Prevent deadlock by not using futures::executor::block_on
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As Alice Rhyl recommended (slightly reworded) > Either you can use `tokio::runtime::Handle::block_on` [instead of > `futures::executor::block_on`] or your program has a subtle > deadlock.
The request guard works by sending on a channel when it is
dropped. Since we can't do async code in `Drop::drop`, I was using
`futures::executor::block_on` under the (false!) impression that
this *specific* usecase would not be a problem as we are always
pulling from that channel. However, sending on a channel can result in
`Pending` when the Tokio worker's cooperative budget is exhausted,
even if the queue is completely empty!
A simple example:
```rust
// Drain the budget
while task::consume_budget().now_or_never().is_some() {}
// Try to use more budget than we have and block forever
futures::executor::block_on(tx.send(42u8)).unwrap();
```
This `Pending` will never resolve because we are no longer running the
Tokio runtime so it will never see an `.await` and replenish the
budget. With enough bad luck, all the worker threads get stuck like
this and the entire system becomes unavailable.
There are a number of possible workarounds, but the least invasive is
to punt the final logging off to a Tokio task and let it happen
out-of-band. Technically this will make the timing a little bit less
accurate, but I was never worried about exact nanoseconds anyway.
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