10.2.1. Forking a New Process

When using fork( ), you should check its return value, since a return of undef it means that the call was unsuccessful and no process was spawned. This can happen for example, when the system is already running too many processes and cannot spawn new ones.

When the process is successfully forked, the parent receives the PID of the newly spawned child as a returned value of the fork( ) call and the child receives 0. Now the program splits into two. In the above example, the code inside the first block after if will be executed by the parent, and the code inside the first block after else will be executed by the child.

It's important not to forget to explicitly call exit( ) at the end of the child code when forking. If you don't and there is some code outside the if...else block, the child process will execute it as well. But under mod_perl there is another nuance—you must use CORE::exit( ) and not exit( ), which would be automatically overriden by Apache::exit( ) if used in conjunction with Apache::Registry and similar modules. You want the spawned process to quit when its work is done, or it'll just stay alive, using resources and doing nothing.

The parent process usually completes its execution and returns to the pool of free servers to wait for a new assignment. If the execution is to be aborted earlier for some reason, you should use Apache::exit( ) or die( ). In the case of Apache::Registry or Apache::PerlRun handlers, a simple exit( ) will do the right thing.

10.2.2. Freeing the Parent Process

Under mod_perl, the spawned process also inherits the file descriptor that's tied to the socket through which all the communications between the server and the client pass. Therefore, you need to free this stream in the forked process. If you don't, the server can't be restarted while the spawned process is still running. If you attempt to restart the server, you will get the following error:

[Mon May 20 23:04:11 2002] [crit] 
(98)Address already in use: make_sock:
  could not bind to address 127.0.0.1 port 8000

Of course, the real code should be placed between freeing the parent code and the child process termination.

10.2.3. Detaching the Forked Process

Now the spawned child process has a life of its own, and it doesn't depend on the parent any more.

10.2.4. Avoiding Zombie Processes

A zombie is a process that doesn't have a parent. When the child quits, it reports the termination to its parent. If no parent wait( )s to collect the exit status of the child, it gets confused and becomes a ghost process that can be seen as a process but not killed. It will be killed only when you stop the parent process that spawned it.

Generally, the ps(1) utility displays these processes with the <defunc> tag, and you may see the zombies counter increment when using top( ). These zombie processes can take up system resources and are generally undesirable.

The simplest solution is to ignore your dead children. Just add this line before the fork( ) call:

$SIG{CHLD} = 'IGNORE';

When you set the CHLD (SIGCHLD in C) signal handler to 'IGNORE', all the processes will be collected by the init process and therefore will be prevented from becoming zombies. This doesn't work everywhere, but it has been proven to work at least on Linux.

Note that you cannot localize this setting with local( ). If you try, it won't have the desired effect.

Note that the waitpid( ) call is gone. The $SIG{CHLD} = 'IGNORE'; statement protects us from zombies, as explained above.

Grandkid becomes a child of init—i.e., a child of the process whose PID is 1.

Note that the previous two solutions do allow you to determine the exit status of the process, but in our example, we don't care about it.

Yet another solution is to use a different SIGCHLD handler:

use POSIX 'WNOHANG';
$SIG{CHLD} = sub { while( waitpid(-1,WNOHANG)>0 ) {  } };

This is useful when you fork( ) more than one process. The handler could call wait( ) as well, but for a variety of reasons involving the handling of stopped processes and the rare event in which two children exit at nearly the same moment, the best technique is to call waitpid( ) in a tight loop with a first argument of -1 and a second argument of WNOHANG. Together these arguments tell waitpid( ) to reap the next child that's available and prevent the call from blocking if there happens to be no child ready for reaping. The handler will loop until waitpid( ) returns a negative number or zero, indicating that no more reapable children remain.

While testing and debugging code that uses one of the above examples, you might want to write debug information to the error_log file so that you know what's happening.

Read the perlipc manpage for more information about signal handlers.

10.2.5. A Complete Fork Example

The script starts with the usual declaration of strict mode, then loads the POSIX and Apache::SubProcess modules and imports the setsid( )symbol from the POSIX package.

The HTTP header is sent next, with the Content-Type of text/plain. To avoid zombies, the parent process gets ready to ignore the child, and the fork is called.

The if condition evaluates to a true value for the parent process and to a false value for the child process; therefore, the first block is executed by the parent and the second by the child.

The parent process announces its PID and the PID of the spawned process, and finishes its block. If there is any code outside the ifstatement, it will be executed by the parent as well.

