From: Ben Weintraub <benweint@...>
Date: 2013-01-18T10:39:16+09:00
Subject: Re: Background threads and Process.daemon

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Ah, I believe I've found the issue. Process.daemon calls daemon(3), which
calls fork(). The POSIX documentation for fork says:

A process shall be created with a single thread. If a multi-threaded
process calls *fork*(), the new process shall contain a replica of the
calling thread and its entire address space, possibly including the states
of mutexes and other resources.

From: http://pubs.opengroup.org/onlinepubs/009695399/functions/fork.html

So this seems a misunderstanding of daemon() semantics on my part rather
than a bug.

Sorry for the noise!
Ben

On Thursday, January 17, 2013, Ben Weintraub wrote:

> Hello all,
>
> I'm seeing some unexpected behavior with background threads and
> Process.daemon, and wondering whether I'm just doing something
> wrong/unsupported, or if I've hit upon a Ruby bug.
>
> Here's a test case that exposes the oddness I'm seeing:
> https://gist.github.com/4561056
>
> This code spawns a background thread that loops calling sleep(1)
> repeatedly, then calls Process.daemon, and finally puts the main thread
> into a sleep loop.
>
> To reproduce the strangeness, run it, find the PID of the daemonized
> process, and send it SIGQUIT. Instead of exiting, the process gets stuck in
> a loop in rb_thread_terminate_all that looks like this:
> https://gist.github.com/4561403
>
> In addition, after daemonization, attaching to the process with gdb shows
> only 2 native threads. Prior to daemonization, three native threads are
> visible from gdb.
>
> Is this (creating a background thread and then daemonizing) expected to
> work? Am I doing something else wrong here?  I'm using ruby 1.9.3p327 on
> Mac OS X 10.8.2.
>
> Thanks!
> Ben
>

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Ah, I believe I&#39;ve found the issue. Process.daemon calls daemon(3), whi=
ch calls fork(). The POSIX documentation for fork says:<div><br></div><div>=
<font><span style=3D"line-height:normal;background-color:rgba(255,255,255,0=
)">A process shall be created with a single thread. If a multi-threaded pro=
cess calls=A0<i>fork</i>(), the new process shall contain a replica of the =
calling thread and its entire address space, possibly including the states =
of mutexes and other resources.</span></font></div>
<div><font><span style=3D"line-height:normal"><br></span></font></div><div>=
<span style=3D"font-family:&#39;.HelveticaNeueUI&#39;;white-space:nowrap">F=
rom:=A0<a href=3D"http://pubs.opengroup.org/onlinepubs/009695399/functions/=
fork.html">http://pubs.opengroup.org/onlinepubs/009695399/functions/fork.ht=
ml</a></span><font><span style=3D"line-height:normal"><br dir=3D"ltr">
</span></font><br>So this seems a misunderstanding of daemon() semantics on=
<span></span>=A0my part rather than a bug.=A0</div><div><br></div><div>Sorr=
y for the noise!</div><div>Ben<br><br>On Thursday, January 17, 2013, Ben We=
intraub  wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr">Hello all,<div><br></div><d=
iv>I&#39;m seeing some unexpected behavior with background threads and Proc=
ess.daemon, and wondering whether I&#39;m just doing something wrong/unsupp=
orted, or if I&#39;ve hit upon a Ruby bug.</div>

<div><br></div><div>Here&#39;s a test case that exposes the oddness I&#39;m=
 seeing:</div><div><a href=3D"https://gist.github.com/4561056" target=3D"_b=
lank">https://gist.github.com/4561056</a><br></div><div><br></div><div>This=
 code spawns a background thread that loops calling sleep(1) repeatedly, th=
en calls Process.daemon, and finally puts the main thread into a sleep loop=
.</div>

<div><br></div><div>To reproduce the strangeness, run it, find the PID of t=
he daemonized process, and send it SIGQUIT. Instead of exiting, the process=
 gets stuck in a loop in rb_thread_terminate_all=A0that looks like this:</d=
iv>

<div><a href=3D"https://gist.github.com/4561403" target=3D"_blank">https://=
gist.github.com/4561403</a><br></div><div><br></div><div>In addition, after=
 daemonization, attaching to the process with gdb shows only 2 native threa=
ds. Prior to daemonization, three native threads are visible from gdb.</div=
>

<div><br></div><div>Is this (creating a background thread and then daemoniz=
ing) expected to work? Am I doing something else wrong here? =A0I&#39;m usi=
ng ruby=A01.9.3p327 on Mac OS X 10.8.2.</div><div><br></div>
<div>Thanks!</div><div>Ben</div></div>
</blockquote></div>

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