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//! Worker pool for zshrs — persistent threads for background work.
//!
//! Port rationale: zsh forks for everything (completion, process subs,
//! command substitution). Each fork copies the entire shell state.
//! We replace that with a fixed-size thread pool + channel dispatch,
//! giving us:
//! - No fork overhead (50-500μs per fork on macOS)
//! - No address space duplication
//! - Warm thread stacks ready to go
//! - Backpressure via bounded channel
//!
//! Pool size = available_parallelism() clamped to [2, 18].
//! Channel capacity = 4 × pool size (bounded backpressure).
//!
//! Audit fixes applied:
//! 1. crossbeam-channel replaces Arc<Mutex<mpsc::Receiver>> — no mutex contention
//! 2. Bounded channel (4×N) provides backpressure
//! 3. catch_unwind wraps every task — panics logged, worker stays alive
//! 4. tracing spans on submit + worker loop
//! 5. Queue depth metric on submit
//! 6. Task cancellation via AtomicBool flag
use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::sync::Arc;
use std::thread;
/// A unit of work the pool can execute.
type Task = Box<dyn FnOnce() + Send + 'static>;
/// Fixed-size thread pool with bounded FIFO task queue.
///
/// Uses crossbeam-channel for lock-free multi-consumer dispatch —
/// each worker calls `recv()` directly, no mutex.
pub struct WorkerPool {
workers: Vec<Worker>,
sender: Option<crossbeam_channel::Sender<Task>>,
size: usize,
/// Shared cancellation flag — when set, workers drop pending tasks
cancelled: Arc<AtomicBool>,
/// Queue depth — incremented on submit, decremented on task start
queued: Arc<AtomicUsize>,
/// Total tasks completed across all workers
completed: Arc<AtomicUsize>,
}
struct Worker {
#[allow(dead_code)]
id: usize,
handle: Option<thread::JoinHandle<()>>,
}
impl WorkerPool {
/// Create a pool with `size` worker threads and bounded channel.
/// Channel capacity = 4 × size (provides backpressure without starving).
pub fn new(size: usize) -> Self {
let capacity = size * 4;
let (sender, receiver) = crossbeam_channel::bounded::<Task>(capacity);
let cancelled = Arc::new(AtomicBool::new(false));
let queued = Arc::new(AtomicUsize::new(0));
let completed = Arc::new(AtomicUsize::new(0));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
let rx = receiver.clone();
let cancelled = Arc::clone(&cancelled);
let queued = Arc::clone(&queued);
let completed = Arc::clone(&completed);
let handle = thread::Builder::new()
.name(format!("zshrs-worker-{}", id))
.spawn(move || {
loop {
let task = match rx.recv() {
Ok(task) => task,
Err(_) => break, // channel closed → shutdown
};
queued.fetch_sub(1, Ordering::Relaxed);
// Check cancellation before running
if cancelled.load(Ordering::Relaxed) {
continue; // drain without executing
}
// catch_unwind keeps the worker alive if a task panics
if let Err(e) = std::panic::catch_unwind(std::panic::AssertUnwindSafe(task))
{
let msg = if let Some(s) = e.downcast_ref::<&str>() {
(*s).to_string()
} else if let Some(s) = e.downcast_ref::<String>() {
s.clone()
} else {
"unknown panic".to_string()
};
tracing::error!(
worker = id,
panic = %msg,
"worker task panicked"
);
}
completed.fetch_add(1, Ordering::Relaxed);
}
tracing::debug!(worker = id, "worker thread exiting");
})
.expect("failed to spawn worker thread");
workers.push(Worker {
id,
handle: Some(handle),
});
}
tracing::info!(
pool_size = size,
channel_capacity = capacity,
"worker pool started"
);
WorkerPool {
workers,
sender: Some(sender),
size,
cancelled,
queued,
completed,
}
}
/// Create a pool sized to the machine's parallelism, clamped [2, 18].
pub fn default_size() -> Self {
let cpus = thread::available_parallelism()
.map(|n| n.get())
.unwrap_or(4);
Self::new(cpus.clamp(2, 18))
}
/// Submit a task to the pool. Blocks if the queue is full (backpressure).
/// Panics if the pool has been shut down.
pub fn submit<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let depth = self.queued.fetch_add(1, Ordering::Relaxed) + 1;
if depth > self.size * 2 {
tracing::debug!(queue_depth = depth, "worker pool queue building up");
}
self.sender
.as_ref()
.expect("pool shut down")
.send(Box::new(f))
.expect("all workers dead");
}
/// Submit a task and get a receiver for its result.
pub fn submit_with_result<F, R>(&self, f: F) -> crossbeam_channel::Receiver<R>
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
let (tx, rx) = crossbeam_channel::bounded(1);
self.submit(move || {
let result = f();
let _ = tx.send(result);
});
rx
}
/// Signal all workers to drop pending tasks.
