum level the collector will enable can change over the
    /// course of its lifetime, it is free to return a different value from
    /// multiple invocations of this method. However, note that changes in the
    /// maximum level will **only** be reflected after the callsite [`Interest`]
    /// cache is rebuilt, by calling the [`callsite::rebuild_interest_cache`][rebuild]
    /// function. Therefore, if the collector will change the value returned by
    /// this method, it is responsible for ensuring that
    /// [`rebuild_interest_cache`][rebuild] is called after the value of the max
    /// level changes.
    ///
    /// [level]: super::Level
    /// [rebuild]: super::callsite::fn.rebuild_interest_cache.html
    fn max_level_hint(&self) -> Option<LevelFilter> {
        None
    }

    /// Visit the construction of a new span, returning a new [span ID] for the
    /// span being constructed.
    ///
    /// The provided [`Attributes`] contains any field values that were provided
    /// when the span was created. The collector may pass a [visitor] to the
    /// `Attributes`' [`record` method] to record these values.
    ///
    /// IDs are used to uniquely identify spans and events within the context of a
    /// collector, so span equality will be based on the returned ID. Thus, if
    /// the collector wishes for all spans with the same metadata to be
    /// considered equal, it should return the same ID every time it is given a
    /// particular set of metadata. Similarly, if it wishes for two separate
    /// instances of a span with the same metadata to *not* be equal, it should
    /// return a distinct ID every time this function is called, regardless of
    /// the metadata.
    ///
    /// Note that the collector is free to assign span IDs based on whatever
    /// scheme it sees fit. Any guarantees about uniqueness, ordering, or ID
    /// reuse are left up to the collector implementation to determine.
    ///
    /// [span ID]: super::span::Id
    /// [`Attributes`]: super::span::Attributes
    /// [visitor]: super::field::Visit
    /// [`record` method]: super::span::Attributes::record
    fn new_span(&self, span: &span::Attributes<'_>) -> span::Id;

    // === Notification methods ===============================================

    /// Record a set of values on a span.
    ///
    /// This method will be invoked when value is recorded on a span.
    /// Recording multiple values for the same field is possible,
    /// but the actual behaviour is defined by the collector implementation.
    ///
    /// Keep in mind that a span might not provide a value
    /// for each field it declares.
    ///
    /// The collector is expected to provide a [visitor] to the `Record`'s
    /// [`record` method] in order to record the added values.
    ///
    /// # Example
    ///  "foo = 3" will be recorded when [`record`] is called on the
    /// `Attributes` passed to `new_span`.
    /// Since values are not provided for the `bar` and `baz` fields,
    /// the span's `Metadata` will indicate that it _has_ those fields,
    /// but values for them won't be recorded at this time.
    ///
    /// ```rust,ignore
    /// #[macro_use]
    /// extern crate tracing;
    ///
    /// let mut span = span!("my_span", foo = 3, bar, baz);
    ///
    /// // `Collector::record` will be called with a `Record`
    /// // containing "bar = false"
    /// span.record("bar", &false);
    ///
    /// // `Collector::record` will be called with a `Record`
    /// // containing "baz = "a string""
    /// span.record("baz", &"a string");
    /// ```
    ///
    /// [visitor]: super::field::Visit
    /// [`record`]: super::span::Attributes::record
    /// [`record` method]: super::span::Record::record
    fn record(&self, span: &span::Id, values: &span::Record<'_>);

    /// Adds an indication that `span` follows from the span with the id
    /// `follows`.
    ///
    /// This relationship differs somewhat from the parent-child relationship: a
    /// span may have any number of prior spans, rather than a single one; and
    /// spans are not considered to be executing _inside_ of the spans they
    /// follow from. This means that a span may close even if subsequent spans
    /// that follow from it are still open, and time spent inside of a
    /// subsequent span should not be included in the time its precedents were
    /// executing. This is used to model causal relationships such as when a
    /// single future spawns several related background tasks, et cetera.
    ///
    /// If the collector has spans corresponding to the given IDs, it should
    /// record this relationship in whatever way it deems necessary. Otherwise,
    /// if one or both of the given span IDs do not correspond to spans that the
    /// collector knows about, or if a cyclical relationship would be created
    /// (i.e., some span _a_ which proceeds some other span _b_ may not also
    /// follow from _b_), it may silently do nothing.
    fn record_follows_from(&self, span: &span::Id, follows: &span::Id);

    /// Records that an [`Event`] has occurred.
    ///
    /// This method will be invoked when an Event is constructed by
    /// the `Event`'s [`dispatch` method]. For example, this happens internally
    /// when an event macro from `tracing` is called.
    ///
    /// The key difference between this method and `record` is that `record` is
    /// called when a value is recorded for a field defined by a span,
    /// while `event` is called when a new event occurs.
    ///
    /// The provided `Event` struct contains any field values attached to the
    /// event. The collector may pass a [visitor] to the `Event`'s
    /// [`record` method] to record these values.
    ///
    /// [`Event`]: super::event::Event
    /// [visitor]: super::field::Visit
    /// [`record` method]: super::event::Event::record
    /// [`dispatch` method]: super::event::Event::dispatch
    fn event(&self, event: &Event<'_>);

