Is this library thread-safe? If not shouldn't the documentation state that?

[enobayram]

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]

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]

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]

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.

[andreasabel]

Ran into this in

• Capturing stdout in a test makes the testsuite fail non-deterministically UnkindPartition/tasty#434

[andreasabel]

I clarified the situation in v1.2.5.4:

• v1.2.5.4: README: failure multi-thread; CI: bump to GHC 9.12.0 #28

[sol]

@andreasabel just in case you're not aware of it, Hspec does not suffer from the same limitations as tasty. With Hspec you can capture stdout in a test cases as long as you don't mark that specific test case safe for parallel execution. Other test cases can still be executed in parallel.