I wish more people (and crate authors) would treat panic!() as it really should be treated: only for absolutely unrecoverable errors that indicate that some sort of state is corrupted and that continuing wouldn't be safe from a data- or program-integrity perspective.
Even then, though, I do see a need to catch panics in some situations: if I'm writing some sort of API or web service, and there's some inconsistency in a particular request (even if it's because of a bug I've written), I probably really would prefer only that request to abort, not for the entire process to be torn down, terminating any other in-flight requests that might be just fine.
But otherwise, you really should just not be catching panics at all.
Unfortunately even the Rust core language doesn't treat them this way.
I think it's arguably the single biggest design mistake in the Rust language. It prevents a ton of useful stuff like temporarily moving out of mutable references.
They've done a shockingly good job with the language overall, but this is definitely a wart.
> I probably really would prefer only that request to abort, not for the entire process to be torn down,
This is a sign you are writing an operating system instead of using one. Your web server should be handling requests from a pool of processes - so that you get real memory isolation and can crash when there is a problem.
Even if you used a pool of processes, that's still not one process per request, and you still don't want one request crashing to tear down unrelated requests.
I question both things. I would first of all handle each request in its own process.
If there was a special case that would not work, then the design dictates that requests are not independent and there must be risk of interference (they are in the same process!)
What I definitely do not want is a bug ridden “crashable async sub task” system built in my web program.
This is simply a wrong idea about how to write web servers. You're giving up scalability massively, only to gain a minor amount of safety - one that is virtually irrelevant in a memory safe language, which you should anyway use. The overhead of process-per-request, or even thread-per-request, is absurd if you're already using a memory safe language.
you’re vastly over estimating the overhead of processes and number of simultaneous web connections.
> only to gain a minor amount of safety
What you’re telling me is performance (memory?) is such a high priority you’re willing to make correctness and security tradeoffs.
And I’m saying thats ok, one of those is crashing might bring down more than one request.
> one that is virtually irrelevant in a memory safe language
Your memory safe language uses C libraries in its process.
Memory safe languages have bugs all the time. The attack surface is every line of your program and runtime.
Memory is only one kind of resource and privilege. Process isolation is key for managing resource access - for example file descriptors.
Chrome is a case study if these principles. Everybody thought isolating JS and HTML pages should be easy - nobody could get it right and chrome instead wrapped each page in a process.
Please find one web server being actively developed using one process per request.
Handling thousands of concurrent requests is table stakes for a simple web server. Handling thousands of concurrent processes is beyond most OSs. The context switching overhead alone would consume much of the CPU of the system. Even hundreds of processes will mean a good fraction of the CPU being spent solely on context switching - which is a terrible place to be.
> Handling thousands of concurrent processes is beyond most OS
It works fine on Linux - the operating system for the internet. Have you tried it?
> good fraction of the CPU being spent solely on context switching
I was waiting for this one. Threads and processes do the same amount of context switching. The overhead of processes switch is a little higher. The main cost is memory.
> Threads and processes do the same amount of context switching.
Yes, therefore real webservers use a limited amount of threads/processes (in the same ballpark as a number of CPU cores). Modern approach is to use green threads which are really cheap to switch, it is like store registers, read registers and jmp.
> The main cost is memory.
The main cost is scheduling, not switching per se. Preemptive multitasking needs to deal with priorities to not waste time, and algorithms that do it are O(N) mostly. All these O(N) calculations needs to be completed multiple times per second, the higher the frequency of switching the more work to do. When you have thousands of processes it is the main cost. If you have tens of thousands it starts to bite hard.
> The main cost is scheduling, not switching per se. Preemptive multitasking needs to deal with priorities to not waste time, and algorithms that do it
The person I am having a conversation with is advocating for threads instead of processes. How do you think threads work?
> Modern approach is to use green threads which are really cheap to switch, it is like store registers, read registers and jmp.
That’s certainly the popular approach. As I said at the beginning this approach is making a mini operating system with more bugs and less security rather than leveraging the capabilities of your operating system.
Once again, im waiting to here about your experience of maxing out processes and after that having to switch to green threads.
> The person I am having a conversation with is advocating for threads instead of processes. How do you think threads work?
I was certainly not, I explicitly said that thread-per-request is as bad as process-per-request. I could even agree that it's the worse of both worlds to some extent - none of the isolation, almost all of the overhead (except if you're using a language with a heavy runtime, like Java, where spawning a new JVM has a huge cost compared to a new thread in an existing JVM).
Modern operating systems provide many mechanisms for doing async IO specifically to prevent the need for spawning and switching between thousands of processes. Linux in particular has invested heavily in this, from select, to poll, to epoll, and now unto io_uring.
