logo
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
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
//! Callsites represent the source locations from which spans or events
//! originate.
//!
//! # What Are Callsites?
//!
//! Every span or event in `tracing` is associated with a [`Callsite`]. A
//! callsite is a small `static` value that is responsible for the following:
//!
//! * Storing the span or event's [`Metadata`],
//! * Uniquely [identifying](Identifier) the span or event definition,
//! * Caching the subscriber's [`Interest`][^1] in that span or event, to avoid
//!   re-evaluating filters,
//! * Storing a [`Registration`] that allows the callsite to be part of a global
//!   list of all callsites in the program.
//!
//! # Registering Callsites
//!
//! When a span or event is recorded for the first time, its callsite
//! [`register`]s itself with the global callsite registry. Registering a
//! callsite calls the [`Subscriber::register_callsite`][`register_callsite`]
//! method with that callsite's [`Metadata`] on every currently active
//! subscriber. This serves two primary purposes: informing subscribers of the
//! callsite's existence, and performing static filtering.
//!
//! ## Callsite Existence
//!
//! If a [`Subscriber`] implementation wishes to allocate storage for each
//! unique span/event location in the program, or pre-compute some value
//! that will be used to record that span or event in the future, it can
//! do so in its [`register_callsite`] method.
//!
//! ## Performing Static Filtering
//!
//! The [`register_callsite`] method returns an [`Interest`] value,
//! which indicates that the subscriber either [always] wishes to record
//! that span or event, [sometimes] wishes to record it based on a
//! dynamic filter evaluation, or [never] wishes to record it.
//!
//! When registering a new callsite, the [`Interest`]s returned by every
//! currently active subscriber are combined, and the result is stored at
//! each callsite. This way, when the span or event occurs in the
//! future, the cached [`Interest`] value can be checked efficiently
//! to determine if the span or event should be recorded, without
//! needing to perform expensive filtering (i.e. calling the
//! [`Subscriber::enabled`] method every time a span or event occurs).
//!
//! ### Rebuilding Cached Interest
//!
//! When a new [`Dispatch`] is created (i.e. a new subscriber becomes
//! active), any previously cached [`Interest`] values are re-evaluated
//! for all callsites in the program. This way, if the new subscriber
//! will enable a callsite that was not previously enabled, the
//! [`Interest`] in that callsite is updated. Similarly, when a
//! subscriber is dropped, the interest cache is also re-evaluated, so
//! that any callsites enabled only by that subscriber are disabled.
//!
//! In addition, the [`rebuild_interest_cache`] function in this module can be
//! used to manually invalidate all cached interest and re-register those
//! callsites. This function is useful in situations where a subscriber's
//! interest can change, but it does so relatively infrequently. The subscriber
//! may wish for its interest to be cached most of the time, and return
//! [`Interest::always`][always] or [`Interest::never`][never] in its
//! [`register_callsite`] method, so that its [`Subscriber::enabled`] method
//! doesn't need to be evaluated every time a span or event is recorded.
//! However, when the configuration changes, the subscriber can call
//! [`rebuild_interest_cache`] to re-evaluate the entire interest cache with its
//! new configuration. This is a relatively costly operation, but if the
//! configuration changes infrequently, it may be more efficient than calling
//! [`Subscriber::enabled`] frequently.
//!
//! # Implementing Callsites
//!
//! In most cases, instrumenting code using `tracing` should *not* require
//! implementing the [`Callsite`] trait directly. When using the [`tracing`
//! crate's macros][macros] or the [`#[instrument]` attribute][instrument], a
//! `Callsite` is automatically generated.
//!
//! However, code which provides alternative forms of `tracing` instrumentation
//! may need to interact with the callsite system directly. If
//! instrumentation-side code needs to produce a `Callsite` to emit spans or
//! events, the [`DefaultCallsite`] struct provided in this module is a
//! ready-made `Callsite` implementation that is suitable for most uses. When
//! possible, the use of `DefaultCallsite` should be preferred over implementing
//! [`Callsite`] for user types, as `DefaultCallsite` may benefit from
//! additional performance optimizations.
//!
//! [^1]: Returned by the [`Subscriber::register_callsite`][`register_callsite`]
//!     method.
//!
//! [`Metadata`]: crate::metadata::Metadata
//! [`Interest`]: crate::subscriber::Interest
//! [`Subscriber`]: crate::subscriber::Subscriber
//! [`register_callsite`]: crate::subscriber::Subscriber::register_callsite
//! [`Subscriber::enabled`]: crate::subscriber::Subscriber::enabled
//! [always]: crate::subscriber::Interest::always
//! [sometimes]: crate::subscriber::Interest::sometimes
//! [never]: crate::subscriber::Interest::never
//! [`Dispatch`]: crate::dispatch::Dispatch
//! [macros]: https://docs.rs/tracing/latest/tracing/#macros
//! [instrument]: https://docs.rs/tracing/latest/tracing/attr.instrument.html
use crate::stdlib::{
    any::TypeId,
    fmt,
    hash::{Hash, Hasher},
    ptr,
    sync::{
        atomic::{AtomicBool, AtomicPtr, AtomicU8, Ordering},
        Mutex,
    },
    vec::Vec,
};
use crate::{
    dispatcher::Dispatch,
    lazy::Lazy,
    metadata::{LevelFilter, Metadata},
    subscriber::Interest,
};

