Reimplement futex manager to use interruptible waits

[jstarks]

Update futex manager to use interruptible waits. As part of this, implement an intrusive linked list to avoid heap
allocations in the futex path.

The futex and linked list tests have been run against miri to check for uses of undefined behavior.

[wdcui]

Thank you for the code change, John. I left some minor comments. Please wait for @CvvT 's review.

[CvvT]

LGTM! With this new design of WaitContext, things got simplifed a lot and futex now can support bitset mask. I only have a high level question: why we switched from hashmap to hashtable with linked list? You mentioned it avoids heap allocation. I wonder it is purely for optimization or has any other reasons.

[jstarks]

LGTM! With this new design of WaitContext, things got simplifed a lot and futex now can support bitset mask. I only have a high level question: why we switched from hashmap to hashtable with linked list? You mentioned it avoids heap allocation. I wonder it is purely for optimization or has any other reasons.

As LiteBox becomes more general-purpose, I think we'll have to confront the reality that "panic on OOM" is not a viable strategy for some scenarios. So, if we can design some of our core data structures to avoid needing allocations in the first place, then the eventual conversion will be that much easier.

It's probably OK to terminate the current process on OOM in cases where the contract doesn't allow us to return ENOMEM, so this is not strictly necessary. But still, better to just avoid allocations when it's straightforward to do so.

In this case, I had to change the data structure for the futex manager anyway--the old hash table was based on a single entry per futex, but with the new wait model (and to correctly support bitsets) we need an entry per waiter. Or at least a list of waiters per entry. So, I figured that we might as well switch to a data structure that's much more efficient for the use case, allocations or no--an external chaining hash table gives us good concurrency (lock per bucket instead of a global lock) while not requiring resizing under high load.

It might be interesting to use this LoanList for event wait queues as well. I have a prototype to do this.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure enum_discriminants_undefined_non_unit_variant: enum's variants no longer have defined discriminants due to non-unit variant ---

Description:
An enum's variants no longer have well-defined discriminant values due to a tuple or struct variant in the enum. This breaks downstream code that accesses discriminants via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/enum_discriminants_undefined_non_unit_variant.ron

Failed in:
  enum FutexError in /home/runner/work/litebox/litebox/litebox/src/sync/futex.rs:187

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/enum_variant_added.ron

Failed in:
  variant FutexError:WaitError in /home/runner/work/litebox/litebox/litebox/src/sync/futex.rs:193

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/enum_variant_missing.ron

Failed in:
  variant FutexError::TimedOut, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/2d724bf73ec7896701693dd0803e8a2cd86f7525/litebox/src/sync/futex.rs:360