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tracing: add Span::none() constructor (#147)
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* tracing: add `Span::none()` constructor

This branch adds a `Span::none()` constructor, which returns a totally
disabled span, without requiring a `Metadata` reference. Unlike
`new_disabled`, this span will *not* log, even if the log-support
feature flag is enabled. It is not constructed by the macors, but is
intended for use by users to represent a situation where a span may or
may not be present without using `Option<Span>`.

A `none` span will silently do nothing when any of its methods are
called.

If an API for getting the current span is added to `Subscriber`, a
`None` span will be returned when there is no currently-entered span, or
when the subscriber does not track a current span.

Closes #140

Signed-off-by: Eliza Weisman <[email protected]>
Co-Authored-By: Jon Gjengset <[email protected]>
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@hawkw @jonhoo
hawkw and jonhoo authored Jul 5, 2019
1 parent 3dc80c9 commit 0e02d6d
Showing 1 changed file with 119 additions and 57 deletions.
176 changes: 119 additions & 57 deletions tracing/src/span.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -343,7 +343,11 @@ pub struct Span {
///
/// If this is `None`, then the span has either closed or was never enabled.
inner: Option<Inner>,
meta: &'static Metadata<'static>,
/// Metadata describing the span.
///
/// This might be `Some` even if `inner` is `None`, in the case that the
/// span is disabled but the metadata is needed for `log` support.
meta: Option<&'static Metadata<'static>>,
}

/// A handle representing the capacity to enter a span which is known to exist.
Expand Down Expand Up @@ -434,10 +438,33 @@ impl Span {
Self::make(meta, new_span)
}

/// Constructs a new disabled span.
/// 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.
#[inline(always)]
pub fn new_disabled(meta: &'static Metadata<'static>) -> Span {
Span { inner: None, meta }
Self {
inner: None,
meta: Some(meta),
}
}

/// 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 const fn none() -> Span {
Self {
inner: None,
meta: None,
}
}

fn make(meta: &'static Metadata<'static>, new_span: Attributes) -> Span {
Expand All @@ -446,8 +473,15 @@ impl Span {
let id = dispatch.new_span(attrs);
Some(Inner::new(id, dispatch))
});
let span = Self { inner, meta };
span.log(format_args!("{}; {}", meta.name(), FmtAttrs(&new_span)));

let span = Self {
inner,
meta: Some(meta),
};

#[cfg(feature = "log")]
span.log(format_args!("{}; {}", meta.name(), FmtAttrs(attrs)));

span
}

Expand Down Expand Up @@ -527,7 +561,14 @@ impl Span {
if let Some(ref inner) = self.inner.as_ref() {
inner.subscriber.enter(&inner.id);
}
self.log(format_args!("-> {}", self.meta.name()));

#[cfg(feature = "log")]
{
if let Some(ref meta) = self.meta {
self.log(format_args!("-> {}", meta.name()));
}
}

Entered { span: self }
}

Expand Down Expand Up @@ -603,13 +644,14 @@ impl Span {
Q: field::AsField,
V: field::Value,
{
if let Some(field) = field.as_field(self.meta) {
self.record_all(
&self
.meta
.fields()
.value_set(&[(&field, Some(value as &dyn field::Value))]),
);
if let Some(ref meta) = self.meta {
if let Some(field) = field.as_field(meta) {
self.record_all(
&meta
.fields()
.value_set(&[(&field, Some(value as &dyn field::Value))]),
);
}
}

self
Expand All @@ -621,7 +663,14 @@ impl Span {
if let Some(ref inner) = self.inner {
inner.record(&record);
}
self.log(format_args!("{}; {}", self.meta.name(), FmtValues(&record)));

#[cfg(feature = "log")]
{
if let Some(ref meta) = self.meta {
self.log(format_args!("{}; {}", meta.name(), FmtValues(&record)));
}
}

self
}

Expand Down Expand Up @@ -662,42 +711,41 @@ impl Span {

/// Returns this span's `Metadata`, if it is enabled.
pub fn metadata(&self) -> Option<&'static Metadata<'static>> {
if self.inner.is_some() {
Some(self.meta)
} else {
None
}
self.meta.clone()
}

#[cfg(feature = "log")]
#[inline]
fn log(&self, message: fmt::Arguments) {
let logger = log::logger();
let log_meta = log::Metadata::builder()
.level(level_to_log!(self.meta.level()))
.target(self.meta.target())
.build();
if logger.enabled(&log_meta) {
logger.log(
&log::Record::builder()
.metadata(log_meta)
.module_path(self.meta.module_path())
.file(self.meta.file())
.line(self.meta.line())
.args(message)
.build(),
);
if let Some(ref meta) = self.meta {
let logger = log::logger();
let log_meta = log::Metadata::builder()
.level(level_to_log!(meta.level()))
.target(meta.target())
.build();
if logger.enabled(&log_meta) {
logger.log(
&log::Record::builder()
.metadata(log_meta)
.module_path(meta.module_path())
.file(meta.file())
.line(meta.line())
.args(message)
.build(),
);
}
}
}

#[cfg(not(feature = "log"))]
#[inline]
fn log(&self, _: fmt::Arguments) {}
}

impl cmp::PartialEq for Span {
fn eq(&self, other: &Self) -> bool {
self.meta.callsite() == other.meta.callsite() && self.inner == other.inner
match (&self.meta, &other.meta) {
(Some(this), Some(that)) => {
this.callsite() == that.callsite() && self.inner == other.inner
}
_ => false,
}
}
}

Expand All @@ -710,26 +758,30 @@ impl Hash for Span {
impl fmt::Debug for Span {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let mut span = f.debug_struct("Span");
span.field("name", &self.meta.name())
.field("level", &self.meta.level())
.field("target", &self.meta.target());

if let Some(ref inner) = self.inner {
span.field("id", &inner.id());
} else {
span.field("disabled", &true);
}
if let Some(ref meta) = self.meta {
span.field("name", &meta.name())
.field("level", &meta.level())
.field("target", &meta.target());

if let Some(ref inner) = self.inner {
span.field("id", &inner.id());
} else {
span.field("disabled", &true);
}

if let Some(ref path) = self.meta.module_path() {
span.field("module_path", &path);
}
if let Some(ref path) = meta.module_path() {
span.field("module_path", &path);
}

if let Some(ref line) = self.meta.line() {
span.field("line", &line);
}
if let Some(ref line) = meta.line() {
span.field("line", &line);
}

if let Some(ref file) = self.meta.file() {
span.field("file", &file);
if let Some(ref file) = meta.file() {
span.field("file", &file);
}
} else {
span.field("none", &true);
}

span.finish()
Expand Down Expand Up @@ -832,12 +884,20 @@ impl<'a> Drop for Entered<'a> {
if let Some(inner) = self.span.inner.as_ref() {
inner.subscriber.exit(&inner.id);
}
self.span.log(format_args!("<- {}", self.span.meta.name()));

#[cfg(feature = "log")]
{
if let Some(ref meta) = self.span.meta {
self.span.log(format_args!("<- {}", meta.name()));
}
}
}
}

#[cfg(feature = "log")]
struct FmtValues<'a>(&'a Record<'a>);

#[cfg(feature = "log")]
impl<'a> fmt::Display for FmtValues<'a> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let mut res = Ok(());
Expand All @@ -848,8 +908,10 @@ impl<'a> fmt::Display for FmtValues<'a> {
}
}

#[cfg(feature = "log")]
struct FmtAttrs<'a>(&'a Attributes<'a>);

#[cfg(feature = "log")]
impl<'a> fmt::Display for FmtAttrs<'a> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let mut res = Ok(());
Expand Down

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