Struct tracing::Span [−][src]
A handle representing a span, with the capability to enter the span if it exists.
If the span was rejected by the current Subscriber
's filter, entering the
span will silently do nothing. Thus, the handle can be used in the same
manner regardless of whether or not the trace is currently being collected.
Implementations
impl Span
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pub fn new(meta: &'static Metadata<'static>, values: &ValueSet<'_>) -> Span
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Constructs a new Span
with the given metadata and set of
field values.
The new span will be constructed by the currently-active Subscriber
,
with the current span as its parent (if one exists).
After the span is constructed, field values and/or follows_from
annotations may be added to it.
pub fn new_root(meta: &'static Metadata<'static>, values: &ValueSet<'_>) -> Span
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Constructs a new Span
as the root of its own trace tree, with the
given metadata and set of field values.
After the span is constructed, field values and/or follows_from
annotations may be added to it.
pub fn child_of(
parent: impl Into<Option<Id>>,
meta: &'static Metadata<'static>,
values: &ValueSet<'_>
) -> Span
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parent: impl Into<Option<Id>>,
meta: &'static Metadata<'static>,
values: &ValueSet<'_>
) -> Span
Constructs a new Span
as child of the given parent span, with the
given metadata and set of field values.
After the span is constructed, field values and/or follows_from
annotations may be added to it.
pub fn new_disabled(meta: &'static Metadata<'static>) -> Span
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Constructs a new disabled span with the given Metadata
.
This should be used when a span is constructed from a known callsite, but the subscriber indicates that it is disabled.
Entering, exiting, and recording values on this span will not notify the
Subscriber
but may record log messages if the log
feature flag is
enabled.
pub const fn none() -> Span
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Constructs a new span that is completely disabled.
This can be used rather than Option<Span>
to represent cases where a
span is not present.
Entering, exiting, and recording values on this span will do nothing.
pub fn current() -> Span
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Returns a handle to the span considered by the Subscriber
to be the
current span.
If the subscriber indicates that it does not track the current span, or that the thread from which this function is called is not currently inside a span, the returned span will be disabled.
pub fn enter(&self) -> Entered<'_>
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Enters this span, returning a guard that will exit the span when dropped.
If this span is enabled by the current subscriber, then this function will
call Subscriber::enter
with the span's Id
, and dropping the guard
will call Subscriber::exit
. If the span is disabled, this does
nothing.
In Asynchronous Code
Warning: in asynchronous code that uses async/await syntax,
Span::enter
should be used very carefully or avoided entirely. Holding
the drop guard returned by Span::enter
across .await
points will
result in incorrect traces.
For example,
async fn my_async_function() { let span = info_span!("my_async_function"); // THIS WILL RESULT IN INCORRECT TRACES let _enter = span.enter(); some_other_async_function().await; // ... }
The drop guard returned by Span::enter
exits the span when it is
dropped. When an async function or async block yields at an .await
point, the current scope is exited, but values in that scope are
not dropped (because the async block will eventually resume
execution from that await point). This means that another task will
begin executing while remaining in the entered span. This results in
an incorrect trace.
