[ViewVC] Annotation of: cvs/AnyEvent-Fork/README

NAME
    AnyEvent::Fork - everything you wanted to use fork() for, but couldn't

SYNOPSIS
       use AnyEvent::Fork;

       ##################################################################
       # create a single new process, tell it to run your worker function

       AnyEvent::Fork
          ->new
          ->require ("MyModule")
          ->run ("MyModule::worker, sub {
             my ($master_filehandle) = @_;

             # now $master_filehandle is connected to the
             # $slave_filehandle in the new process.
          });

       # MyModule::worker might look like this
       sub MyModule::worker {
          my ($slave_filehandle) = @_;

          # now $slave_filehandle is connected to the $master_filehandle
          # in the original prorcess. have fun!
       }

       ##################################################################
       # create a pool of server processes all accepting on the same socket

       # create listener socket
       my $listener = ...;

       # create a pool template, initialise it and give it the socket
       my $pool = AnyEvent::Fork
                     ->new
                     ->require ("Some::Stuff", "My::Server")
                     ->send_fh ($listener);

       # now create 10 identical workers
       for my $id (1..10) {
          $pool
             ->fork
             ->send_arg ($id)
             ->run ("My::Server::run");
       }

       # now do other things - maybe use the filehandle provided by run
       # to wait for the processes to die. or whatever.

       # My::Server::run might look like this
       sub My::Server::run {
          my ($slave, $listener, $id) = @_;

          close $slave; # we do not use the socket, so close it to save resources

          # we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
          # or anything we usually couldn't do in a process forked normally.
          while (my $socket = $listener->accept) {
             # do sth. with new socket
          }
       }

DESCRIPTION
    This module allows you to create new processes, without actually forking
    them from your current process (avoiding the problems of forking), but
    preserving most of the advantages of fork.

    It can be used to create new worker processes or new independent
    subprocesses for short- and long-running jobs, process pools (e.g. for
    use in pre-forked servers) but also to spawn new external processes
    (such as CGI scripts from a webserver), which can be faster (and more
    well behaved) than using fork+exec in big processes.

    Special care has been taken to make this module useful from other
    modules, while still supporting specialised environments such as
    App::Staticperl or PAR::Packer.

PROBLEM STATEMENT
    There are two ways to implement parallel processing on UNIX like
    operating systems - fork and process, and fork+exec and process. They
    have different advantages and disadvantages that I describe below,
    together with how this module tries to mitigate the disadvantages.

    Forking from a big process can be very slow (a 5GB process needs 0.05s
    to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is
    often shared with exec (because you have to fork first), but in some
    circumstances (e.g. when vfork is used), fork+exec can be much faster.
        This module can help here by telling a small(er) helper process to
        fork, or fork+exec instead.

    Forking usually creates a copy-on-write copy of the parent process.
    Memory (for example, modules or data files that have been will not take
    additional memory). When exec'ing a new process, modules and data files
    might need to be loaded again, at extra cpu and memory cost. Likewise
    when forking, all data structures are copied as well - if the program
    frees them and replaces them by new data, the child processes will
    retain the memory even if it isn't used.
        This module allows the main program to do a controlled fork, and
        allows modules to exec processes safely at any time. When creating a
        custom process pool you can take advantage of data sharing via fork
        without risking to share large dynamic data structures that will
        blow up child memory usage.

    Exec'ing a new perl process might be difficult and slow. For example, it
    is not easy to find the correct path to the perl interpreter, and all
    modules have to be loaded from disk again. Long running processes might
    run into problems when perl is upgraded for example.
        This module supports creating pre-initialised perl processes to be
        used as template, and also tries hard to identify the correct path
        to the perl interpreter. With a cooperative main program, exec'ing
        the interpreter might not even be necessary.