    /// Records that a span has been entered.
    ///
    /// When entering a span, this method is called to notify the collector
    /// that the span has been entered. The collector is provided with the
    /// [span ID] of the entered span, and should update any internal state
    /// tracking the current span accordingly.
    ///
    /// [span ID]: super::span::Id
    fn enter(&self, span: &span::Id);

    /// Records that a span has been exited.
    ///
    /// When exiting a span, this method is called to notify the collector
    /// that the span has been exited. The collector is provided with the
    /// [span ID] of the exited span, and should update any internal state
    /// tracking the current span accordingly.
    ///
    /// Exiting a span does not imply that the span will not be re-entered.
    ///
    /// [span ID]: super::span::Id
    fn exit(&self, span: &span::Id);

    /// Notifies the collector that a [span ID] has been cloned.
    ///
    /// This function is guaranteed to only be called with span IDs that were
    /// returned by this collector's `new_span` function.
    ///
    /// Note that the default implementation of this function this is just the
    /// identity function, passing through the identifier. However, it can be
    /// used in conjunction with [`try_close`] to track the number of handles
    /// capable of `enter`ing a span. When all the handles have been dropped
    /// (i.e., `try_close` has been called one more time than `clone_span` for a
    /// given ID), the collector may assume that the span will not be entered
    /// again. It is then free to deallocate storage for data associated with
    /// that span, write data from that span to IO, and so on.
    ///
    /// For more unsafe situations, however, if `id` is itself a pointer of some
    /// kind this can be used as a hook to "clone" the pointer, depending on
    /// what that means for the specified pointer.
    ///
    /// [span ID]: super::span::Id
    /// [`try_close`]: Collect::try_close
    fn clone_span(&self, id: &span::Id) -> span::Id {
        id.clone()
    }

    /// **This method is deprecated.**
    ///
    /// Using `drop_span` may result in collectors composed using
    /// `tracing-subscriber` crate's `Subscriber` trait from observing close events.
    /// Use [`try_close`] instead.
    ///
    /// The default implementation of this function does nothing.
    ///
    /// [`try_close`]: Collect::try_close
    #[deprecated(since = "0.1.2", note = "use `Collector::try_close` instead")]
    fn drop_span(&self, _id: span::Id) {}

    /// Notifies the collector that a [`span ID`] has been dropped, and returns
    /// `true` if there are now 0 IDs that refer to that span.
    ///
    /// Higher-level libraries providing functionality for composing multiple
    /// collector implementations may use this return value to notify any
    /// "layered" collectors that this collector considers the span closed.
    ///
    /// The default implementation of this method calls the collector's
    /// [`drop_span`] method and returns `false`. This means that, unless the
    /// collector overrides the default implementation, close notifications
    /// will never be sent to any layered collectors. In general, if the
    /// collector tracks reference counts, this method should be implemented,
    /// rather than `drop_span`.
    ///
    /// This function is guaranteed to only be called with span IDs that were
    /// returned by this collector's `new_span` function.
    ///
    /// It's guaranteed that if this function has been called once more than the
    /// number of times `clone_span` was called with the same `id`, then no more
    /// handles that can enter the span with that `id` exist. This means that it
    /// can be used in conjunction with [`clone_span`] to track the number of
    /// handles capable of `enter`ing a span. When all the handles have been
    /// dropped (i.e., `try_close` has been called one more time than
    /// `clone_span` for a given ID), the collector may assume that the span
    /// will not be entered again, and should return `true`. It is then free to
    /// deallocate storage for data associated with that span, write data from
    /// that span to IO, and so on.
    ///
    /// **Note**: since this function is called when spans are dropped,
    /// implementations should ensure that they are unwind-safe. Panicking from
    /// inside of a `try_close` function may cause a double panic, if the span
    /// was dropped due to a thread unwinding.
    ///
    /// [`span ID`]: super::span::Id
    /// [`clone_span`]: Collect::clone_span
    /// [`drop_span`]: Collect::drop_span
    fn try_close(&self, id: span::Id) -> bool {
        #[allow(deprecated)]
        self.drop_span(id);
        false
    }

    /// Returns a type representing this collector's view of the current span.
    ///
    /// If collectors track a current span, they should override this function
    /// to return [`Current::new`] if the thread from which this method is
    /// called is inside a span, or [`Current::none`] if the thread is not
    /// inside a span.
    ///
    /// By default, this returns a value indicating that the collector
    /// does **not** track what span is current. If the collector does not
    /// implement a current span, it should not override this method.
    ///
    /// [`Current::new`]: super::span::Current::new
    /// [`Current::none`]: super::span::Current::none
    fn current_span(&self) -> span::Current {
        span::Current::unknown()
    }

