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List: apache-modperl
Subject: RE: mp2, IPC::Run3 && Environment Variables giving "variable not set" error
From: "Berg, Eric" <eric.berg () lehman ! com>
Date: 2008-08-16 19:35:27
Message-ID: 5CAFF755525FC5418CCF35FAEB2683BE12A319F4 () njpcmg1exms305 ! leh ! lbcorp ! lehman ! com
[Download RAW message or body]
I've taken a few minutes to regurgitate this into something that I can submit to the \
documentation project. Mind taking a look and giving me some feedback (anyone) \
before I do?
> > > > > > > > > > > > > >
=head1 Issues with Environment Variables
There are several thread-safety issues with setting environment
variables.
=head2 Environment Variables and C Code
Environment variables set during request time won't be seen by C
code. See the L<DBD::Oracle
issue|docs::2.0::user::troubleshooting::troubleshooting/C_Libraries_Don_t_See_C__ENV__Entries_Set_by_Perl_Code> \
for possible workarounds.
=head2 Environment Variables and Forked Processes
Forked processes (accomplished using backticks, system, or exec )
won't see CGI emulation environment variables. That means that the
environment variables as set in the calling context will not be set in
the forked process. This is different from the behavior in Apache
1.3. (META: This will hopefully be resolved in the future, it's
documented in modperl_env.c:modperl_env_magic_set_all.)
Apache, at the time of this writing, does not distinguish between
threaded and forked MPM types, and the processes as which they run.
So all MPMs have a standard behavior which is that the environment is
not passed to the forked process.
The environment is a process-global resource. In Apache terms, that
means that when using a threaded MPM, all of the children are threads
of the parent apache server process, whereas in the preforked MPM, the
children are each their own separate process.
The environment for CGI scripts is kept in a request-specific data
structure called C<subprocess_env>. Remembering that in a threaded
MPM, a forked process is handled by another thread of the parent
process, we know that if the environment in C<subprocess_env> is
changed, it will also change the environment for all other threads of
it's parent process. But not so for a non-threaded MPM (i.e.,
preforked), because each subprocess -- such as the Apache children or
processes forked off of them -- has it's own process, so changing that
doesn't affect the environment of any other process.
That said, if you're running the prefork MPM, then there are several
work arounds that will allow you to pass environment variables to
forked processes from within mod_perl code.
=head3 The Env::C Solution
You can manipulate the environment using the C-level functions. There
is a module called Env::C on CPAN that provides a perl interface to
them.
# To set a single variable for the subprocess
Env::C::setenv( 'ENVTEST', $ENV{ENVTEST}, 1);
# To set all environment variables for the subprocess unless
# they're already set. map { Env::C::setenv( '$_', $ENV{$_} ) } keys %ENV;
=head3 The Apache2::SubProcess Solution
Again, for non-threaded MPMs only: If you use the Apache interface
directly to spawn subprocesses (Apache2::SubProcess) you'll notice
that variables set in subprocess_env do appear as environment
variables of the spawned process. Under mod_perl %ENV is tied to
$r->subprocess_env.
=head3 The "Passing-The-Environment-Variable-On-The-Command-Line" Solution
Again, for non-threaded MPMs only: This is probably just a UNIX thing,
but you can set variables for a process by passing them on the command
line before calling the command. For example, you may have seen this
in the README file of any number of software packages:
C<CC=gcc make>
Again, for non-threaded MPMs only: This tells the shell to set the
environment variable C<CC> to the value "gcc" in the process
in which you run C<make>.
You can do that with your own environment varibles too:
my $cmd = 'MYVAR1="My value" MYVAR2="your value" do_that_thing.cgi';
system( $cmd );
Then MYVAR1 and MYVAR2 will be set to "My value" and "your value",
respectively in the environment of the process in which
do_that_thing.cgi runs.
Links:
http://perl.apache.org/docs/2.0/user/troubleshooting/troubleshooting.html#C_Libraries_Don_t_See_C__ENV__Entries_Set_by_Perl_Code
<<<<<<<<<<<<<<<<<<<<<<<<
Thanks.
Eric
> -----Original Message-----
> From: Torsten Foertsch [mailto:torsten.foertsch@gmx.net]
> Sent: Friday, August 15, 2008 3:15 PM
> To: modperl@perl.apache.org
> Cc: Berg, Eric
> Subject: Re: mp2, IPC::Run3 && Environment Variables giving
> "variable not set" error
>
> On Fri 15 Aug 2008, Berg, Eric wrote:
> > This issue of the environment variables' not being passed to forked
> > processes is turning into a fairly substantial problem for me. We
> > have a number of places in our code -- both the modules that are
> > tightly controlled and fairly easy to manage, as well as a bunch of
> > scripts and CGI's that use them -- that use system or backticks
> > (sometimes via IPC::Run3) to get info from external tools.
> >
> > Does anybody know anything about this issue or about the code
> > involved? How difficult would it be to address this? I took a look
> > at the code in modperl_env.c as suggested in the docs, but
> I'm a Perl
> > guy, not a C guy.
> >
> > This is a deal killer for us.
>
> Let's have a look at the reason for this behavior first. It's always
> better to know what you are doing. Apache 2.x can use various MPMs.
> Some of them use threads instead of processes to achieve parallelism.
> Now, the environment is a process global resource. So, if
> those values
> are changed all threads are affected. This is surely no what you want.
>
> Therefore there is a request specific data structure called
> subprocess_env where environment variables for CGI scripts
> are kept. If
> you use the Apache interface to spawn subprocesses
> (Apache2::SubProcess) you'll notice that variables set in
> subprocess_env appear as environment variables of the spawned
> process.
> Under mod_perl %ENV is tied to $r->subprocess_env.
>
> If you are using the prefork MPM (as you probably do) you can still
> manipulate the environment without affecting other requests. But you
> have to use the C-level functions. There is a module called Env::C on
> CPAN that provides a perl interface to them.
>
> Torsten
>
> --
> Need professional mod_perl support?
> Just hire me: torsten.foertsch@gmx.net
>
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