sync: fix panic in broadcast::Receiver drop #3434
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This test case causes `broadcast::Receiver` to panic on drop
with "unexpected empty broadcast channel" when the tests are
executed with this command:
```
LOOM_MAX_PREEMPTIONS=1 RUSTFLAGS="--cfg loom" \
cargo test --lib --release --features full -- --test-threads=1 --nocapture
```
Refs: tokio-rs#2533
`self.next` can be set to a value greater than `until` when the channel overflows with messages but the drop implementation does not account for it. Fixes: tokio-rs#2533
Darksonn
approved these changes
Jan 16, 2021
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Motivation
Fixes: #2533
I came across this problem in my program with multiple worker threads and an insufficient channel capacity. With a debugger, I noticed that
self.nextcan be set a greater value thanuntilbyself.recv_ref()when it's called multiple times in the loop in drop.Here is another program that reproduces the panic:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d09c0b70aeb7187b87a01db544c6b821
Solution
self.recv_refwhenself.nextis smaller thanuntil.