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sync: forget mpsc Permit before sending value - #8560
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Currently, `mpsc::Permit::send` calls `self.chan.send(value)` before calling `mem::forget(self)`. Because `chan.send(value)` enqueues the message and then wakes the receiver waker, a panic in the receiver's `Waker::wake` implementation causes unwinding to drop `Permit` after the value has already been enqueued. Dropping `Permit` returns a permit to the semaphore, and receiving the enqueued message returns the permit a second time, permanently inflating the channel's capacity. Fix this by copying the `self.chan` reference and forgetting `self` before calling `chan.send(value)`. Fixes: tokio-rs#8559
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martin-g
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Oct 2, 2026
This was referenced Oct 3, 2026
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Currently,
mpsc::Permit::sendcallsself.chan.send(value)beforecalling
mem::forget(self). Becausechan.send(value)enqueues themessage and then wakes the receiver waker, a panic in the receiver's
Waker::wakeimplementation causes unwinding to dropPermitafter thevalue has already been enqueued.
Dropping
Permitreturns a permit to the semaphore, and receiving theenqueued message returns the permit a second time, permanently inflating
the channel's capacity.
Fix this by copying the
self.chanreference and forgettingselfbefore calling
chan.send(value).Fixes: #8559