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#[cfg(feature = "trace")]
use crate::trace::context::SynchronizedSpan;
use std::any::{Any, TypeId};
use std::cell::RefCell;
use std::collections::HashMap;
use std::fmt;
use std::hash::{BuildHasherDefault, Hasher};
use std::marker::PhantomData;
use std::sync::Arc;
thread_local! {
static CURRENT_CONTEXT: RefCell<Context> = RefCell::new(Context::default());
}
/// An execution-scoped collection of values.
///
/// A [`Context`] is a propagation mechanism which carries execution-scoped
/// values across API boundaries and between logically associated execution
/// units. Cross-cutting concerns access their data in-process using the same
/// shared context object.
///
/// [`Context`]s are immutable, and their write operations result in the creation
/// of a new context containing the original values and the new specified values.
///
/// ## Context state
///
/// Concerns can create and retrieve their local state in the current execution
/// state represented by a context through the [`get`] and [`with_value`]
/// methods. It is recommended to use application-specific types when storing new
/// context values to avoid unintentionally overwriting existing state.
///
/// ## Managing the current context
///
/// Contexts can be associated with the caller's current execution unit on a
/// given thread via the [`attach`] method, and previous contexts can be restored
/// by dropping the returned [`ContextGuard`]. Context can be nested, and will
/// restore their parent outer context when detached on drop. To access the
/// values of the context, a snapshot can be created via the [`Context::current`]
/// method.
///
/// [`Context::current`]: Context::current()
/// [`get`]: Context::get()
/// [`with_value`]: Context::with_value()
/// [`attach`]: Context::attach()
///
/// # Examples
///
/// ```
/// use opentelemetry::Context;
///
/// // Application-specific `a` and `b` values
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
/// #[derive(Debug, PartialEq)]
/// struct ValueB(u64);
///
/// let _outer_guard = Context::new().with_value(ValueA("a")).attach();
///
/// // Only value a has been set
/// let current = Context::current();
/// assert_eq!(current.get::<ValueA>(), Some(&ValueA("a")));
/// assert_eq!(current.get::<ValueB>(), None);
///
/// {
/// let _inner_guard = Context::current_with_value(ValueB(42)).attach();
/// // Both values are set in inner context
/// let current = Context::current();
/// assert_eq!(current.get::<ValueA>(), Some(&ValueA("a")));
/// assert_eq!(current.get::<ValueB>(), Some(&ValueB(42)));
/// }
///
/// // Resets to only the `a` value when inner guard is dropped
/// let current = Context::current();
/// assert_eq!(current.get::<ValueA>(), Some(&ValueA("a")));
/// assert_eq!(current.get::<ValueB>(), None);
/// ```
#[derive(Clone, Default)]
pub struct Context {
#[cfg(feature = "trace")]
pub(super) span: Option<Arc<SynchronizedSpan>>,
entries: HashMap<TypeId, Arc<dyn Any + Sync + Send>, BuildHasherDefault<IdHasher>>,
}
impl Context {
/// Creates an empty `Context`.
///
/// The context is initially created with a capacity of 0, so it will not
/// allocate. Use [`with_value`] to create a new context that has entries.
///
/// [`with_value`]: Context::with_value()
pub fn new() -> Self {
Context::default()
}
/// Returns an immutable snapshot of the current thread's context.
///
/// # Examples
///
/// ```
/// use opentelemetry::Context;
///
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
///
/// fn do_work() {
/// assert_eq!(Context::current().get(), Some(&ValueA("a")));
/// }
///
/// let _guard = Context::new().with_value(ValueA("a")).attach();
/// do_work()
/// ```
pub fn current() -> Self {
Context::map_current(|cx| cx.clone())
}
/// Applies a function to the current context returning its value.
///
/// This can be used to build higher performing algebraic expressions for
/// optionally creating a new context without the overhead of cloning the
/// current one and dropping it.
