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7922: runtime: fix race condition during the blocking pool shutdown #67

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7922: runtime: fix race condition during the blocking pool shutdown #67

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26 changes: 18 additions & 8 deletions tokio/src/runtime/blocking/pool.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -507,6 +507,8 @@ impl Inner {

let mut shared = self.shared.lock();
let mut join_on_thread = None;
// is this thread currently counted in `num_idle_threads`?
let mut is_counted_idle;

'main: loop {
// BUSY
Expand All @@ -520,6 +522,8 @@ impl Inner {

// IDLE
self.metrics.inc_num_idle_threads();
// mark this thread as currently counted in `num_idle_threads`.
is_counted_idle = true;

while !shared.shutdown {
let lock_result = self.condvar.wait_timeout(shared, self.keep_alive).unwrap();
Expand All @@ -532,6 +536,9 @@ impl Inner {
// acknowledge it by decrementing the counter
// and transition to the BUSY state.
shared.num_notify -= 1;
// since this is a legitimate wakeup,
// the `Spawner::spawn_task` has already decremented `num_idle_threads`.
is_counted_idle = false;
break;
}

Expand Down Expand Up @@ -568,14 +575,17 @@ impl Inner {
// Thread exit
self.metrics.dec_num_threads();

// `num_idle_threads` should now be tracked exactly, panic
// with a descriptive message if it is not the
// case.
let prev_idle = self.metrics.dec_num_idle_threads();
assert_ne!(
prev_idle, 0,
"`num_idle_threads` underflowed on thread exit"
);
// Is this thread currently counted in `num_idle_threads`?
if is_counted_idle {
// `num_idle_threads` should now be tracked exactly, panic
// with a descriptive message if it is not the
// case.
let prev_idle = self.metrics.dec_num_idle_threads();
assert_ne!(
prev_idle, 0,
"`num_idle_threads` underflowed on thread exit"
);
}

if shared.shutdown && self.metrics.num_threads() == 0 {
self.condvar.notify_one();
Expand Down