Re: FcgidAccessChecker hangs.

Peter Danenberg <pcd@xxxxxxxxxxx> · Tue, 13 Nov 2012 19:59:10 -0600

mod_fcgid doesn't appear to have an external process manager; compare
this process tree from mod_fastcgi where the access checker
(872695a60a6d82aa9fc041bae44803af) is a child of fcgi is a child of
httpd:

  /usr/local/apache2/bin/httpd -k start
   \_ /usr/local/apache2/bin/fcgi- -k start
   |   \_ 872695a60a6d82aa9fc041bae44803af

to this one from mod_fcgid where the access checker is a direct
descendant of httpd:

  /usr/local/apache2/bin/httpd -k start
   |   \_ 872695a60a6d82aa9fc041bae44803af

I've attached straces from mod_fastcgi's process manager; as well as
straces from 872695a60a6d82aa9fc041bae44803af running under mod_fcgid
and mod_fastcgi, if that's helpful.

Do you still have the original httpd-strace for mod_fcgid? If so, grep
for access.scm; you'll see the script there.

PS. I've since compiled mod_fastcgi against httpd-2.4.3; the only
reason I was screwing around with mod_fcgid was that mod_fastcgi had
stopped compiling against 2.4.3.

It may be time to consider mod_fcgid abandonware, since I haven't
heard from the authors; and go back to using mod_fastcgi in
production.

> On Sun, Nov 11, 2012 at 8:41 PM, Peter Danenberg <pcd@xxxxxxxxxxx> wrote:
> 
> > Thanks for looking into this, Jeff; I've attached an strace with
> > mod_fcgid (strace-fcgid), one with mod_fastcgi (strace-fastcgi) and
> > one with mod_fcgid and relative time stamps so you can see where it
> > hangs (strace-fcgid-with-relative-time).
> >
> 
> Ouch.  Is your script running in there somewhere?  I just saw httpd
> activity but no activity of your script.  (s/script/application/)
> 
> Perhaps the most straightforward way to get a trace is to start httpd,
> identify the mod_fastcgi or mod_fcgid daemon process, start strace -fp
> DAEMON-PID, and then make the request.
> 
> How to identify the mod_fcgid daemon: At startup, it logs a message at
> LogLevel info which says "mod_fcgid: Process manager XXXX started", where
> XXXX is the pid.
> 
> mod_fastcgi?  I don't know.  But you may be very used to gdb and strace
> before this is over, so maybe you can pick the child that doesn't look like
> the others ;)
> 
> --/--
> 
> OTOH, how much effort is required to get your chicken-scheme script running
> on Linux?
> 
> 
> 
> >
> > It looks like the immediate call is:
> >
> >   rt_sigtimedwait(~[ILL TRAP ABRT BUS FPE KILL SEGV USR2 PIPE CONT
> >   STOP SYS RTMIN RT_1], NULL, NULL, 8) = 1
> >
> 
> 
> 
> >
> > > On Sunday, November 11, 2012, Peter Danenberg wrote:
> > >
> > > > Digging a little deeper, mod_fcgid hangs on line 318 of
> > > > fcgid_bridge.c:
> > > >
> > > >   ap_scan_script_header_err_core(r, sbuf, getsfunc_fcgid_BRIGADE,
> > > >                                  brigade_stdout)
> > > >
> > > > I don't know enough about ap_scan_script_header_err_core yet to figure
> > > > out why this might be the case.
> > >
> > >
> > > Can You compare an strace/truss/dtruss of your script running with the
> > > alternative modules?
> > >
> > > >
> > > > > The following trivial script,[1] which simply returns a 200 header,
> > > > > hangs under FcgidAccessChecker:
> > > > >
> > > > >   (call-with-dynamic-fastcgi-query
> > > > >    (λ (query)
> > > > >      (display-status-&c.)))
> > > > >
> > > > > The same script works fine under mod_fastcgi with
> > > > > FastCgiAccessChecker; or handled as an fcgid-script or
> > fastcgi-script.
> > > > >
> > > > > The output from error_log is as follows:
> > > > >
> > > > >   [Sun Nov 11 01:39:44 2012] [info] mod_fcgid: server
> > > > >   localhost:/usr/local/var/html/scm/api/access.scm(15965)
> > > > >   started
> > > > >
> > > > >   [Sun Nov 11 01:40:24 2012] [warn] [client 127.0.0.1] mod_fcgid:
> > read
> > > > >   data timeout in 40 seconds
> > > > >
> > > > >   [Sun Nov 11 01:40:24 2012] [error] [client 127.0.0.1] Premature
> > end of
> > > > >   script headers: access.scm
> > > > >
> > > > >   [Sun Nov 11 01:40:24 2012] [warn] [client 127.0.0.1] mod_fcgid:
> > user
> > > > >   (null) access check failed, respond 500, URI /scm/api/access.scm
> > > > >
> > > > >   [Sun Nov 11 01:40:26 2012] [debug] fcgid_pm_main.c(357): mod_fcgid:
> > > > >   gracefully terminated 1 processes
> > > > >
> > > > > How can I diagnose the problem? I find it interesting that it works
> > > > > under mod_fastcgi but not mod_fcgid; I'm not sure whether that
> > implies
> > > > > the problem is in mod_fcgid, however.
> > > > >
> > > > >
> > > > > Footnotes:
> > > > > [1]  The script is written in Chicken Scheme, by the way.
> > > > >
> > > > >
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> > > --
> > > Born in Roswell... married an alien...
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> >
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> 
> 
> -- 
> Born in Roswell... married an alien...
> http://emptyhammock.com/