The child process starts its code by disconnecting from the socket, changing its current directory to /, and opening the STDIN and STDOUTstreams to /dev/null (this has the effect of closing them both before opening them). In fact, in this example we don't need either of these, so we could just close( ) both. The child process completes its disengagement from the parent process by opening the STDERRstream to /tmp/log, so it can write to that file, and creates a new session with the help of setsid( ). Now the child process has nothing to do with the parent process and can do the actual processing that it has to do. In our example, it outputs a series of warnings, which are logged to /tmp/log:

my $oldfh = select STDERR;
local $| = 1;
select $oldfh;
warn "started\n";
# do something time-consuming
sleep 1, warn "$_\n" for 1..20;
warn "completed\n";

We set $|=1 to unbuffer the STDERRstream, so we can immediately see the debug output generated by the program. We use the keyword localso that buffering in other processes is not affected. In fact, we don't really need to unbuffer output when it is generated by warn( ). You want it if you use print( ) to debug.

Finally, the child process terminates by calling:

CORE::exit(0);

which makes sure that it terminates at the end of the block and won't run some code that it's not supposed to run.

This code example will allow you to verify that indeed the spawned child process has its own life, and that its parent is free as well. Simply issue a request that will run this script, see that the process starts writing warnings to the file /tmp/log, and issue a complete server stop and start. If everything is correct, the server will successfully restart and the long-term process will still be running. You will know that it's still running if the warnings are still being written into /tmp/log. If Apache takes a long time to stop and restart, you may need to raise the number of warnings to make sure that you don't miss the end of the run.

If there are only five warnings to be printed, you should see the following output in the /tmp/log file:

started
1
2
3
4
5
completed

10.2.6. Starting a Long-Running External Program

When using any of these methods, and when taint mode is enabled, we must also add the following code to untaint the PATH environment variable and delete a few other insecure environment variables. This information can be found in the perlsec manpage.

$ENV{'PATH'} = '/bin:/usr/bin';
delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};

Now all we have to do is reuse the code from the previous section.

Notice that exec( ) never returns unless it fails to start the process. Therefore you shouldn't put any code after exec( )—it will not be executed in the case of success. Use system( ) or backticks instead if you want to continue doing other things in the process. But then you probably will want to terminate the process after the program has finished, so you will have to write:

system "/home/httpd/perl/external.pl"
    or die "Cannot execute system: $!";
CORE::exit(0);

Another important nuance is that we have to close all STDstreams in the forked process, even if the called program does that.

10.2.7. Starting a Short-Running External Program

system "perl -e 'print 5+5'"

You would never call the Perl interperter for doing a simple calculation like this, but for the sake of a simple example it's good enough.

The problem with this approach is that we cannot get the results printed to STDOUT. That's where backticks or qx( ) can help. If you use either:

my $result = `perl -e 'print 5+5'`;

or:

my $result = qx{perl -e 'print 5+5'};

the whole output of the external program will be stored in the $result variable.

Of course, you can use other solutions, such as opening a pipe (|) to the program if you need to submit many arguments. And there are more evolved solutions provided by other Perl modules, such as IPC::Open2 and IPC::Open3, that allow you to open a process for reading, writing, and error handling.

10.2.8. Executing system( ) or exec( ) in the Right Way

system("echo", "Hi");

Perl will use the first argument as a program to execute, find the echo executable along the search path, invoke it directly, and pass the string "Hi" as an argument.

Note that Perl's system( ) is not the same as the standard libc system(3) call.

If there is more than one argument to system( ) or exec( ), or the argument is an array with more than one element in it, the arguments are passed directly to the C-level functions. When the argument is a single scalar or an array with only a single scalar in it, it will first be checked to see if it contains any shell metacharacters (e.g., *, ?). If there are any, the Perl interpreter invokes a real shell program (/bin/sh -c on Unix platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to the C level, which is more efficient.

In other words, only if you do:

system "echo *"

will Perl actually exec( ) a copy of /bin/sh to parse your command, which may incur a slight overhead on certain OSes.

It's especially important to remember to run your code with taint mode enabled when system( ) or exec( ) is called using a single argument. There can be bad consequences if user input gets to the shell without proper laundering first. Taint mode will alert you when such a condition happens.

Perl will try to do the most efficient thing no matter how the arguments are passed, and the additional overhead may be incurred only if you need the shell to expand some metacharacters before doing the actual call.