use self::dispatchers::Dispatchers;

/// Trait implemented by callsites.
///
/// These functions are only intended to be called by the callsite registry, which
/// correctly handles determining the common interest between all subscribers.
///
/// See the [module-level documentation](crate::callsite) for details on
/// callsites.
pub trait Callsite: Sync {
    /// Sets the [`Interest`] for this callsite.
    ///
    /// See the [documentation on callsite interest caching][cache-docs] for
    /// details.
    ///
    /// [`Interest`]: super::subscriber::Interest
    /// [cache-docs]: crate::callsite#performing-static-filtering
    fn set_interest(&self, interest: Interest);

    /// Returns the [metadata] associated with the callsite.
    ///
    /// <div class="example-wrap" style="display:inline-block">
    /// <pre class="ignore" style="white-space:normal;font:inherit;">
    ///
    /// **Note:** Implementations of this method should not produce [`Metadata`]
    /// that share the same callsite [`Identifier`] but otherwise differ in any
    /// way (e.g., have different `name`s).
    ///
    /// </pre></div>
    ///
    /// [metadata]: super::metadata::Metadata
    fn metadata(&self) -> &Metadata<'_>;

    /// This method is an *internal implementation detail* of `tracing-core`. It
    /// is *not* intended to be called or overridden from downstream code.
    ///
    /// The `Private` type can only be constructed from within `tracing-core`.
    /// Because this method takes a `Private` as an argument, it cannot be
    /// called from (safe) code external to `tracing-core`. Because it must
    /// *return* a `Private`, the only valid implementation possible outside of
    /// `tracing-core` would have to always unconditionally panic.
    ///
    /// THIS IS BY DESIGN. There is currently no valid reason for code outside
    /// of `tracing-core` to override this method.
    // TODO(eliza): this could be used to implement a public downcasting API
    // for `&dyn Callsite`s in the future.
    #[doc(hidden)]
    #[inline]
    fn private_type_id(&self, _: private::Private<()>) -> private::Private<TypeId>
    where
        Self: 'static,
    {
        private::Private(TypeId::of::<Self>())
    }
}

/// Uniquely identifies a [`Callsite`]
///
/// Two `Identifier`s are equal if they both refer to the same callsite.
///
/// [`Callsite`]: super::callsite::Callsite
#[derive(Clone)]
pub struct Identifier(
    /// **Warning**: The fields on this type are currently `pub` because it must
    /// be able to be constructed statically by macros. However, when `const
    /// fn`s are available on stable Rust, this will no longer be necessary.
    /// Thus, these fields are *not* considered stable public API, and they may
    /// change warning. Do not rely on any fields on `Identifier`. When
    /// constructing new `Identifier`s, use the `identify_callsite!` macro
    /// instead.
    #[doc(hidden)]
    pub &'static dyn Callsite,
);

/// A default [`Callsite`] implementation.
#[derive(Debug)]
pub struct DefaultCallsite {
    interest: AtomicU8,
    registration: AtomicU8,
    meta: &'static Metadata<'static>,
    next: AtomicPtr<Self>,
}