OS process schedulers are really a poor tool for doing massively parallel IO. They are a general purpose algorithm that has to keep in mind many possible types of heterogeneous processes, and has no insight into the plausible behaviors of those. For a constrained problem like parallel IO, it's a much better idea to use a purpose-built algorithm and tool. And they have simply not been optimized with this kind of scale in mind, because it's much more important and common use case to run quickly for a small number of processes than it is to scale up to thousands. There's a reason typical ulimit configurations are limited to around 1000 threads/processes per system for all common distros.
> The person I am having a conversation with is advocating for threads instead of processes. How do you think threads work?
Are they? I looked back and I've found this quote of them: "The overhead of process-per-request, or even thread-per-request, is absurd if you're already using a memory safe language." Doesn't seem as an advocacy for thread-per-request to me.
> As I said at the beginning this approach is making a mini operating system with more bugs and less security rather than leveraging the capabilities of your operating system.
Lets look at Apache for example. It starts a few processes and/or threads, but then each thread deals with a lot of connections. The threads Apache starts are for spreading work over several CPUs and maybe to overcome some limits of select/poll/epoll. The main approach is to track a state of a connection, and when something happens on a socket, Apache find the state of the connection and deals with events on the socket. Then it stores the new state and moves to deal with other sockets in the same manner.
It is like green threads but without green threads. Green threads streamlines all this state keeping by allowing each connection to have it's own stack. And I'd say it is easier to do right than to write a finite automata for HTTP/HTTPS.
> Once again, im waiting to here about your experience of maxing out processes and after that having to switch to green threads.
Oh, I didn't. A long long time ago I was reading stuff on networking. All of it was in one opinion: 10k kernel tasks maybe a tolerable solution, but 100k is bad. IIRC Apache had a document describing its internal architecture and explaining why it is as it is.
So I wouldn't even try to start thousands of threads. I mean I tried to start 1000s of processes when I was young and learned about fork-bombs, and this experience confirmed it for me, that 1000s of processes is not a really good idea.
Moreover I completely agree with them: if you use a memory-safe language, then it is strange to pay costs for preemptive multitasking just to have separate virtual address spaces. I mean, it will be better to get a virtual machine with JIT compiler, and run code for different connection on different instances of a virtual machine. O(1) complexity of cooperative switching will beat O(N) complexity of preemptive switching. To my mind hardware memory management is overrated.
Apache has years of engineering work - and almost weekly patches to fix issues related to security. Many of these security issues would go away if they were not using special technique to optimize performance.
But the best part of the web is its modular. So now your application doesn’t need to that. It can leverage those benefits without complexity cascade.
For example, Apache can manage more connections than your application needs running processes for.
> I was reading stuff on networking….
That’s exactly my point. Too many people are repeating advice from Google or Facebook and not actually thinking about real problems they face.
Can you serve more requests using specialized task management? Yes. You can make a mini-OS with fewer features to squeeze out more scheduling performance and that’s what some big companies did.
But you will pay for that with reduced security and reliability. To bring it back to my original complaint - you must accept that a crash can bring down multiple requests.
And it’s an insane default to design Rust around. It’s especially confusing to make all these arguments about how “unsafe” languages are, but then ignore OS safety in hopes of squeezing out a little more perf.
> So I wouldn't even try to start thousands of threads.
Please try it before arguing it doesn’t work. Fork bombing is recursive and unrelated.
> if you use a memory-safe language, then it is strange to pay costs for preemptive multitasking just to have separate virtual address spaces
Then why do these “memory-safe” languages need constant security patches? Why does chrome need to wrap each page’s JS in its own process?
In theory you’re right. If they are actually memory-safe then you don’t need to consider address spaces. But in practice the attack surface is massive and processes give you stronger invariants.
> you’re vastly over estimating the overhead of processes and number of simultaneous web connections.
It's less the actual overhead of the process but the savings you get from sharing. You can reuse database connections, have in-memory caches, in-memory rate limits and various other things. You can use shared memory which is very difficult to manage or an additional common process, but either way you are effectively back to square one with regards to shared state that can be corrupted.
Using a Rust lib from Swift on macOS I definitely want to catch panics - to access security scoped resources in Rust I need the Rust code to execute in process (I believe) but I’d also like it not to crash the entire app.
would you consider panics acceptable when you think it cannot panic in practice?
e.g. unwraping/expecting a value for a key in a map when you inserted that value before and know it hasn't been removed?
you could have a panic though, if you wrongly make assumptions
Obviously yes. For the same reason it's acceptable that myvec[i] panics (it will panic if i is out of bounds - but you already figured out that i is in bounds) and a / b panic for a and b integers (it will panic if b is zero, but if your code is not buggy you already tested if b is zero prior to dividing right?)
