Like I said, it isn’t clear to me that all uses of `semaphore-wait` when breaks 
are enabled are incorrect. You could argue that then you should have a 
`semaphore-wait/trust-me-even-though-breaks-are-enabled`, and sure, I don’t 
think that would necessarily be bad. I just imagine the API just wasn’t 
originally designed that way for some reason or another, possibly simply 
because it wasn’t considered at the time. Maybe Matthew can give a more 
satisfying answer, but I don’t know; I’m just speculating.

> On Jan 18, 2020, at 03:10, Jack Firth <> wrote:
> I don't see how it has to do with semaphores being low-level. If waiting on a 
> semaphore while breaks are enabled is almost certainly wrong, checking 
> whether breaks are enabled and raising an error seems like a way more 
> sensible default behavior than just silently doing something that's almost 
> certainly wrong. If car and cdr can check their arguments by default, 
> shouldn't semaphores guard against misuse too?
> On Sat, Jan 18, 2020 at 1:04 AM Alexis King < 
> <>> wrote:
> It is guaranteed to leave the semaphore in a consistent state, from the 
> perspective of the implementation of semaphores. No matter what you do, you 
> won’t ever corrupt a semaphore (assuming you’re not using unsafe operations 
> and assuming the runtime is not buggy).
> But perhaps you mean inconsistent from the point of view of the application, 
> not from the point of view of the Racket runtime. In that case, it’s true 
> that when using semaphores as locks, using them in a context where breaks are 
> enabled is almost certainly wrong. It’s not immediately clear to me that 
> there aren’t any valid uses of semaphores where you would want breaks to be 
> enabled, but I admit, I have no idea what they are.
> Semaphores are low-level primitives, though, so I think it makes some sense 
> for them to just do the minimal possible thing. Perhaps a library ought to 
> offer a slightly more specialized “critical section” abstraction a la Windows 
> (or perhaps something like Haskell’s MVars) that manages disabling interrupts 
> in the critical section for you. (Why doesn’t this exist already? My guess is 
> that most Racket programmers don’t worry about these details, since they 
> don’t call `break-thread` anywhere, and they want SIGINT to just kill their 
> process, anyway.)
>> On Jan 18, 2020, at 02:54, Jack Firth < 
>> <>> wrote:
>> I do understand all of that, and you're right that "kill-safe" isn't what I 
>> meant.
>> What I'm confused about is why, if it's inherently not guaranteed to leave 
>> the semaphore in a consistent state, semaphore-wait attempts to work at all 
>> if breaks are enabled. Why not raise some helpful error like "it's unsafe to 
>> wait on a semaphore while breaks are enabled, did you forget to disable 
>> breaks?". What's the actual use case for calling semaphore-wait (and not 
>> semaphore-wait/enable-break) while breaks are enabled?
>> On Sat, Jan 18, 2020 at 12:47 AM Alexis King < 
>> <>> wrote:
>> Killing a thread is different from breaking a thread. Killing a thread kills 
>> the thread unrecoverably, and no cleanup actions are run. This usually isn’t 
>> what you want, but there’s always a tension between these kinds of things: 
>> defensive programmers ask “How do I make myself unkillable so I can safely 
>> clean up?” but then implementors of a dynamic environment (like, say, 
>> DrRacket) find themselves asking “How do I kill a runaway thread?” Assuming 
>> you’re not DrRacket, you usually want `break-thread`, not `kill-thread`.
>> But perhaps you know that already, and your question is just about breaking, 
>> so by “kill-safe” you mean “break-safe.” You ask why `semaphore-break` 
>> doesn’t just disable breaking, but that wouldn’t help with the problem the 
>> documentation alludes to. The problem is that there’s fundamentally a race 
>> condition in code like this:
>>     (semaphore-wait sem)
>>     ; do something important
>>     (semaphore-post sem)
>> If this code is executed in a context where breaks are enabled, it’s not 
>> break-safe whether or not `semaphore-wait` were to disable breaks while 
>> waiting on the semaphore. As soon as `semaphore-wait` returns, the queued 
>> break would be delivered, the stack would unwind, and the matching 
>> `semaphore-post` call would never execute, potentially holding a lock 
>> forever. So the issue isn’t that the semaphore’s internal state gets somehow 
>> corrupted, but that the state no longer reflects the value you want.
>> The right way to write that code is to disable breaks in the critical 
>> section:
>>     (parameterize-break #f
>>       (semaphore-wait sem)
>>       ; do something important
>>       (semaphore-post sem))
>> This eliminates the race condition, since a break cannot be delivered until 
>> the `semaphore-post` executes (and synchronous, non-break exceptions can be 
>> protected against via `dynamic-wind` or an exception handler). But this 
>> creates a new problem, since if a break is delivered while the code is 
>> blocked on the semaphore, it won’t be delivered until the semaphore is 
>> posted/unlocked, which may be a very long time. You’d really rather just 
>> break the thread, since it hasn’t entered the critical section yet, anyway.
>> This is what `semaphore-wait/enable-break` is for. You can think of it as a 
>> version of `semaphore-wait` that re-enables breaks internally, inside its 
>> implementation, and it installs an exception handler to ensure that if a 
>> break is delivered at the worst possible moment (after the count has been 
>> decremented but before breaks are disabled again), it reverses the change 
>> and re-raises the break exception. (I have no idea if this is how it’s 
>> actually implemented, but I think it’s an accurate model of its behavior.) 
>> This does exactly what we want, since it ensures that if we do enter the 
>> critical section, breaks are disabled until we exit it, but we can still be 
>> interrupted if we’re blocked waiting to enter it.
>> So it’s not so much that there’s anything really special going on here, but 
>> more that break safety is inherently anti-modular where state is involved, 
>> and you can’t implement `semaphore-wait/enable-break`-like constructs if you 
>> only have access to the `semaphore-wait`-like sibling.
>> > On Jan 17, 2020, at 22:37, Jack Firth < 
>> > <>> wrote:
>> > 
>> > The docs for semaphores say this:
>> > 
>> > In general, it is impossible using only semaphore-wait to implement the 
>> > guarantee that either the semaphore is decremented or an exception is 
>> > raised, but not both. Racket therefore supplies 
>> > semaphore-wait/enable-break (see Semaphores), which does permit the 
>> > implementation of such an exclusive guarantee.
>> > 
>> > I understand the purpose of semaphore-wait/enable-break, but there's 
>> > something about semaphore-wait that confuses me: why does it allow 
>> > breaking at all? My understanding is that if breaks are enabled, 
>> > semaphore-wait still tries to block and decrement the counter, even though 
>> > a break at any time could destroy the integrity of the semaphore. Does 
>> > that mean it's not kill-safe to use a semaphore as a lock? Wouldn't it be 
>> > safer if semaphore-wait automatically disabled breaks while waiting?

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