///
/// Note: This function will panic if you attempt to attach another context
/// while the current one is still borrowed.
pub fn map_current<T>(f: impl FnOnce(&Context) -> T) -> T {
CURRENT_CONTEXT.with(|cx| f(&cx.borrow()))
}
/// Returns a clone of the current thread's context with the given value.
///
/// This is a more efficient form of `Context::current().with_value(value)`
/// as it avoids the intermediate context clone.
///
/// # Examples
///
/// ```
/// use opentelemetry::Context;
///
/// // Given some value types defined in your application
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
/// #[derive(Debug, PartialEq)]
/// struct ValueB(u64);
///
/// // You can create and attach context with the first value set to "a"
/// let _guard = Context::new().with_value(ValueA("a")).attach();
///
/// // And create another context based on the fist with a new value
/// let all_current_and_b = Context::current_with_value(ValueB(42));
///
/// // The second context now contains all the current values and the addition
/// assert_eq!(all_current_and_b.get::<ValueA>(), Some(&ValueA("a")));
/// assert_eq!(all_current_and_b.get::<ValueB>(), Some(&ValueB(42)));
/// ```
pub fn current_with_value<T: 'static + Send + Sync>(value: T) -> Self {
let mut new_context = Context::current();
new_context
.entries
.insert(TypeId::of::<T>(), Arc::new(value));
new_context
}
/// Returns a reference to the entry for the corresponding value type.
///
/// # Examples
///
/// ```
/// use opentelemetry::Context;
///
/// // Given some value types defined in your application
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
/// #[derive(Debug, PartialEq)]
/// struct MyUser();
///
/// let cx = Context::new().with_value(ValueA("a"));
///
/// // Values can be queried by type
/// assert_eq!(cx.get::<ValueA>(), Some(&ValueA("a")));
///
/// // And return none if not yet set
/// assert_eq!(cx.get::<MyUser>(), None);
/// ```
pub fn get<T: 'static>(&self) -> Option<&T> {
self.entries
.get(&TypeId::of::<T>())
.and_then(|rc| rc.downcast_ref())
}
/// Returns a copy of the context with the new value included.
///
/// # Examples
///
/// ```
/// use opentelemetry::Context;
///
/// // Given some value types defined in your application
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
/// #[derive(Debug, PartialEq)]
/// struct ValueB(u64);
///
/// // You can create a context with the first value set to "a"
/// let cx_with_a = Context::new().with_value(ValueA("a"));
///
/// // And create another context based on the fist with a new value
/// let cx_with_a_and_b = cx_with_a.with_value(ValueB(42));
///
/// // The first context is still available and unmodified
/// assert_eq!(cx_with_a.get::<ValueA>(), Some(&ValueA("a")));
/// assert_eq!(cx_with_a.get::<ValueB>(), None);
///
/// // The second context now contains both values
/// assert_eq!(cx_with_a_and_b.get::<ValueA>(), Some(&ValueA("a")));
/// assert_eq!(cx_with_a_and_b.get::<ValueB>(), Some(&ValueB(42)));
/// ```
pub fn with_value<T: 'static + Send + Sync>(&self, value: T) -> Self {
let mut new_context = self.clone();
new_context
.entries
.insert(TypeId::of::<T>(), Arc::new(value));
new_context
}
/// Replaces the current context on this thread with this context.
///
/// Dropping the returned [`ContextGuard`] will reset the current context to the
/// previous value.
///
///
/// # Examples
///
/// ```
/// use opentelemetry::Context;
///
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
///
/// let my_cx = Context::new().with_value(ValueA("a"));
///
/// // Set the current thread context
/// let cx_guard = my_cx.attach();
/// assert_eq!(Context::current().get::<ValueA>(), Some(&ValueA("a")));
///
/// // Drop the guard to restore the previous context
/// drop(cx_guard);
/// assert_eq!(Context::current().get::<ValueA>(), None);
/// ```
///
/// Guards do not need to be explicitly dropped:
///
/// ```
/// use opentelemetry::Context;
///
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
///
/// fn my_function() -> String {
/// // attach a context the duration of this function.
