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//! Subscribers collect and record trace data. use crate::{span, Event, LevelFilter, Metadata}; use crate::stdlib::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 `Subscriber` 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 /// subscriber implementations more modular or improve the ergonomics of writing /// subscribers. /// /// A subscriber is responsible for the following: /// - Registering new spans as they are created, and providing them with span /// IDs. Implicitly, this means the subscriber 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 subscriber is provided only with the /// [ID] with which it tagged that span when it was created. This means /// that it is up to the subscriber 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 subscriber _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 subscriber /// may freely discard that data without allocating space to store it. /// /// ## Overriding default impls /// /// Some trait methods on `Subscriber` have default implementations, either in /// order to reduce the surface area of implementing `Subscriber`, or for /// backward-compatibility reasons. However, many subscribers 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 subscriber 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 subscriber's interest can change dynamically at runtime, /// it may want to override this function to return `Interest::sometimes()`. /// Additionally, subscribers 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 subscribers to free storage for span data and to determine /// when a span has _closed_ permanently (rather than being exited). /// Subscribers which store per-span data or which need to track span closures /// should override these functions together. /// /// [ID]: ../span/struct.Id.html /// [`new_span`]: trait.Subscriber.html#method.new_span /// [`register_callsite`]: trait.Subscriber.html#method.register_callsite /// [`Interest`]: struct.Interest.html /// [`enabled`]: trait.Subscriber.html#method.enabled /// [`clone_span`]: trait.Subscriber.html#method.clone_span /// [`try_close`]: trait.Subscriber.html#method.try_close pub trait Subscriber: 'static { // === Span registry methods ============================================== /// Registers a new callsite with this subscriber, returning whether or not /// the subscriber is interested in being notified about the callsite. /// /// By default, this function assumes that the subscriber's [filter] /// represents an unchanging view of its interest in the callsite. However, /// if this is not the case, subscribers 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 subscriber. The subscriber 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 subscriber 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 subscriber always /// wishes to be notified about the callsite, and its filter need not be /// re-evaluated; `Sometimes`, indicating that the subscriber may sometimes /// care about the callsite but not always (such as when sampling), or /// `Never`, indicating that the subscriber never wishes to be notified about /// that callsite. If all active subscribers return `Never`, a callsite will /// never be enabled unless a new subscriber expresses interest in it. /// /// `Subscriber`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 subscriber 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 subscriber 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 subscriber 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`. /// /// `Subscriber`s that need to change their filters occasionally should call /// [`rebuild_interest_cache`] to re-evaluate `register_callsite` for all /// callsites. /// /// Similarly, if a `Subscriber` 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 subscriber which manages fanout to multiple other subscribers /// should proxy this decision to all of its child subscribers, /// returning `Interest::never` only if _all_ such children return /// `Interest::never`. If the set of subscribers to which spans are /// broadcast may change dynamically, the subscriber should also never /// return `Interest::Never`, as a new subscriber may be added that _is_ /// interested. /// /// # Notes /// This function may be called again when a new subscriber is created or /// when the registry is invalidated. /// /// If a subscriber returns `Interest::never` for a particular callsite, it /// _may_ still see spans and events originating from that callsite, if /// another subscriber expressed interest in it. /// /// [filter]: #method.enabled /// [metadata]: ../metadata/struct.Metadata.html /// [`Interest`]: struct.Interest.html /// [`enabled`]: #method.enabled /// [`rebuild_interest_cache`]: ../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 subscriber /// 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 subscriber'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]: ../metadata/struct.Metadata.html /// [interested]: struct.Interest.html /// [`Interest::sometimes`]: struct.Interest.html#method.sometimes /// [`register_callsite`]: #method.register_callsite fn enabled(&self, metadata: &Metadata<'_>) -> bool; /// Returns the highest [verbosity level][level] that this `Subscriber` will /// enable, or `None`, if the subscriber 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. Subscribers which perform filtering are strongly /// encouraged to provide an implementation of this method. /// /// If the maximum level the subscriber 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 subscriber 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]: ../struct.Level.html /// [`Interest`]: struct.Interest.html /// [rebuild]: ../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 subscriber 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 /// subscriber, so span equality will be based on the returned ID. Thus, if /// the subscriber 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 subscriber 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 subscriber implementation to determine. /// /// [span ID]: ../span/struct.Id.html /// [`Attributes`]: ../span/struct.Attributes.html /// [visitor]: ../field/trait.Visit.html /// [`record` method]: ../span/struct.Attributes.html#method.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 subscriber implementation. /// /// Keep in mind that a span might not provide a value /// for each field it declares. /// /// The subscriber 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); /// /// // `Subscriber::record` will be called with a `Record` /// // containing "bar = false" /// span.record("bar", &false); /// /// // `Subscriber::record` will be called with a `Record` /// // containing "baz = "a string"" /// span.record("baz", &"a string"); /// ``` /// /// [visitor]: ../field/trait.Visit.html /// [`record`]: ../span/struct.Attributes.html#method.record /// [`record` method]: ../span/struct.Record.html#method.