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use crate::time::driver::Entry;
use crate::time::error::Error;
use std::ptr;
use std::sync::atomic::AtomicPtr;
use std::sync::atomic::Ordering::SeqCst;
use std::sync::Arc;
/// A stack of `Entry` nodes
#[derive(Debug)]
pub(crate) struct AtomicStack {
/// Stack head
head: AtomicPtr<Entry>,
}
/// Entries that were removed from the stack
#[derive(Debug)]
pub(crate) struct AtomicStackEntries {
ptr: *mut Entry,
}
/// Used to indicate that the timer has shutdown.
const SHUTDOWN: *mut Entry = 1 as *mut _;
impl AtomicStack {
pub(crate) fn new() -> AtomicStack {
AtomicStack {
head: AtomicPtr::new(ptr::null_mut()),
}
}
/// Pushes an entry onto the stack.
///
/// Returns `true` if the entry was pushed, `false` if the entry is already
/// on the stack, `Err` if the timer is shutdown.
pub(crate) fn push(&self, entry: &Arc<Entry>) -> Result<bool, Error> {
// First, set the queued bit on the entry
let queued = entry.queued.fetch_or(true, SeqCst);
if queued {
// Already queued, nothing more to do
return Ok(false);
}
let ptr = Arc::into_raw(entry.clone()) as *mut _;
let mut curr = self.head.load(SeqCst);
loop {
if curr == SHUTDOWN {
// Don't leak the entry node
let _ = unsafe { Arc::from_raw(ptr) };
return Err(Error::shutdown());
}
// Update the `next` pointer. This is safe because setting the queued
// bit is a "lock" on this field.
unsafe {
*(entry.next_atomic.get()) = curr;
}
let actual = self.head.compare_and_swap(curr, ptr, SeqCst);
if actual == curr {
break;
}
curr = actual;
}
Ok(true)
}
/// Takes all entries from the stack
pub(crate) fn take(&self) -> AtomicStackEntries {
let ptr = self.head.swap(ptr::null_mut(), SeqCst);
AtomicStackEntries { ptr }
}
/// Drains all remaining nodes in the stack and prevent any new nodes from
/// being pushed onto the stack.
pub(crate) fn shutdown(&self) {
// Shutdown the processing queue
let ptr = self.head.swap(SHUTDOWN, SeqCst);
// Let the drop fn of `AtomicStackEntries` handle draining the stack
drop(AtomicStackEntries { ptr });
}
}
// ===== impl AtomicStackEntries =====
impl Iterator for AtomicStackEntries {
type Item = Arc<Entry>;
fn next(&mut self) -> Option<Self::Item> {
if self.ptr.is_null() || self.ptr == SHUTDOWN {
return None;
}
// Convert the pointer to an `Arc<Entry>`
let entry = unsafe { Arc::from_raw(self.ptr) };
// Update `self.ptr` to point to the next element of the stack
self.ptr = unsafe { *entry.next_atomic.get() };
// Unset the queued flag
let res = entry.queued.fetch_and(false, SeqCst);
debug_assert!(res);
// Return the entry
Some(entry)
}
}
impl Drop for AtomicStackEntries {
fn drop(&mut self) {
for entry in self {
// Flag the entry as errored
entry.error(Error::shutdown());
}
}
}
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