/// let my_cx = Context::new().with_value(ValueA("a"));
/// // NOTE: a variable name after the underscore is **required** or rust
/// // will drop the guard, restoring the previous context _immediately_.
/// let _guard = my_cx.attach();
///
/// // anything happening in functions we call can still access my_cx...
/// 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:
///
/// ```
/// use opentelemetry::Context;
///
/// #[derive(Debug, PartialEq)]
/// struct ValueA(&'static str);
///
/// let my_cx = Context::new().with_value(ValueA("a"));
///
/// {
/// let _guard = my_cx.attach();
///
/// // the current context can access variables in
/// assert_eq!(Context::current().get::<ValueA>(), Some(&ValueA("a")));
///
/// // exiting the scope drops the guard, detaching the context.
/// }
///
/// // this is back in the default empty context
/// assert_eq!(Context::current().get::<ValueA>(), None);
/// ```
pub fn attach(self) -> ContextGuard {
let previous_cx = CURRENT_CONTEXT
.try_with(|current| current.replace(self))
.ok();
ContextGuard {
previous_cx,
_marker: PhantomData,
}
}
#[cfg(feature = "trace")]
pub(super) fn current_with_synchronized_span(value: SynchronizedSpan) -> Self {
Context {
span: Some(Arc::new(value)),
entries: Context::map_current(|cx| cx.entries.clone()),
}
}
#[cfg(feature = "trace")]
pub(super) fn with_synchronized_span(&self, value: SynchronizedSpan) -> Self {
Context {
span: Some(Arc::new(value)),
entries: self.entries.clone(),
}
}
}
impl fmt::Debug for Context {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Context")
.field("entries", &self.entries.len())
.finish()
}
}
/// A guard that resets the current context to the prior context when dropped.
#[allow(missing_debug_implementations)]
pub struct ContextGuard {
previous_cx: Option<Context>,
// ensure this type is !Send as it relies on thread locals
_marker: PhantomData<*const ()>,
}
impl Drop for ContextGuard {
fn drop(&mut self) {
if let Some(previous_cx) = self.previous_cx.take() {
let _ = CURRENT_CONTEXT.try_with(|current| current.replace(previous_cx));
}
}
}
/// With TypeIds as keys, there's no need to hash them. They are already hashes
/// themselves, coming from the compiler. The IdHasher holds the u64 of
/// the TypeId, and then returns it, instead of doing any bit fiddling.
#[derive(Clone, Default, Debug)]
struct IdHasher(u64);
impl Hasher for IdHasher {
fn write(&mut self, _: &[u8]) {
unreachable!("TypeId calls write_u64");
}
#[inline]
fn write_u64(&mut self, id: u64) {
self.0 = id;
}
#[inline]
fn finish(&self) -> u64 {
self.0
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn nested_contexts() {
#[derive(Debug, PartialEq)]
struct ValueA(&'static str);
#[derive(Debug, PartialEq)]
struct ValueB(u64);
let _outer_guard = Context::new().with_value(ValueA("a")).attach();
// Only value `a` is set
let current = Context::current();
assert_eq!(current.get(), Some(&ValueA("a")));
assert_eq!(current.get::<ValueB>(), None);
{
let _inner_guard = Context::current_with_value(ValueB(42)).attach();
// Both values are set in inner context
let current = Context::current();
assert_eq!(current.get(), Some(&ValueA("a")));
assert_eq!(current.get(), Some(&ValueB(42)));
assert!(Context::map_current(|cx| {
assert_eq!(cx.get(), Some(&ValueA("a")));
assert_eq!(cx.get(), Some(&ValueB(42)));
true
}));
}
// Resets to only value `a` when inner guard is dropped
let current = Context::current();
assert_eq!(current.get(), Some(&ValueA("a")));
assert_eq!(current.get::<ValueB>(), None);
assert!(Context::map_current(|cx| {
assert_eq!(cx.get(), Some(&ValueA("a")));
assert_eq!(cx.get::<ValueB>(), None);
true
}));
}
}