    // === Downcasting methods ================================================

    /// If `self` is the same type as the provided `TypeId`, returns an untyped
    /// `*const` pointer to that type. Otherwise, returns `None`.
    ///
    /// If you wish to downcast a `Collector`, it is strongly advised to use
    /// the safe API provided by [`downcast_ref`] instead.
    ///
    /// This API is required for `downcast_raw` to be a trait method; a method
    /// signature like [`downcast_ref`] (with a generic type parameter) is not
    /// object-safe, and thus cannot be a trait method for `Collector`. This
    /// means that if we only exposed `downcast_ref`, `Collector`
    /// implementations could not override the downcasting behavior
    ///
    /// This method may be overridden by "fan out" or "chained" collector
    /// implementations which consist of multiple composed types. Such
    /// collectors might allow `downcast_raw` by returning references to those
    /// component if they contain components with the given `TypeId`.
    ///
    /// # Safety
    ///
    /// The [`downcast_ref`] method expects that the pointer returned by
    /// `downcast_raw` is non-null and points to a valid instance of the type
    /// with the provided `TypeId`. Failure to ensure this will result in
    /// undefined behaviour, so implementing `downcast_raw` is unsafe.
    ///
    /// [`downcast_ref`]: #method.downcast_ref
    unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> {
        if id == TypeId::of::<Self>() {
            Some(self as *const Self as *const ())
        } else {
            None
        }
    }
}

impl dyn Collect {
    /// Returns `true` if this `Collector` is the same type as `T`.
    pub fn is<T: Any>(&self) -> bool {
        self.downcast_ref::<T>().is_some()
    }

    /// Returns some reference to this `Collector` value if it is of type `T`,
    /// or `None` if it isn't.
    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
        unsafe {
            let raw = self.downcast_raw(TypeId::of::<T>())?;
            if raw.is_null() {
                None
            } else {
                Some(&*(raw as *const _))
            }
        }
    }
}

/// Indicates a [`Collect`]'s interest in a particular callsite.
///
/// Collectors return an `Interest` from their [`register_callsite`] methods
/// in order to determine whether that span should be enabled or disabled.
///
/// [`Collect`]: super::Collect
/// [`register_callsite`]: super::Collect::register_callsite
#[derive(Clone, Debug)]
pub struct Interest(InterestKind);

#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
enum InterestKind {
    Never = 0,
    Sometimes = 1,
    Always = 2,
}

impl Interest {
    /// Returns an `Interest` indicating that the collector is never interested
    /// in being notified about a callsite.
    ///
    /// If all active collectors are `never()` interested in a callsite, it will
    /// be completely disabled unless a new collector becomes active.
    #[inline]
    pub fn never() -> Self {
        Interest(InterestKind::Never)
    }

    /// Returns an `Interest` indicating the collector is sometimes interested
    /// in being notified about a callsite.
    ///
    /// If all active collectors are `sometimes` or `never` interested in a
    /// callsite, the currently active collector will be asked to filter that
    /// callsite every time it creates a span. This will be the case until a new
    /// collector expresses that it is `always` interested in the callsite.
    #[inline]
    pub fn sometimes() -> Self {
        Interest(InterestKind::Sometimes)
    }

    /// Returns an `Interest` indicating the collector is always interested in
    /// being notified about a callsite.
    ///
    /// If any collector expresses that it is `always()` interested in a given
    /// callsite, then the callsite will always be enabled.
    #[inline]
    pub fn always() -> Self {
        Interest(InterestKind::Always)
    }

    /// Returns `true` if the collector is never interested in being notified
    /// about this callsite.
    #[inline]
    pub fn is_never(&self) -> bool {
        matches!(self.0, InterestKind::Never)
    }

    /// Returns `true` if the collector is sometimes interested in being notified
    /// about this callsite.
    #[inline]
    pub fn is_sometimes(&self) -> bool {
        matches!(self.0, InterestKind::Sometimes)
    }

    /// Returns `true` if the collector is always interested in being notified
    /// about this callsite.
    #[inline]
    pub fn is_always(&self) -> bool {
        matches!(self.0, InterestKind::Always)
    }

    /// Returns the common interest between these two Interests.
    ///
    /// If both interests are the same, this propagates that interest.
    /// Otherwise, if they differ, the result must always be
    /// `Interest::sometimes` --- if the two collectors differ in opinion, we
    /// will have to ask the current collector what it thinks, no matter what.
    // Only needed when combining interest from multiple collectors.
    #[cfg(feature = "std")]
    pub(crate) fn and(self, rhs: Interest) -> Self {
        if self.0 == rhs.0 {
            self
        } else {
            Interest::sometimes()
        }
    }
}