    Forking might be impossible when a program is running. For example,
    POSIX makes it almost impossible to fork from a multithreaded program
    and do anything useful in the child - strictly speaking, if your perl
    program uses posix threads (even indirectly via e.g. IO::AIO or
    threads), you cannot call fork on the perl level anymore, at all.
        This module can safely fork helper processes at any time, by caling
        fork+exec in C, in a POSIX-compatible way.

    Parallel processing with fork might be inconvenient or difficult to
    implement. For example, when a program uses an event loop and creates
    watchers it becomes very hard to use the event loop from a child
    program, as the watchers already exist but are only meaningful in the
    parent. Worse, a module might want to use such a system, not knowing
    whether another module or the main program also does, leading to
    problems.
        This module only lets the main program create pools by forking
        (because only the main program can know when it is still safe to do
        so) - all other pools are created by fork+exec, after which such
        modules can again be loaded.

CONCEPTS
    This module can create new processes either by executing a new perl
    process, or by forking from an existing "template" process.

    Each such process comes with its own file handle that can be used to
    communicate with it (it's actually a socket - one end in the new
    process, one end in the main process), and among the things you can do
    in it are load modules, fork new processes, send file handles to it, and
    execute functions.

    There are multiple ways to create additional processes to execute some
    jobs:

    fork a new process from the "default" template process, load code, run
    it
        This module has a "default" template process which it executes when
        it is needed the first time. Forking from this process shares the
        memory used for the perl interpreter with the new process, but
        loading modules takes time, and the memory is not shared with
        anything else.

        This is ideal for when you only need one extra process of a kind,
        with the option of starting and stipping it on demand.

        Example:

           AnyEvent::Fork
              ->new
              ->require ("Some::Module")
              ->run ("Some::Module::run", sub {
                 my ($fork_fh) = @_;
              });

    fork a new template process, load code, then fork processes off of it
    and run the code
        When you need to have a bunch of processes that all execute the same
        (or very similar) tasks, then a good way is to create a new template
        process for them, loading all the modules you need, and then create
        your worker processes from this new template process.

        This way, all code (and data structures) that can be shared (e.g.
        the modules you loaded) is shared between the processes, and each
        new process consumes relatively little memory of its own.

        The disadvantage of this approach is that you need to create a
        template process for the sole purpose of forking new processes from
        it, but if you only need a fixed number of proceses you can create
        them, and then destroy the template process.

        Example:

           my $template = AnyEvent::Fork->new->require ("Some::Module");
   
           for (1..10) {
              $template->fork->run ("Some::Module::run", sub {
                 my ($fork_fh) = @_;
              });
           }

           # at this point, you can keep $template around to fork new processes
           # later, or you can destroy it, which causes it to vanish.

    execute a new perl interpreter, load some code, run it
        This is relatively slow, and doesn't allow you to share memory
        between multiple processes.

        The only advantage is that you don't have to have a template process
        hanging around all the time to fork off some new processes, which
        might be an advantage when there are long time spans where no extra
        processes are needed.

        Example:

           AnyEvent::Fork
              ->new_exec
              ->require ("Some::Module")
              ->run ("Some::Module::run", sub {
                 my ($fork_fh) = @_;
              });

FUNCTIONS
    my $pool = new AnyEvent::Fork key => value...
        Create a new process pool. The following named parameters are
        supported:

    my $proc = new AnyEvent::Fork
        Create a new "empty" perl interpreter process and returns its
        process object for further manipulation.

        The new process is forked from a template process that is kept
        around for this purpose. When it doesn't exist yet, it is created by
        a call to "new_exec" and kept around for future calls.

        When the process object is destroyed, it will release the file
        handle that connects it with the new process. When the new process
        has not yet called "run", then the process will exit. Otherwise,
        what happens depends entirely on the code that is executed.

    $new_proc = $proc->fork
        Forks $proc, creating a new process, and returns the process object
        of the new process.