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 subscriber 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 /// subscriber 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 subscriber may pass a [visitor] to the `Event`'s /// [`record` method] to record these values. /// /// [`Event`]: ../event/struct.Event.html /// [visitor]: ../field/trait.Visit.html /// [`record` method]: ../event/struct.Event.html#method.record /// [`dispatch` method]: ../event/struct.Event.html#method.dispatch fn event(&self, event: &Event<'_>); /// Records that a span has been entered. /// /// When entering a span, this method is called to notify the subscriber /// that the span has been entered. The subscriber is provided with the /// [span ID] of the entered span, and should update any internal state /// tracking the current span accordingly. /// /// [span ID]: ../span/struct.Id.html fn enter(&self, span: &span::Id); /// Records that a span has been exited. /// /// When exiting a span, this method is called to notify the subscriber /// that the span has been exited. The subscriber 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]: ../span/struct.Id.html fn exit(&self, span: &span::Id); /// Notifies the subscriber that a [span ID] has been cloned. /// /// This function is guaranteed to only be called with span IDs that were /// returned by this subscriber'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 subscriber 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]: ../span/struct.Id.html /// [`try_close`]: trait.Subscriber.html#method.try_close fn clone_span(&self, id: &span::Id) -> span::Id { id.clone() } /// **This method is deprecated.** /// /// Using `drop_span` may result in subscribers composed using /// `tracing-subscriber` crate's `Layer` trait from observing close events. /// Use [`try_close`] instead. /// /// The default implementation of this function does nothing. /// /// [`try_close`]: trait.Subscriber.html#method.try_close #[deprecated(since = "0.1.2", note = "use `Subscriber::try_close` instead")] fn drop_span(&self, _id: span::Id) {} /// Notifies the subscriber 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 /// subscriber implementations may use this return value to notify any /// "layered" subscribers that this subscriber considers the span closed. /// /// The default implementation of this method calls the subscriber's /// [`drop_span`] method and returns `false`. This means that, unless the /// subscriber overrides the default implementation, close notifications /// will never be sent to any layered subscribers. In general, if the /// subscriber 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 subscriber'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 subscriber 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]: ../span/struct.Id.html /// [`clone_span`]: trait.Subscriber.html#method.clone_span /// [`drop_span`]: trait.Subscriber.html#method.drop_span fn try_close(&self, id: span::Id) -> bool { #[allow(deprecated)] self.drop_span(id); false } /// Returns a type representing this subscriber's view of the current span. /// /// If subscribers 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 subscriber /// does **not** track what span is current. If the subscriber does not /// implement a current span, it should not override this method. /// /// [`Current::new`]: ../span/struct.Current.html#tymethod.new /// [`Current::none`]: ../span/struct.Current.html#tymethod.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 `Subscriber`, 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 `Subscriber`. This /// means that if we only exposed `downcast_ref`, `Subscriber` /// implementations could not override the downcasting behavior /// /// This method may be overridden by "fan out" or "chained" subscriber /// implementations which consist of multiple composed types. Such /// subscribers 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 Subscriber { /// Returns `true` if this `Subscriber` is the same type as `T`. pub fn is<T: Any>(&self) -> bool { self.downcast_ref::<T>().is_some() } /// Returns some reference to this `Subscriber` 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 [`Subscriber`]'s interest in a particular callsite. /// /// `Subscriber`s return an `Interest` from their [`register_callsite`] methods /// in order to determine whether that span should be enabled or disabled. /// /// [`Subscriber`]: ../trait.Subscriber.html /// [`register_callsite`]: ../trait.Subscriber.html#method.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 subscriber is never interested /// in being notified about a callsite. /// /// If all active subscribers are `never()` interested in a callsite, it will /// be completely disabled unless a new subscriber becomes active. #[inline] pub fn never() -> Self { Interest(InterestKind::Never) } /// Returns an `Interest` indicating the subscriber is sometimes interested /// in being notified about a callsite. /// /// If all active subscribers are `sometimes` or `never` interested in a /// callsite, the currently active subscriber will be asked to filter that /// callsite every time it creates a span. This will be the case until a new /// subscriber expresses that it is `always` interested in the callsite. #[inline] pub fn sometimes() -> Self { Interest(InterestKind::Sometimes) } /// Returns an `Interest` indicating the subscriber is always interested in /// being notified about a callsite. /// /// If any subscriber 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 subscriber is never interested in being notified /// about this callsite. #[inline] pub fn is_never(&self) -> bool { match self.0 { InterestKind::Never => true, _ => false, } } /// Returns `true` if the subscriber is sometimes interested in being notified /// about this callsite. #[inline] pub fn is_sometimes(&self) -> bool { match self.0 { InterestKind::Sometimes => true, _ => false, } } /// Returns `true` if the subscriber is always interested in being notified /// about this callsite. #[inline] pub fn is_always(&self) -> bool { match self.0 { InterestKind::Always => true, _ => false, } } /// 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 subscribers differ in opinion, we /// will have to ask the current subscriber what it thinks, no matter what. pub(crate) fn and(self, rhs: Interest) -> Self { if self.0 == rhs.0 { self } else { Interest::sometimes() } } }