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Support writing to block devices on MacOS #827

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Support writing to block devices on MacOS #827

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@aaronjeline aaronjeline commented Jan 8, 2026

If we get POLLNVAL, wake all of our readers/writers. MacOS will report POLLNVAL for any block device. Solves #757

talex5
talex5 reviewed Jan 9, 2026
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I assume this means that, if the resource isn't ready, then it will spin? That's not a problem for /dev/null but it might be for other things.

Possibly that's still better than the current behaviour, but needs a comment in the code that it should be improved if it turns out to be a problem.

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aaronjeline commented Jan 9, 2026 •
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but needs a comment in the code that it should be improved if it turns out to be a problem.

Added a note about this.

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talex5 commented Jan 11, 2026

This results in us blocking on block devices

Does it block? I'd expect it to keep trying at 100% CPU instead (since we normally open things as non-blocking, it should get EAGAIN and try to wait again).

I wonder if we should just use select on macOS, since apparently you can avoid the 1024 FD limit there (with _DARWIN_UNLIMITED_SELECT), and I doubt people are using macOS to run e.g. high-performance servers with huge numbers of connections anyway.

Any other mac users have thoughts about this?

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aaronjeline commented Jan 12, 2026 •
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I wonder if we should just use select on macOS

Do you mean just switch from poll() to select() for all files, or keep a separate thread up for monitoring non-pollable files with select.

I believe (2) is what libuv does

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talex5 commented Jan 12, 2026

Either would be fine I think.

@aaronjeline
Adds support for interacting with devices on macOS
6778624 
Block devices currently did not work on macOS,
due to a bug that prevents poll(2) from working with
block devices. This fixes that by creating a new thread
that monitors block devices using select(2). The thread is
created the first time a user computation attempts to read
or write to a block devices. The implementation of the new
select manager is in select.ml

Additionally, the select manager also needed an eventfd system.
The eventfd system from sched.ml was refactoring into a
re-usable model that encapsulates some of the internal state
in posix_eventfd.ml.
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