readFile "/proc/mounts" hangs on an amd64 machine

[dsf]

readFile "/proc/mounts" hangs on an amd64 machine with ghc 6.10.1, it worked find in ghc 6.8.3.

[dsf]

I'm guessing from the responses on the mailing list that this affects amd64 machines and not i386 (or amd64 machines running an i386 kernel.)

[dsf]

Here is the related thread on the mailing list:  http://www.haskell.org/pipermail/haskell-cafe/2009-January/054126.html

[dsf]

No, I can now confirm that this bug affects i386 as well.

[rm]

SBCL ran into this problem some time ago. It turned out the Linux kernel doesn't support standard select semantics on files in procfs and sysfs:  http://bugzilla.kernel.org/show_bug.cgi?id=11014

[duncan]

The question is why did it work in 6.8 and not work now in 6.10?

[dsf]

Replying to duncan:

The question is why did it work in 6.8 and not work now in 6.10?

If you try reading it using a ByteString? function in 6.8 you will get a similar failure. I suspect that that code has moved into the regular IO module.

[rm]

I think I've tracked this down to libraries/base/GHC/Handle.hs. In order to fix #2363, calls to fdReady were added before every IO operation between 6.8.3 and 6.10.1. For real files, this works fine. For the special not-really-files files in /proc and /sys it can fail.

[igloo]

Thanks for tracking down the problem

[simonmar]

I messed up when I modified the IO library to avoid using O_NONBLOCK for external file descriptors. Fixing it now.

[simonmar]

Fixed:

Fri Feb  6 16:59:12 GMT 2009  Simon Marlow <marlowsd@gmail.com>
  * Fix #2971: we had lost the non-blocking flag on Handles created by openFile
  This code is a mess, fortunately the new IO library cleans it up.

reading /proc/mounts works:

Prelude> readFile "/proc/mounts"
"rootfs / rootfs rw 0 0\n/dev/root / ext3 ...

[igloo]

Merged