extensions.rs

// taken from https://github.com/hyperium/http/blob/master/src/extensions.rs.

use crate::sync::{RwLockReadGuard, RwLockWriteGuard};
use std::{
    any::{Any, TypeId},
    collections::HashMap,
    fmt,
    hash::{BuildHasherDefault, Hasher},
};

#[allow(warnings)]
type AnyMap = HashMap<TypeId, Box<dyn Any + Send + Sync>, BuildHasherDefault<IdHasher>>;

/// 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(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
    }
}

/// An immutable, read-only reference to a Span's extensions.
#[derive(Debug)]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub struct Extensions<'a> {
    inner: RwLockReadGuard<'a, ExtensionsInner>,
}

impl<'a> Extensions<'a> {
    #[cfg(feature = "registry")]
    pub(crate) fn new(inner: RwLockReadGuard<'a, ExtensionsInner>) -> Self {
        Self { inner }
    }

    /// Immutably borrows a type previously inserted into this `Extensions`.
    pub fn get<T: 'static>(&self) -> Option<&T> {
        self.inner.get::<T>()
    }
}

/// An mutable reference to a Span's extensions.
#[derive(Debug)]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub struct ExtensionsMut<'a> {
    inner: RwLockWriteGuard<'a, ExtensionsInner>,
}

impl<'a> ExtensionsMut<'a> {
    #[cfg(feature = "registry")]
    pub(crate) fn new(inner: RwLockWriteGuard<'a, ExtensionsInner>) -> Self {
        Self { inner }
    }

    /// Insert a type into this `Extensions`.
    ///
    /// Note that extensions are _not_
    /// [subscriber]-specific—they are _span_-specific. This means that
    /// other subscribers can access and mutate extensions that
    /// a different Subscriber recorded. For example, an application might
    /// have a subscriber that records execution timings, alongside a subscriber
    /// that reports spans and events to a distributed
    /// tracing system that requires timestamps for spans.
    /// Ideally, if one subscriber records a timestamp _x_, the other subscriber
    /// should be able to reuse timestamp _x_.
    ///
    /// Therefore, extensions should generally be newtypes, rather than common
    /// types like [`String`](std::string::String), to avoid accidental
    /// cross-`Subscriber` clobbering.
    ///
    /// ## Panics
    ///
    /// If `T` is already present in `Extensions`, then this method will panic.
    ///
    /// [subscriber]: crate::subscribe::Subscribe
    pub fn insert<T: Send + Sync + 'static>(&mut self, val: T) {
        assert!(self.replace(val).is_none())
    }

    /// Replaces an existing `T` into this extensions.
    ///
    /// If `T` is not present, `Option::None` will be returned.
    pub fn replace<T: Send + Sync + 'static>(&mut self, val: T) -> Option<T> {
        self.inner.insert(val)
    }

    /// Get a mutable reference to a type previously inserted on this `ExtensionsMut`.
    pub fn get_mut<T: 'static>(&mut self) -> Option<&mut T> {
        self.inner.get_mut::<T>()
    }

    /// Remove a type from this `Extensions`.
    ///
    /// If a extension of this type existed, it will be returned.
    pub fn remove<T: Send + Sync + 'static>(&mut self) -> Option<T> {
        self.inner.remove::<T>()
    }
}

/// A type map of span extensions.
///
/// [ExtensionsInner] is used by [Data] to store and
/// span-specific data. A given [Subscriber] can read and write
/// data that it is interested in recording and emitting.
#[derive(Default)]
pub(crate) struct ExtensionsInner {
    map: AnyMap,
}

impl ExtensionsInner {
    /// Create an empty `Extensions`.
    #[cfg(any(test, feature = "registry"))]
    #[inline]
    pub(crate) fn new() -> ExtensionsInner {
        ExtensionsInner {
            map: AnyMap::default(),
        }
    }

