Before this PR, macOS builds would get clippy warning
```
warning: `tokio_epoll_uring::thread_local_system` does not refer to an existing function
```
The reason is that the `thread_local_system` function is only defined on
Linux.
Add `allow-invalid = true` to make macOS clippy pass, and manually test
that on Linux builds, clippy still fails when we use it.
refs
- https://databricks.slack.com/archives/C09254R641L/p1751917655527099
Co-authored-by: Christian Schwarz <Christian Schwarz>
## Problem
The proxy denies using `unwrap()`s in regular code, but we want to use
it in test code
and so have to allow it for each test block.
## Summary of changes
Set `allow-unwrap-in-tests = true` in clippy.toml and remove all
exceptions.
refs https://github.com/neondatabase/neon/issues/7136
Problem
-------
Before this PR, we were using
`tokio_epoll_uring::thread_local_system()`,
which panics on tokio_epoll_uring::System::launch() failure
As we've learned in [the
past](https://github.com/neondatabase/neon/issues/6373#issuecomment-1905814391),
some older Linux kernels account io_uring instances as locked memory.
And while we've raised the limit in prod considerably, we did hit it
once on 2024-03-11 16:30 UTC.
That was after we enabled tokio-epoll-uring fleet-wide, but before
we had shipped release-5090 (c6ed86d3d0)
which did away with the last mass-creation of tokio-epoll-uring
instances as per
commit 3da410c8fe
Author: Christian Schwarz <christian@neon.tech>
Date: Tue Mar 5 10:03:54 2024 +0100
tokio-epoll-uring: use it on the layer-creating code paths (#6378)
Nonetheless, it highlighted that panicking in this situation is probably
not ideal, as it can leave the pageserver process in a semi-broken
state.
Further, due to low sampling rate of Prometheus metrics, we don't know
much about the circumstances of this failure instance.
Solution
--------
This PR implements a custom thread_local_system() that is
pageserver-aware
and will do the following on failure:
- dump relevant stats to `tracing!`, hopefully they will be useful to
understand the circumstances better
- if it's the locked memory failure (or any other ENOMEM): abort() the
process
- if it's ENOMEM, retry with exponential back-off, capped at 3s.
- add metric counters so we can create an alert
This makes sense in the production environment where we know that
_usually_, there's ample locked memory allowance available, and we know
the failure rate is rare.
`std` has had `pin!` macro for some time, there is no need for us to use
the older alternatives. Cannot disallow `tokio::pin` because tokio
macros use that.
