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hspec / silently

Prevent or capture output to stdout or other handles in Haskell
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Is this library thread-safe? If not shouldn't the documentation state that? #6

Open enobayram opened 5 years ago

enobayram commented 5 years ago

As far as I know, it's impossible to implement silently in a thread-safe manner, but I'd love to be told otherwise. So I'm assuming that the functions in this library aren't thread-safe. In that case, I think the documentation should state that since it may not be obvious to everyone.

saurabhnanda commented 4 years ago

I have a similar question related to how silently works with async threads. Is it possible to "silence" specific asyncs without affecting the others?

soenkehahn commented 4 years ago

As far as I understand this library is not thread-safe, at least on linux. I.e. when one thread wraps an operation with silence, while that operation is running, it will silence all output to stdout, even from other threads. And yes, I think it'd be impossible to implement this in a thread-safe way, @enobayram. At least with how putStrLn and other functions are currently implemented. This is caused by the file-descriptors being shared between OS threads. (See e.g. here.)

@saurabhnanda: Because of this it's not possible to silence one async thread without affecting others.

Re: documentation: Sorry, don't have many opinions on this. Just wanted to at least answer the question about thread-safety.

nh2 commented 4 years ago

What @soenkehahn says is correct.

This library uses hDuplicateTo which uses the POSIX dup2() function to redirect file descriptors (FDs), and FDs are per process (thus shared by all OS threads in the process, and thus shared by all Haskell threads).

I'm sure the project would gladly accept a PR to make this as clear as possible.