Instead of using Span::enter
in asynchronous code, prefer the
following:
-
To enter a span for a synchronous section of code within an async block or function, prefer
Span::in_scope
. Sincein_scope
takes a synchronous closure and exits the span when the closure returns, the span will always be exited before the next await point. For example:async fn my_async_function() { let span = info_span!("my_async_function"); let some_value = span.in_scope(|| { // run some synchronous code inside the span... }); // This is okay! The span has already been exited before we reach // the await point. some_other_async_function(some_value).await; // ... }
-
For instrumenting asynchronous code,
tracing
provides theFuture::instrument
combinator for attaching a span to a future (async function or block). This will enter the span every time the future is polled, and exit it whenever the future yields.Instrument
can be used with an async block inside an async function:ⓘuse tracing::Instrument; async fn my_async_function() { let span = info_span!("my_async_function"); async move { // This is correct! If we yield here, the span will be exited, // and re-entered when we resume. some_other_async_function().await; //more asynchronous code inside the span... } // instrument the async block with the span... .instrument(span) // ...and await it. .await }
It can also be used to instrument calls to async functions at the callsite:
ⓘuse tracing::Instrument; async fn my_async_function() { let some_value = some_other_async_function() .instrument(debug_span!("some_other_async_function")) .await; // ... }
-
The
#[instrument]
attribute macro can automatically generate correct code when used on an async function:ⓘ#[tracing::instrument(level = "info")] async fn my_async_function() { // This is correct! If we yield here, the span will be exited, // and re-entered when we resume. some_other_async_function().await; // ... }
Examples
#[macro_use] extern crate tracing; let span = span!(Level::INFO, "my_span"); let guard = span.enter(); // code here is within the span drop(guard); // code here is no longer within the span
Guards need not be explicitly dropped:
#[macro_use] extern crate tracing; fn my_function() -> String { // enter a span for the duration of this function. let span = trace_span!("my_function"); let _enter = span.enter(); // anything happening in functions we call is still inside the span... my_other_function(); // returning from the function drops the guard, exiting the span. return "Hello world".to_owned(); } fn my_other_function() { // ... }
Sub-scopes may be created to limit the duration for which the span is entered:
#[macro_use] extern crate tracing; let span = info_span!("my_great_span"); { let _enter = span.enter(); // this event occurs inside the span. info!("i'm in the span!"); // exiting the scope drops the guard, exiting the span. } // this event is not inside the span. info!("i'm outside the span!")
pub fn in_scope<F: FnOnce() -> T, T>(&self, f: F) -> T
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Executes the given function in the context of this span.
If this span is enabled, then this function enters the span, invokes f
and then exits the span. If the span is disabled, f
will still be
invoked, but in the context of the currently-executing span (if there is
one).
Returns the result of evaluating f
.
Examples
let my_span = span!(Level::TRACE, "my_span"); my_span.in_scope(|| { // this event occurs within the span. trace!("i'm in the span!"); }); // this event occurs outside the span. trace!("i'm not in the span!");
Calling a function and returning the result:
fn hello_world() -> String { "Hello world!".to_owned() } let span = info_span!("hello_world"); // the span will be entered for the duration of the call to // `hello_world`. let a_string = span.in_scope(hello_world);
pub fn field<Q: ?Sized>(&self, field: &Q) -> Option<Field> where
Q: AsField,
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Q: AsField,
Returns a Field
for the field with the
given name
, if one exists,
pub fn has_field<Q: ?Sized>(&self, field: &Q) -> bool where
Q: AsField,
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Q: AsField,
Returns true if this Span
has a field for the given
Field
or field name.
pub fn record<Q: ?Sized, V>(&self, field: &Q, value: &V) -> &Self where
Q: AsField,
V: Value,
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Q: AsField,
V: Value,
Records that the field described by field
has the value value
.
This may be used with field::Empty
to declare fields whose values
are not known when the span is created, and record them later:
use tracing::{trace_span, field}; // Create a span with two fields: `greeting`, with the value "hello world", and // `parting`, without a value. let span = trace_span!("my_span", greeting = "hello world", parting = field::Empty); // ... // Now, record a value for parting as well. // (note that the field name is passed as a string slice) span.record("parting", &"goodbye world!");
However, it may also be used to record a new value for a field whose value was already recorded:
use tracing::info_span; // Initially, let's assume that our attempt to do something is going okay... let span = info_span!("doing_something", is_okay = true); let _e = span.enter(); match do_something() { Ok(something) => { // ... } Err(_) => { // Things are no longer okay! span.record("is_okay", &false); } }
Note: The fields associated with a span are part of itsMetadata
. TheMetadata
. describing a particular span is constructed statically when the span is created and cannot be extended later to add new fields. Therefore, you cannot record a value for a field that was not specified when the span was created:
use tracing::{trace_span, field}; // Create a span with two fields: `greeting`, with the value "hello world", and // `parting`, without a value. let span = trace_span!("my_span", greeting = "hello world", parting = field::Empty); // ... // Now, you try to record a value for a new field, `new_field`, which was not // declared as `Empty` or populated when you created `span`. // You won't get any error, but the assignment will have no effect! span.record("new_field", &"interesting_value_you_really_need"); // Instead, all fields that may be recorded after span creation should be declared up front, // using field::Empty when a value is not known, as we did for `parting`. // This `record` call will indeed replace field::Empty with "you will be remembered". span.record("parting", &"you will be remembered");
pub fn record_all(&self, values: &ValueSet<'_>) -> &Self
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Records all the fields in the provided ValueSet
.
pub fn is_disabled(&self) -> bool
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Returns true
if this span was disabled by the subscriber and does not
exist.