/// Clear and reregister interest on every [`Callsite`]
///
/// This function is intended for runtime reconfiguration of filters on traces
/// when the filter recalculation is much less frequent than trace events are.
/// The alternative is to have the [`Subscriber`] that supports runtime
/// reconfiguration of filters always return [`Interest::sometimes()`] so that
/// [`enabled`] is evaluated for every event.
///
/// This function will also re-compute the global maximum level as determined by
/// the [`max_level_hint`] method. If a [`Subscriber`]
/// implementation changes the value returned by its `max_level_hint`
/// implementation at runtime, then it **must** call this function after that
/// value changes, in order for the change to be reflected.
///
/// See the [documentation on callsite interest caching][cache-docs] for
/// additional information on this function's usage.
///
/// [`max_level_hint`]: super::subscriber::Subscriber::max_level_hint
/// [`Callsite`]: super::callsite::Callsite
/// [`enabled`]: super::subscriber::Subscriber#tymethod.enabled
/// [`Interest::sometimes()`]: super::subscriber::Interest::sometimes
/// [`Subscriber`]: super::subscriber::Subscriber
/// [cache-docs]: crate::callsite#rebuilding-cached-interest
pub fn rebuild_interest_cache() {
    CALLSITES.rebuild_interest(DISPATCHERS.rebuilder());
}

/// Register a new [`Callsite`] with the global registry.
///
/// This should be called once per callsite after the callsite has been
/// constructed.
///
/// See the [documentation on callsite registration][reg-docs] for details
/// on the global callsite registry.
///
/// [`Callsite`]: crate::callsite::Callsite
/// [reg-docs]: crate::callsite#registering-callsites
pub fn register(callsite: &'static dyn Callsite) {
    rebuild_callsite_interest(callsite, &DISPATCHERS.rebuilder());

    // Is this a `DefaultCallsite`? If so, use the fancy linked list!
    if callsite.private_type_id(private::Private(())).0 == TypeId::of::<DefaultCallsite>() {
        let callsite = unsafe {
            // Safety: the pointer cast is safe because the type id of the
            // provided callsite matches that of the target type for the cast
            // (`DefaultCallsite`). Because user implementations of `Callsite`
            // cannot override `private_type_id`, we can trust that the callsite
            // is not lying about its type ID.
            &*(callsite as *const dyn Callsite as *const DefaultCallsite)
        };
        CALLSITES.push_default(callsite);
        return;
    }

    CALLSITES.push_dyn(callsite);
}

static CALLSITES: Callsites = Callsites {
    list_head: AtomicPtr::new(ptr::null_mut()),
    has_locked_callsites: AtomicBool::new(false),
};

static DISPATCHERS: Dispatchers = Dispatchers::new();

static LOCKED_CALLSITES: Lazy<Mutex<Vec<&'static dyn Callsite>>> = Lazy::new(Default::default);

struct Callsites {
    list_head: AtomicPtr<DefaultCallsite>,
    has_locked_callsites: AtomicBool,
}

// === impl DefaultCallsite ===

impl DefaultCallsite {
    const UNREGISTERED: u8 = 0;
    const REGISTERING: u8 = 1;
    const REGISTERED: u8 = 2;

    const INTEREST_NEVER: u8 = 0;
    const INTEREST_SOMETIMES: u8 = 1;
    const INTEREST_ALWAYS: u8 = 2;

    /// Returns a new `DefaultCallsite` with the specified `Metadata`.
    pub const fn new(meta: &'static Metadata<'static>) -> Self {
        Self {
            interest: AtomicU8::new(0xFF),
            meta,
            next: AtomicPtr::new(ptr::null_mut()),
            registration: AtomicU8::new(Self::UNREGISTERED),
        }
    }

    /// Registers this callsite with the global callsite registry.
    ///
    /// If the callsite is already registered, this does nothing. When using
    /// [`DefaultCallsite`], this method should be preferred over
    /// [`tracing_core::callsite::register`], as it ensures that the callsite is
    /// only registered a single time.
    ///
    /// Other callsite implementations will generally ensure that
    /// callsites are not re-registered through another mechanism.
    ///
    /// See the [documentation on callsite registration][reg-docs] for details
    /// on the global callsite registry.
    ///
    /// [`Callsite`]: crate::callsite::Callsite
    /// [reg-docs]: crate::callsite#registering-callsites
    #[inline(never)]
    // This only happens once (or if the cached interest value was corrupted).
    #[cold]
    pub fn register(&'static self) -> Interest {
        // Attempt to advance the registration state to `REGISTERING`...
        match self.registration.compare_exchange(
            Self::UNREGISTERED,
            Self::REGISTERING,
            Ordering::AcqRel,
            Ordering::Acquire,
        ) {
            Ok(_) => {
                // Okay, we advanced the state, try to register the callsite.
                rebuild_callsite_interest(self, &DISPATCHERS.rebuilder());
                CALLSITES.push_default(self);
                self.registration.store(Self::REGISTERED, Ordering::Release);
            }
            // Great, the callsite is already registered! Just load its
            // previous cached interest.
            Err(Self::REGISTERED) => {}
            // Someone else is registering...
            Err(_state) => {
                debug_assert_eq!(
                    _state,
                    Self::REGISTERING,
                    "weird callsite registration state"
                );
                // Just hit `enabled` this time.
                return Interest::sometimes();
            }
        }

        match self.interest.load(Ordering::Relaxed) {
            Self::INTEREST_NEVER => Interest::never(),
            Self::INTEREST_ALWAYS => Interest::always(),
            _ => Interest::sometimes(),
        }
    }