        If any of the "send_" functions have been called before fork, then
        they will be cloned in the child. For example, in a pre-forked
        server, you might "send_fh" the listening socket into the template
        process, and then keep calling "fork" and "run".

    my $proc = new_exec AnyEvent::Fork
        Create a new "empty" perl interpreter process and returns its
        process object for further manipulation.

        Unlike the "new" method, this method *always* spawns a new perl
        process (except in some cases, see AnyEvent::Fork::Early for
        details). This reduces the amount of memory sharing that is
        possible, and is also slower.

        You should use "new" whenever possible, except when having a
        template process around is unacceptable.

        The path to the perl interpreter is divined usign various methods -
        first $^X is investigated to see if the path ends with something
        that sounds as if it were the perl interpreter. Failing this, the
        module falls back to using $Config::Config{perlpath}.

    $proc = $proc->eval ($perlcode, @args)
        Evaluates the given $perlcode as ... perl code, while setting @_ to
        the strings specified by @args.

        This call is meant to do any custom initialisation that might be
        required (for example, the "require" method uses it). It's not
        supposed to be used to completely take over the process, use "run"
        for that.

        The code will usually be executed after this call returns, and there
        is no way to pass anything back to the calling process. Any
        evaluation errors will be reported to stderr and cause the process
        to exit.

        Returns the process object for easy chaining of method calls.

    $proc = $proc->require ($module, ...)
        Tries to load the given module(s) into the process

        Returns the process object for easy chaining of method calls.

    $proc = $proc->send_fh ($handle, ...)
        Send one or more file handles (*not* file descriptors) to the
        process, to prepare a call to "run".

        The process object keeps a reference to the handles until this is
        done, so you must not explicitly close the handles. This is most
        easily accomplished by simply not storing the file handles anywhere
        after passing them to this method.

        Returns the process object for easy chaining of method calls.

        Example: pass an fh to a process, and release it without closing. it
        will be closed automatically when it is no longer used.

           $proc->send_fh ($my_fh);
           undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT

    $proc = $proc->send_arg ($string, ...)
        Send one or more argument strings to the process, to prepare a call
        to "run". The strings can be any octet string.

        Returns the process object for easy chaining of emthod calls.

    $proc->run ($func, $cb->($fh))
        Enter the function specified by the fully qualified name in $func in
        the process. The function is called with the communication socket as
        first argument, followed by all file handles and string arguments
        sent earlier via "send_fh" and "send_arg" methods, in the order they
        were called.

        If the called function returns, the process exits.

        Preparing the process can take time - when the process is ready, the
        callback is invoked with the local communications socket as
        argument.

        The process object becomes unusable on return from this function.

        If the communication socket isn't used, it should be closed on both
        sides, to save on kernel memory.

        The socket is non-blocking in the parent, and blocking in the newly
        created process. The close-on-exec flag is set on both. Even if not
        used otherwise, the socket can be a good indicator for the existance
        of the process - if the other process exits, you get a readable
        event on it, because exiting the process closes the socket (if it
        didn't create any children using fork).

        Example: create a template for a process pool, pass a few strings,
        some file handles, then fork, pass one more string, and run some
        code.

           my $pool = AnyEvent::Fork
                         ->new
                         ->send_arg ("str1", "str2")
                         ->send_fh ($fh1, $fh2);

           for (1..2) {
              $pool
                 ->fork
                 ->send_arg ("str3")
                 ->run ("Some::function", sub {
                    my ($fh) = @_;

                    # fh is nonblocking, but we trust that the OS can accept these
                    # extra 3 octets anyway.
                    syswrite $fh, "hi #$_\n";

                    # $fh is being closed here, as we don't store it anywhere
                 });
           }

           # Some::function might look like this - all parameters passed before fork
           # and after will be passed, in order, after the communications socket.
           sub Some::function {
              my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;

              print scalar <$fh>; # prints "hi 1\n" and "hi 2\n"
           }

TYPICAL PROBLEMS
    This section lists typical problems that remain. I hope by recognising
    them, most can be avoided.