    /// Insert a type into this `Extensions`.
    ///
    /// If a extension of this type already existed, it will
    /// be returned.
    pub(crate) fn insert<T: Send + Sync + 'static>(&mut self, val: T) -> Option<T> {
        self.map
            .insert(TypeId::of::<T>(), Box::new(val))
            .and_then(|boxed| {
                #[allow(warnings)]
                {
                    (boxed as Box<dyn Any + 'static>)
                        .downcast()
                        .ok()
                        .map(|boxed| *boxed)
                }
            })
    }

    /// Get a reference to a type previously inserted on this `Extensions`.
    pub(crate) fn get<T: 'static>(&self) -> Option<&T> {
        self.map
            .get(&TypeId::of::<T>())
            .and_then(|boxed| (&**boxed as &(dyn Any + 'static)).downcast_ref())
    }

    /// Get a mutable reference to a type previously inserted on this `Extensions`.
    pub(crate) fn get_mut<T: 'static>(&mut self) -> Option<&mut T> {
        self.map
            .get_mut(&TypeId::of::<T>())
            .and_then(|boxed| (&mut **boxed as &mut (dyn Any + 'static)).downcast_mut())
    }

    /// Remove a type from this `Extensions`.
    ///
    /// If a extension of this type existed, it will be returned.
    pub(crate) fn remove<T: Send + Sync + 'static>(&mut self) -> Option<T> {
        self.map.remove(&TypeId::of::<T>()).and_then(|boxed| {
            #[allow(warnings)]
            {
                (boxed as Box<dyn Any + 'static>)
                    .downcast()
                    .ok()
                    .map(|boxed| *boxed)
            }
        })
    }

    /// Clear the `ExtensionsInner` in-place, dropping any elements in the map but
    /// retaining allocated capacity.
    ///
    /// This permits the hash map allocation to be pooled by the registry so
    /// that future spans will not need to allocate new hashmaps.
    #[cfg(any(test, feature = "registry"))]
    pub(crate) fn clear(&mut self) {
        self.map.clear();
    }
}

impl fmt::Debug for ExtensionsInner {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Extensions")
            .field("len", &self.map.len())
            .field("capacity", &self.map.capacity())
            .finish()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[derive(Debug, PartialEq)]
    struct MyType(i32);

    #[test]
    fn test_extensions() {
        let mut extensions = ExtensionsInner::new();

        extensions.insert(5i32);
        extensions.insert(MyType(10));

        assert_eq!(extensions.get(), Some(&5i32));
        assert_eq!(extensions.get_mut(), Some(&mut 5i32));

        assert_eq!(extensions.remove::<i32>(), Some(5i32));
        assert!(extensions.get::<i32>().is_none());

        assert_eq!(extensions.get::<bool>(), None);
        assert_eq!(extensions.get(), Some(&MyType(10)));
    }

    #[test]
    fn clear_retains_capacity() {
        let mut extensions = ExtensionsInner::new();
        extensions.insert(5i32);
        extensions.insert(MyType(10));
        extensions.insert(true);

        assert_eq!(extensions.map.len(), 3);
        let prev_capacity = extensions.map.capacity();
        extensions.clear();

        assert_eq!(
            extensions.map.len(),
            0,
            "after clear(), extensions map should have length 0"
        );
        assert_eq!(
            extensions.map.capacity(),
            prev_capacity,
            "after clear(), extensions map should retain prior capacity"
        );
    }

    #[test]
    fn clear_drops_elements() {
        use std::sync::Arc;
        struct DropMePlease(Arc<()>);
        struct DropMeTooPlease(Arc<()>);

        let mut extensions = ExtensionsInner::new();
        let val1 = DropMePlease(Arc::new(()));
        let val2 = DropMeTooPlease(Arc::new(()));

        let val1_dropped = Arc::downgrade(&val1.0);
        let val2_dropped = Arc::downgrade(&val2.0);
        extensions.insert(val1);
        extensions.insert(val2);

        assert!(val1_dropped.upgrade().is_some());
        assert!(val2_dropped.upgrade().is_some());

        extensions.clear();
        assert!(
            val1_dropped.upgrade().is_none(),
            "after clear(), val1 should be dropped"
        );
        assert!(
            val2_dropped.upgrade().is_none(),
            "after clear(), val2 should be dropped"
        );
    }
}