/// Already-running tasks will finish, but queued tasks are skipped.
/// Reset with `reset_cancel()`.
pub fn cancel(&self) {
self.cancelled.store(true, Ordering::Relaxed);
tracing::info!("worker pool: cancel requested");
}
/// Clear the cancellation flag — pool resumes normal execution.
pub fn reset_cancel(&self) {
self.cancelled.store(false, Ordering::Relaxed);
}
/// Number of worker threads.
pub fn size(&self) -> usize {
self.size
}
/// Approximate number of tasks waiting in the queue.
pub fn queue_depth(&self) -> usize {
self.queued.load(Ordering::Relaxed)
}
/// Total tasks completed since pool creation.
pub fn completed(&self) -> usize {
self.completed.load(Ordering::Relaxed)
}
}
impl Drop for WorkerPool {
fn drop(&mut self) {
// Signal workers to skip remaining queued tasks
self.cancelled.store(true, Ordering::Relaxed);
// Drop the sender → channel closes → recv() returns Err → threads exit
drop(self.sender.take());
// Give workers a brief window to finish their current task.
// Don't block indefinitely — the process is exiting.
for w in &mut self.workers {
if let Some(handle) = w.handle.take() {
// Detach the thread — OS cleans up on process exit.
// join() would block if a worker is mid-parse on a 500-line
// completion function. Not worth the wait on Ctrl-D/exit.
drop(handle);
}
}
tracing::info!(
tasks_completed = self.completed.load(Ordering::Relaxed),
"worker pool shut down"
);
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_pool_executes_tasks() {
let pool = WorkerPool::new(2);
let counter = Arc::new(AtomicUsize::new(0));
for _ in 0..100 {
let c = Arc::clone(&counter);
pool.submit(move || {
c.fetch_add(1, Ordering::Relaxed);
});
}
drop(pool); // waits for all tasks to finish
assert_eq!(counter.load(Ordering::Relaxed), 100);
}
#[test]
fn test_submit_with_result() {
let pool = WorkerPool::new(2);
let rx = pool.submit_with_result(|| 42);
assert_eq!(rx.recv().unwrap(), 42);
}
#[test]
fn test_default_size() {
let pool = WorkerPool::default_size();
assert!(pool.size() >= 2);
assert!(pool.size() <= 18);
}
#[test]
fn test_panic_does_not_kill_worker() {
let pool = WorkerPool::new(2);
let counter = Arc::new(AtomicUsize::new(0));
// Submit a task that panics
pool.submit(|| panic!("intentional test panic"));
// Submit tasks after the panic — they should still run
for _ in 0..10 {
let c = Arc::clone(&counter);
pool.submit(move || {
c.fetch_add(1, Ordering::Relaxed);
});
}
drop(pool);
assert_eq!(counter.load(Ordering::Relaxed), 10);
}
#[test]
fn test_cancel_skips_queued_tasks() {
let pool = WorkerPool::new(1); // single worker to control ordering
let barrier = Arc::new(std::sync::Barrier::new(2));
let counter = Arc::new(AtomicUsize::new(0));
// Block the worker on a barrier so tasks queue up
let b = Arc::clone(&barrier);
pool.submit(move || {
b.wait();
});
// Queue tasks that should be skipped
for _ in 0..5 {
let c = Arc::clone(&counter);
pool.submit(move || {
c.fetch_add(1, Ordering::Relaxed);
});
}
// Cancel, then unblock the worker
pool.cancel();
barrier.wait();
// Give workers time to drain
std::thread::sleep(std::time::Duration::from_millis(50));
// Queued tasks should have been skipped
assert_eq!(counter.load(Ordering::Relaxed), 0);
// Reset and verify pool still works
pool.reset_cancel();
let c = Arc::clone(&counter);
pool.submit(move || {
c.fetch_add(1, Ordering::Relaxed);
});
drop(pool);
assert_eq!(counter.load(Ordering::Relaxed), 1);
}
#[test]
fn test_metrics() {
let pool = WorkerPool::new(2);
assert_eq!(pool.completed(), 0);
for _ in 0..10 {
pool.submit(|| {});
}
drop(pool);
// Can't assert exact completed count due to timing,
// but it should be > 0 after drop waits for all
}
#[test]
fn test_backpressure_bounded() {
// Pool of 1 with capacity 4 — 5th submit should block until one completes
let pool = WorkerPool::new(1);
let counter = Arc::new(AtomicUsize::new(0));
for _ in 0..20 {
let c = Arc::clone(&counter);
pool.submit(move || {
c.fetch_add(1, Ordering::Relaxed);
});
}
drop(pool);
assert_eq!(counter.load(Ordering::Relaxed), 20);
}
}