Panic is absolutely fine for bugs, and it's indeed what should happen when code is buggy. That's because buggy code can make absolutely no guarantees on whether it is okay to continue (arbitrary data structures may be corrupted for instance)
Indeed it's hard to "treat an error" when the error means code is buggy. Because you can rarely do anything meaningful about that.
This is of course a problem for code that can't be interrupted.. which include the Linux kernel (they note the bug, but continue anyway) and embedded systems.
Note that if panic=unwind you have the opportunity to catch the panic. This is usually done by systems that process multiple unrelated requests in the same program: in this case it's okay if only one such request will be aborted (in HTTP, it would return a 5xx error), provided you manually verify that no data structure shared by requests would possibly get corrupted. If you do one thread per request, Rust does this automatically; if you have a smaller threadpool with an async runtime, then the runtime need to catch panics for this to work.
> Note that if panic=unwind you have the opportunity to catch the panic.
And now your language has exceptions - which break control flow and make reasoning about a program very difficult - and hard to optimize for a compiler.
Yeah, but this isn't the only bad thing about unwinding. Much worse than just catching panics is the fact that a panic in a thread takes down only that thread (except if it is in the main thread). If your program is multithreaded, panic=unwind makes it much harder to understand how it reacts to errors, unless you take measures to shut down the program if any thread panic (which again, requires catch_unwind if you have unwinding). Also: that's why locks in Rust have poisoning, they exist so that panics propagate between threads: if a thread panics while holding a lock, any other thread attempting to acquire this lock will panic too (which is better than a deadlock for sure)
And that's why my programs get compiled with panic=abort, that makes panics just quit the program, with no ability to catch them, and no programs in zombie states where some threads panicked and others keep going on.
But see, catch_panic is an escape hatch. It's not meant to be used as a general error handling mechanism and even when doing FFI, Rust code typically converts exceptions in other languages into Results (at a performance cost, but who cares). But Rust needs a escape right, it is a low level language.
And there is at least one case where the catch_unwind is fully warranted: when you have an async web server with multiple concurrent requests and you need panics to take down only a single request, and not the whole server (that would be a DoS vector). If that weren't possible, then async Rust couldn't have feature parity with sync Rust (which uses a thread-per-request model, and where panics kill the thread corresponding to the request)
I don't speak for anyone else but I'm not using `unwrap` and `expect`. I understand the scenario you outlined but I've accepted it as a compromise and will `match` on a map's fetching function and will have an `Err` branch.
I will fight against program aborts as hard as I possibly can. I don't mind boilerplate to be the price paid and will provide detailed error messages even in such obscure error branches.
Again, speaking only for myself. My philosophy is: the program is no good for me dead.
Agreed, that's why I don't catch panics either -- if we get to that point I'm viewing the program as corrupted. I'm simply saying that I do my utmost to never use potentially panicking Rust API and prefer to add boilerplate for `Err` branching.
This implies that every function in your library that ever has to do anything that might error out - e.g. integer arithmetic or array indexing - has to be declared as returning the corresponding Result to propagate the error. Which means that you are now imposing this requirement (to check for internal logic bugs in library code) onto the user of your library.
Well, I don't write as huge a code as this though, nor does it have as many layers.
Usually I just use the `?` and `.map_err` (or `anyhow` / `thiserror`) to delegate and move on with life.
I have a few places where I do pattern-matches to avoid exactly what you described: imposing the extra internal complexity to users. Which is indeed a bad thing and I am trying to fight it. Not always succeeding.
Honestly, I don't think libraries should ever panic. Just return an UnspecifiedError with some sort of string. I work daily with rust, but I wish no_std and an arbitrary no_panic would have better support.
Example docs for `foo() -> Result<(), UnspecifiedError>`:
# Errors
`foo` returns an error called `UnspecifiedError`, but this only
happens when an anticipated bug in the implementation occurs. Since
there are no known such bugs, this API never returns an error. If
an error is ever returned, then that is proof that there is a bug
in the implementation. This error should be rendered differently
to end users to make it clear they've hit a bug and not just a
normal error condition.
Imagine if I designed `regex`'s API like this. What a shit show that would be.
If you want a less flippant take down of this idea and a more complete description of my position, please see: https://burntsushi.net/unwrap/
> Honestly, I don't think libraries should ever panic. Just return an UnspecifiedError with some sort of string.
The latter is not a solution to the former. The latter is a solution to libraries having panicking branches. But panics or other logically incorrect behavior can still occur as a result of bugs.
Even then, though, I do see a need to catch panics in some situations: if I'm writing some sort of API or web service, and there's some inconsistency in a particular request (even if it's because of a bug I've written), I probably really would prefer only that request to abort, not for the entire process to be torn down, terminating any other in-flight requests that might be just fine.
But otherwise, you really should just not be catching panics at all.