Debug help - ! in data sharing concurrency

[Steven Schveighoffer]

On Sunday, 31 August 2025 at 12:44:33 UTC, Brother Bill wrote:
> On Sunday, 31 August 2025 at 01:27:57 UTC, Steven Schveighoffer 
> wrote:
>> Why would your second iteration make a difference? Purely by 
>> chance! In fact, on my machine, it does not exit in either 
>> case.
>>
>> Welcome to the wonderful world of race conditions and 
>> multithreading!
>
> So this was just 'bad' luck with 'race conditions'.
> It is not a failure of the D compiler.

Yes, the (bad) luck seemed to suggest your changes were causing a 
different behavior, whereas actually it was just hitting the race 
condition differently. I've spent many hours on issues such as 
this. You think some behavior is happening because you changed x, 
but really it's due to y, and the change just happens to be 
triggering a difference in y, or some coincidence is happening.

Race conditions are really hard to figure out. Couple that with 
memory safety issues, you are in big trouble.

> FWIW, given that D supports Message Passing Concurrency, is 
> Data Sharing Concurrency just there for D completeness, for 
> those that want to live close to the iron.

Correct. You are not absolved of the responsibility of ensuring 
you don't use pointers after free.

If you want this extra check, you need to mark your function as 
`@safe`. This will prevent, among other things, taking the 
address of a local variable as you have done here.

>
> It would seem that Message Passing Concurrency should be our 
> first, second and third choice for concurrency.

Yes, aside from the issue with lifetime here, you technically are 
not properly synchronizing access to your boolean. But in this 
case, it should properly work. Other cases may not be correct 
without more synchronization (mutexes or using atomics).

`std.concurrency` is meant to make all these control flow systems 
easy to avoid races for, even in `@safe` code. Sending the 
boolean instead of keeping a pointer to a shared boolean can 
alleviate a lot of these problems.

-Steve