    /// Returns the callsite's cached `Interest`, or registers it for the
    /// first time if it has not yet been registered.
    #[inline]
    pub fn interest(&'static self) -> Interest {
        match self.interest.load(Ordering::Relaxed) {
            Self::INTEREST_NEVER => Interest::never(),
            Self::INTEREST_SOMETIMES => Interest::sometimes(),
            Self::INTEREST_ALWAYS => Interest::always(),
            _ => self.register(),
        }
    }
}

impl Callsite for DefaultCallsite {
    fn set_interest(&self, interest: Interest) {
        let interest = match () {
            _ if interest.is_never() => Self::INTEREST_NEVER,
            _ if interest.is_always() => Self::INTEREST_ALWAYS,
            _ => Self::INTEREST_SOMETIMES,
        };
        self.interest.store(interest, Ordering::SeqCst);
    }

    #[inline(always)]
    fn metadata(&self) -> &Metadata<'static> {
        self.meta
    }
}

// ===== impl Identifier =====

impl PartialEq for Identifier {
    fn eq(&self, other: &Identifier) -> bool {
        core::ptr::eq(
            self.0 as *const _ as *const (),
            other.0 as *const _ as *const (),
        )
    }
}

impl Eq for Identifier {}

impl fmt::Debug for Identifier {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Identifier({:p})", self.0)
    }
}

impl Hash for Identifier {
    fn hash<H>(&self, state: &mut H)
    where
        H: Hasher,
    {
        (self.0 as *const dyn Callsite).hash(state)
    }
}

// === impl Callsites ===

impl Callsites {
    /// Rebuild `Interest`s for all callsites in the registry.
    ///
    /// This also re-computes the max level hint.
    fn rebuild_interest(&self, dispatchers: dispatchers::Rebuilder<'_>) {
        let mut max_level = LevelFilter::OFF;
        dispatchers.for_each(|dispatch| {
            // If the subscriber did not provide a max level hint, assume
            // that it may enable every level.
            let level_hint = dispatch.max_level_hint().unwrap_or(LevelFilter::TRACE);
            if level_hint > max_level {
                max_level = level_hint;
            }
        });

        self.for_each(|callsite| {
            rebuild_callsite_interest(callsite, &dispatchers);
        });
        LevelFilter::set_max(max_level);
    }

    /// Push a `dyn Callsite` trait object to the callsite registry.
    ///
    /// This will attempt to lock the callsites vector.
    fn push_dyn(&self, callsite: &'static dyn Callsite) {
        let mut lock = LOCKED_CALLSITES.lock().unwrap();
        self.has_locked_callsites.store(true, Ordering::Release);
        lock.push(callsite);
    }

    /// Push a `DefaultCallsite` to the callsite registry.
    ///
    /// If we know the callsite being pushed is a `DefaultCallsite`, we can push
    /// it to the linked list without having to acquire a lock.
    fn push_default(&self, callsite: &'static DefaultCallsite) {
        let mut head = self.list_head.load(Ordering::Acquire);

        loop {
            callsite.next.store(head, Ordering::Release);

            assert_ne!(
                callsite as *const _, head,
                "Attempted to register a `DefaultCallsite` that already exists! \
                This will cause an infinite loop when attempting to read from the \
                callsite cache. This is likely a bug! You should only need to call \
                `DefaultCallsite::register` once per `DefaultCallsite`."
            );

            match self.list_head.compare_exchange(
                head,
                callsite as *const _ as *mut _,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => {
                    break;
                }
                Err(current) => head = current,
            }
        }
    }

