From: William Morgan <wmorgan-ruby-talk@...>
Date: 2005-02-21T04:06:22+09:00
Subject: Re: Lock on some special file reading within XML-RPC

Excerpts from Pascal Terjan's mail of 20 Feb 2005 (EST):
> I was having a timeout in a client/server program using xml-rpc under
> Linux.

[snip]

> [pterjan@cmoi tmp]$ strace -p 12756
> Process 12756 attached - interrupt to quit
> select(6, [3 5], [], [], NULL
> 
> If I'm not wrong, 3 is the socket and 5 the file I'm trying to read.
> The problem also occurs on some other files in /proc, replacing the read
> (any sort of it will fail) by `cat /proc/cmdline` is OK. I guess these
> files do not like select...
> 
> The read works fine outside xmlrpc :
> 
> [pterjan@cmoi tmp]$ ruby -e 'puts File.readlines("/proc/cmdline")'
> auto BOOT_IMAGE=linux ro root=301 acpi=on resume=/dev/hda6 splash=silent

This is a known problem with no solution, only workarounds.

xmlrpc uses threads. In threading mode, Ruby uses the select system call
rather than read. But in kernel 2.6, many /proc files do not respond to
select. For example, the following slight modification of the above hangs
indefinitely:

  ruby -e 'Thread.new { sleep 0 }; puts File.readlines("/proc/cmdline")'

To the best of my knowledge, your only option is to say:

  IO.popen("cat /proc/cmdline")

Gory details here: http://www.all-thing.net/Ruby/select_and_procfs.html

-- 
William <wmorgan-ruby-talk@masanjin.net>