But, also, Go has way worse semantics around various things, like mutexes, making it much more likely deadlocks happen. Like in go, you see all sorts of "mu.Lock(); f(); mu.Unlock()" type code, where if it's called inside an `http.Handler` and 'f' panics, the program's deadlocked forever. In go, panics are the expected way for an http middleware to abort the server ("panic(http.ErrAbortHandler)"). In rust, panics are expected to actually be fatal.
Rust's mutexes also gate "ownership" of the inner object, which make a lot of trivial deadlocks compiler errors, while go makes it absolutely trivial to forget a "mu.Unlock" in a specific codepath and call 'Lock' twice in a case rust's ownership rules would have caught.
In practice, for similarly sized codebases and similarly experienced engineers, I see only a tiny fraction of deadlocks in concurrent rust code when compared to concurrent go code, so like regardless that it's an "unsolved problem", it's clear that in reality, there's something that's at least sorta working.
I don't see `mu.Lock(); f(); mu.Unlock()` anywhere really.
`mu.Lock(); defer mu.Unlock(); f();` is how everyone does it to prevent that possibility.
1. https://github.com/golang/tools/blob/f7d99c1a286d6ec8bd4516a...
2. https://github.com/golang/sync/blob/7fad2c9213e0821bd78435a9...
There are dozens and dozens throughout the stdlib and other popular go code.
The singleflight case is quite common, if you have:
mu.Lock()
if something() {
mu.Unlock()
moreWorkThatDoesntWantMutex()
return
}
mu.Unlock()
I see manual lock/unlock all the time, and semi-regularly run into deadlocks caused by it. Maybe you don't use any third-party open source libraries, in which case, good for you congrats.
At a minimum now you’ve also got to scan for nil dereferences. Maybe that simple code between the mutex is pulled out into a function in a later commit. Then the function body is changed in the next commit to do something less simple.
This is all depressingly reminiscent of listening to die-hard C enthusiasts insist that avoiding use-after-free bugs and out of bounds reads and other issues are easy as long as you write code perfectly all the time. It’s easy as long as you just always do the right thing.
It’s okay to admit your language of choice has weaknesses. You don’t have to stop using go just because you can acknowledge that data races can creep in if you’re not diligent enough.
The examples pointed to in the stdlib to complain about that usage pattern could have easily done just that -- but they were optimized to shrink the window of the locking time. My take is that the code paths were simple enough to verify that it was safe to do, and being in the stdlib had both. enough eyes to validate and a driver (being foundational code).
I recognize that Go is far from perfect but think some of the pearl clutching is manufactured (OMG! nil pointers! -- they technically can happen but in my many years of coding in Go it hasn't been a problem of note.)
And of course we should recognize that the locking issue being brought up was a simple one and the story gets worse when talking about complicated concurrency. Then the Go story of "just use the race detector!" is tempered by the fact that it is only best effort and no guarantees that it's found everything (like a bloom filter of certainty).
So year, Go has footguns and other inadequacies (FFI is a bummer). But it's good enough for a lot of things. Rust is on my list to learn, so my only complaint about it is the steep learning curve and I'm an old lazy dev.
Now you either refactor into multiple functions, while ensuring all copies of possibly shared data when passing function arguments are correctly guarded or ”manually” unlock when you don’t need the mutex access anymore.
In rust you would just throw a block around the mutex access changing the scoping and ensuring it is dropped before the slow function is called.
Call it a minimally intrusive manual unlock.
drop(foo); // Now foo doesn't exist, it was dropped, thus unlocking anything which was kept locked while foo exists
Specifically for Go, I'd try to address the problem in CSP style, so as to avoid explicit locks unless absolutely necessary.
Now for the case you mention, one can actually achieve the same in Go, it just takes a bit of prior work to set up the infra.
type Foo struct {sync.Mutex; s string}
func doLocked[T sync.Locker](data T, fn func(data T)) {
data.Lock(); defer data.Unlock(); fn(data)
}
func main() {
foo := &Foo{s: "Hello"}
doLocked(foo, func(foo *Foo) {
/* ... */
})
/* do the slow stuff */
}It’s absolutely a go-specific problem from defer being function scoped. Which could be ignored if Unlock was idempotent but it’s not.
This alleviates all these problems of unlocks within if bodies at the cost of an indent (and maybe slight performance penalty).
I personally haven’t worked with any of these perfect programmers, but I’m glad that you do.
I have heard positive things about the loom crate[1] for detecting races in general, but I have not used it much myself.
But in general I agree, writing correct (and readable) concurrent and/or parallel programs is hard. No language has "solved" the problem completely.
The Arc is only needed when you truly need to mutably share data.
Rust like Go has the full suite of different channels and what other patterns to share data.
Comparing writing a web service in Go and rust you would likely also utilize Tokio which has a wide variety of well designed sync primitives.
Or you can just avoid shared mutable state, or use channels, or many of the other patterns for avoiding data races in Rust. The fun thing is that you can be sure that no matter what you do, as long as it's not unsafe, it will not cause a data race.
Also, don’t people know that a Mutex implies lower throughput depending on how long said Mutex is held?
Lock-free data structures/algorithms are attempt to address the drawbacks of Mutexes.
https://en.wikipedia.org/wiki/Lock_(computer_science)#Disadv...
The performance of high-contention code is a really tricky to reason about and depends on a lot of factors. Just replacing a mutex with a lock-free data structure will not magically speed up your code. Eliminating the contention completely is typically much better in general.
This is independent of using a mutex, a lock free algorithm or message passing (because at the end of the day a queue is a memory location).
As for your statement that concurrency is generally hard - yes it is. But it is even harder with data races.
Congrats, rustc forced you to wrap all your types in Arc<Mutex<_>>, and you no longer have data races. As a gift, you will get logical race conditions instead, that are even more difficult to detect, while being equally difficult to reproduce reliably in unit tests and patch.
Don’t get me wrong, Rust has done a ton for safety and pushed other languages to do better. I love probably 50% of Rust. But Rust doesn’t protect against logical races, lovelocks, deadlocks, and so on.
To write concurrent programs that have the same standards of testable, composable, expressive etc as we are expecting with sequential programs is really really difficult. Either we need new languages, frameworks or (best case) design- and architectural patterns that are easy to apply. As far as I’m concerned large scale general purpose concurrent software development is an unsolved problem.