triple-buffer VS rsevents

Great write up! I believe the colloquial name for this algorithm is a "lock-free triple buffer". Here's an implementation in Rust (I couldn't find any c/c++ examples) that has extremely thorough comments that might help completely wrap your head around the synchronization ordering. Rust uses the same semantics for atomic primitives as C11, so it should be pretty easy to match up with your implementation. I came to the same conclusion as you to solve an issue I had with passing arbitrarily large data between two threads in an RTOS system I was working with at my day job. It was an extremely satisfying moment, realizing the index variable was sufficient to communicate all the needed information between the two threads.

Rust marks cross-thread shared memory as immutable in the general case, and allows you to define your own shared mutability constructs out of primitives like mutexes, atomics, and UnsafeCell. As a result you don't get rope to hang yourself with by default, but atomic orderings are more than enough rope to devise incorrect synchronizations (especially with more than 2 threads or memory locations). To quote an earlier post of mine:

In terms of shared-memory threading concurrency, Send and Sync, and the distinction between &T and &Mutex and &mut T, were a revelation when I first learned them. It was a principled approach to shared-memory threading, with Send/Sync banning nearly all of the confusing and buggy entangled-state codebases I've seen and continue to see in C++ (much to my frustration and exasperation), and &Mutex providing a cleaner alternative design (there's an excellent article on its design at http://cliffle.com/blog/rust-mutexes/).

My favorite simple concurrent data structure is https://docs.rs/triple_buffer/latest/triple_buffer/struct.Tr.... It beautifully demonstrates how you can achieve principled shared mutability, by defining two "handle" types (living on different threads), each carrying thread-local state (not TLS) and a pointer to shared memory, and only allowing each handle to access shared memory in a particular way. This statically prevents one thread from calling a method intended to run on another thread, or accessing fields local to another thread (since the methods and fields now live on the other handle). It also demonstrates the complexity of reasoning about lock-free algorithms (https://github.com/HadrienG2/triple-buffer/issues/14).

I find that writing C++ code the Rust way eliminates data races practically as effectively as writing Rust code upfront, but C++ makes the Rust way of thread-safe code extra work (no Mutex unless you make one yourself, and you have to simulate &(T: Sync) yourself using T const* coupled with mutable atomic/mutex fields), whereas the happy path of threaded C++ (raw non-Arc pointers to shared mutable memory) leads to pervasive data races caused by missing or incorrect mutex locking or atomic synchronization.

In terms of shared-memory threading concurrency, Send and Sync, and the distinction between &T and &Mutex and &mut T, were a revelation when I first learned them. It was a principled approach to shared-memory threading, with Send/Sync banning nearly all of the confusing and buggy entangled-state codebases I've seen and continue to see in C++ (much to my frustration and exasperation), and &Mutex providing a cleaner alternative design (there's an excellent article on its design at http://cliffle.com/blog/rust-mutexes/).

My favorite simple concurrent data structure is https://docs.rs/triple_buffer/latest/triple_buffer/struct.Tr.... It beautifully demonstrates how you can achieve principled shared mutability, by defining two "handle" types (living on different threads), each carrying thread-local state (not TLS) and a pointer to shared memory, and only allowing each handle to access shared memory in a particular way. This statically prevents one thread from calling a method intended to run on another thread, or accessing fields local to another thread (since the methods and fields now live on the other handle). It also demonstrates the complexity of reasoning about lock-free algorithms (https://github.com/HadrienG2/triple-buffer/issues/14).

I suppose &/&mut is also a safeguard against event-loop and reentrancy bugs (like https://github.com/quotient-im/Quaternion/issues/702). I don't think Rust solves the general problem of preventing deadlocks within and between processes (which often cross organizational boundaries between projects and distinct codebases, with no clear contract on allowed behavior and which party in a deadlock is at fault), and non-atomicity between processes on a single machine (see my PipeWire criticism at https://news.ycombinator.com/item?id=31519951). File saving is also difficult (https://danluu.com/file-consistency/), though I find that fsync-then-rename works well enough if you don't need to preserve metadata or write through file (not folder) symlinks.

Thanks. Perhaps I did go overboard with that disclaimer.. probably because I myself made the mistake of initially using [0] the oh-so-convenient tokio::io::copy() instead of writing my own copy method that would drop the other half of the connection when one side was closed.

The copy_with_abort() routine is still taking the easy way out in this not-optimized-for-heavy-production-use sample because it uses a broadcast channel per connection to reactively signal that the other half of the connection should be closed (rather than timing out every x ms to see if an abort flag has been set). In the real world, I'd probably replace the join! macro with a manual event loop to be able to do the same but without creating a broadcast channel per-connection.

(I maintain an extremely lightweight "awaitable bools" library for rust [1] that is perfect for this kind of thing (roughly equivalent to a "bounded broadcast_channel<()> of queue length 1, but each "channel" is only a single (optionally stack-allocated) byte) — but it's for event loops in synchronous code and not async executor compatible.)

[0]: https://github.com/mqudsi/tcpproxy/commit/0164ef836a49f2f738...

[1]: https://github.com/neosmart/rsevents

The AutoResetEvent takes care of that, doing Acquire and Release as needed. Source code here if you’re interested, not too long: https://github.com/neosmart/rsevents/blob/master/src/lib.rs

I wrote my own FOSS signals/events library in C++ [0] and in rust [1] (atop of parking lot as a futex shoe-in) and I disagree. This has nothing to do with the language and everything to do with the semantics of the locks. Writing concurrency primitives is HARD and the more functionality your API exposes, the more room there is for nuanced bugs in how everything interplays with everything else.

[0]: https://github.com/neosmart/pevents

[1]: https://github.com/neosmart/rsevents

rsevents-extra types are built on top of the low-level synchronization types from the rsevents crate, which are fast and tiny (one-byte) Send + Sync synchronization types for signalling one or more waiting threads (à la WIN32 events), doing everything but putting a thread to sleep in userland.