See also is_none
.
pub fn is_none(&self) -> bool
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Returns true
if this span was constructed by Span::none
and is
empty.
If is_none
returns true
for a given span, then is_disabled
will
also return true
. However, when a span is disabled by the subscriber
rather than constructed by Span::none
, this method will return
false
, while is_disabled
will return true
.
pub fn follows_from(&self, from: impl Into<Option<Id>>) -> &Self
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Indicates that the span with the given ID has an indirect causal relationship with this span.
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 this span is disabled, or the resulting follows-from relationship would be invalid, this function will do nothing.
Examples
Setting a follows_from
relationship with a Span
:
let span1 = span!(Level::INFO, "span_1"); let span2 = span!(Level::DEBUG, "span_2"); span2.follows_from(span1);
Setting a follows_from
relationship with the current span:
let span = span!(Level::INFO, "hello!"); span.follows_from(Span::current());
Setting a follows_from
relationship with a Span
reference:
let span = span!(Level::INFO, "hello!"); let curr = Span::current(); span.follows_from(&curr);
Setting a follows_from
relationship with an Id
:
let span = span!(Level::INFO, "hello!"); let id = span.id(); span.follows_from(id);
pub fn id(&self) -> Option<Id>
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Returns this span's Id
, if it is enabled.
pub fn metadata(&self) -> Option<&'static Metadata<'static>>
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Returns this span's Metadata
, if it is enabled.
pub fn with_subscriber<T>(
&self,
f: impl FnOnce((&Id, &Dispatch)) -> T
) -> Option<T>
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&self,
f: impl FnOnce((&Id, &Dispatch)) -> T
) -> Option<T>
Invokes a function with a reference to this span's ID and subscriber.
if this span is enabled, the provided function is called, and the result is returned.
If the span is disabled, the function is not called, and this method returns None
instead.
Trait Implementations
impl Clone for Span
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impl Debug for Span
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impl Drop for Span
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impl Hash for Span
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fn hash<H: Hasher>(&self, hasher: &mut H)
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pub fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,
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H: Hasher,
impl<'a> Into<Option<&'a Id>> for &'a Span
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impl<'a> Into<Option<Id>> for &'a Span
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impl Into<Option<Id>> for Span
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impl PartialEq<Span> for Span
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Auto Trait Implementations
impl !RefUnwindSafe for Span
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impl Send for Span
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impl Sync for Span
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impl Unpin for Span
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impl !UnwindSafe for Span
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Blanket Implementations
impl<T> Any for T where
T: 'static + ?Sized,
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T: 'static + ?Sized,
impl<T> Borrow<T> for T where
T: ?Sized,
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T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
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T: ?Sized,
pub fn borrow_mut(&mut self) -> &mut T
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impl<T> From<T> for T
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impl<T> Instrument for T
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fn instrument(self, span: Span) -> Instrumented<Self>ⓘNotable traits for Instrumented<T>
impl<T: Future> Future for Instrumented<T> type Output = T::Output;
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Notable traits for Instrumented<T>
impl<T: Future> Future for Instrumented<T> type Output = T::Output;
fn in_current_span(self) -> Instrumented<Self>ⓘNotable traits for Instrumented<T>
impl<T: Future> Future for Instrumented<T> type Output = T::Output;
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Notable traits for Instrumented<T>
impl<T: Future> Future for Instrumented<T> type Output = T::Output;
impl<T, U> Into<U> for T where
U: From<T>,
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U: From<T>,
impl<T> ToOwned for T where
T: Clone,
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T: Clone,
type Owned = T
The resulting type after obtaining ownership.
pub fn to_owned(&self) -> T
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pub fn clone_into(&self, target: &mut T)
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impl<T, U> TryFrom<U> for T where
U: Into<T>,
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U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
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impl<T, U> TryInto<U> for T where
U: TryFrom<T>,
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U: TryFrom<T>,