    /// Invokes the provided closure `f` with each callsite in the registry.
    fn for_each(&self, mut f: impl FnMut(&'static dyn Callsite)) {
        let mut head = self.list_head.load(Ordering::Acquire);

        while let Some(cs) = unsafe { head.as_ref() } {
            f(cs);

            head = cs.next.load(Ordering::Acquire);
        }

        if self.has_locked_callsites.load(Ordering::Acquire) {
            let locked = LOCKED_CALLSITES.lock().unwrap();
            for &cs in locked.iter() {
                f(cs);
            }
        }
    }
}

pub(crate) fn register_dispatch(dispatch: &Dispatch) {
    let dispatchers = DISPATCHERS.register_dispatch(dispatch);
    CALLSITES.rebuild_interest(dispatchers);
}

fn rebuild_callsite_interest(
    callsite: &'static dyn Callsite,
    dispatchers: &dispatchers::Rebuilder<'_>,
) {
    let meta = callsite.metadata();

    let mut interest = None;
    dispatchers.for_each(|dispatch| {
        let this_interest = dispatch.register_callsite(meta);
        interest = match interest.take() {
            None => Some(this_interest),
            Some(that_interest) => Some(that_interest.and(this_interest)),
        }
    });

    let interest = interest.unwrap_or_else(Interest::never);
    callsite.set_interest(interest)
}

mod private {
    /// Don't call this function, it's private.
    #[allow(missing_debug_implementations)]
    pub struct Private<T>(pub(crate) T);
}

#[cfg(feature = "std")]
mod dispatchers {
    use crate::{dispatcher, lazy::Lazy};
    use std::sync::{
        atomic::{AtomicBool, Ordering},
        RwLock, RwLockReadGuard, RwLockWriteGuard,
    };

    pub(super) struct Dispatchers {
        has_just_one: AtomicBool,
    }

    static LOCKED_DISPATCHERS: Lazy<RwLock<Vec<dispatcher::Registrar>>> =
        Lazy::new(Default::default);

    pub(super) enum Rebuilder<'a> {
        JustOne,
        Read(RwLockReadGuard<'a, Vec<dispatcher::Registrar>>),
        Write(RwLockWriteGuard<'a, Vec<dispatcher::Registrar>>),
    }

    impl Dispatchers {
        pub(super) const fn new() -> Self {
            Self {
                has_just_one: AtomicBool::new(true),
            }
        }

        pub(super) fn rebuilder(&self) -> Rebuilder<'_> {
            if self.has_just_one.load(Ordering::SeqCst) {
                return Rebuilder::JustOne;
            }
            Rebuilder::Read(LOCKED_DISPATCHERS.read().unwrap())
        }

        pub(super) fn register_dispatch(&self, dispatch: &dispatcher::Dispatch) -> Rebuilder<'_> {
            let mut dispatchers = LOCKED_DISPATCHERS.write().unwrap();
            dispatchers.retain(|d| d.upgrade().is_some());
            dispatchers.push(dispatch.registrar());
            self.has_just_one
                .store(dispatchers.len() <= 1, Ordering::SeqCst);
            Rebuilder::Write(dispatchers)
        }
    }

    impl Rebuilder<'_> {
        pub(super) fn for_each(&self, mut f: impl FnMut(&dispatcher::Dispatch)) {
            let iter = match self {
                Rebuilder::JustOne => {
                    dispatcher::get_default(f);
                    return;
                }
                Rebuilder::Read(vec) => vec.iter(),
                Rebuilder::Write(vec) => vec.iter(),
            };
            iter.filter_map(dispatcher::Registrar::upgrade)
                .for_each(|dispatch| f(&dispatch))
        }
    }
}

#[cfg(not(feature = "std"))]
mod dispatchers {
    use crate::dispatcher;

    pub(super) struct Dispatchers(());
    pub(super) struct Rebuilder<'a>(Option<&'a dispatcher::Dispatch>);

    impl Dispatchers {
        pub(super) const fn new() -> Self {
            Self(())
        }

        pub(super) fn rebuilder(&self) -> Rebuilder<'_> {
            Rebuilder(None)
        }

        pub(super) fn register_dispatch<'dispatch>(
            &self,
            dispatch: &'dispatch dispatcher::Dispatch,
        ) -> Rebuilder<'dispatch> {
            // nop; on no_std, there can only ever be one dispatcher
            Rebuilder(Some(dispatch))
        }
    }

    impl Rebuilder<'_> {
        #[inline]
        pub(super) fn for_each(&self, mut f: impl FnMut(&dispatcher::Dispatch)) {
            if let Some(dispatch) = self.0 {
                // we are rebuilding the interest cache because a new dispatcher
                // is about to be set. on `no_std`, this should only happen
                // once, because the new dispatcher will be the global default.
                f(dispatch)
            } else {
                // otherwise, we are rebuilding the cache because the subscriber
                // configuration changed, so use the global default.
                // on no_std, there can only ever be one dispatcher
                dispatcher::get_default(f)
            }
        }
    }
}