    "leaked" file descriptors for exec'ed processes
        POSIX systems inherit file descriptors by default when exec'ing a
        new process. While perl itself laudably sets the close-on-exec flags
        on new file handles, most C libraries don't care, and even if all
        cared, it's often not possible to set the flag in a race-free
        manner.

        That means some file descriptors can leak through. And since it
        isn't possible to know which file descriptors are "good" and
        "neccessary" (or even to know which file descreiptors are open),
        there is no good way to close the ones that might harm.

        As an example of what "harm" can be done consider a web server that
        accepts connections and afterwards some module uses AnyEvent::Fork
        for the first time, causing it to fork and exec a new process, which
        might inherit the network socket. When the server closes the socket,
        it is still open in the child (which doesn't even know that) and the
        client might conclude that the connection is still fine.

        For the main program, there are multiple remedies available -
        AnyEvent::Fork::Early is one, creating a process early and not using
        "new_exec" is another, as in both cases, the first process can be
        exec'ed well before many random file descriptors are open.

        In general, the solution for these kind of problems is to fix the
        libraries or the code that leaks those file descriptors.

        Fortunately, most of these lekaed descriptors do no harm, other than
        sitting on some resources.

    "leaked" file descriptors for fork'ed processes
        Normally, AnyEvent::Fork does start new processes by exec'ing them,
        which closes file descriptors not marked for being inherited.

        However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
        way to create these processes by forking, and this leaks more file
        descriptors than exec'ing them, as there is no way to mark
        descriptors as "close on fork".

        An example would be modules like EV, IO::AIO or Gtk2. Both create
        pipes for internal uses, and Gtk2 might open a connection to the X
        server. EV and IO::AIO can deal with fork, but Gtk2 might have
        trouble with a fork.

        The solution is to either not load these modules before use'ing
        AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
        initialising them, for example, by calling "init Gtk2" manually.

PORTABILITY NOTES
    Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a
    nop, and ::Template is not going to work), and it cost a lot of blood
    and sweat to make it so, mostly due to the bloody broken perl that
    nobody seems to care about. The fork emulation is a bad joke - I have
    yet to see something useful that you cna do with it without running into
    memory corruption issues or other braindamage. Hrrrr.

    Cygwin perl is not supported at the moment, as it should implement fd
    passing, but doesn't, and rolling my own is hard, as cygwin doesn't
    support enough functionality to do it.

SEE ALSO
    AnyEvent::Fork::Early (to avoid executing a perl interpreter),
    AnyEvent::Fork::Template (to create a process by forking the main
    program at a convenient time).

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://home.schmorp.de/

Revision:	1.3
Committed:	Fri Apr 5 19:10:10 2013 UTC (11 years, 1 month ago) by root
Branch:	MAIN
CVS Tags:	rel-0_2
Changes since 1.2:	+57 -3 lines
Log Message:	0.2
#	User	Rev	Content
1	root	1.2	NAME
2			AnyEvent::Fork - everything you wanted to use fork() for, but couldn't
3
4			SYNOPSIS
5			use AnyEvent::Fork;
6
7			##################################################################
8			# create a single new process, tell it to run your worker function
9
10			AnyEvent::Fork
11			->new
12			->require ("MyModule")
13			->run ("MyModule::worker, sub {
14			my ($master_filehandle) = @_;
15
16			# now $master_filehandle is connected to the
17			# $slave_filehandle in the new process.
18			});
19
20			# MyModule::worker might look like this
21			sub MyModule::worker {
22			my ($slave_filehandle) = @_;
23
24			# now $slave_filehandle is connected to the $master_filehandle
25			# in the original prorcess. have fun!
26			}
27
28			##################################################################
29			# create a pool of server processes all accepting on the same socket
30
31			# create listener socket
32			my $listener = ...;
33
34			# create a pool template, initialise it and give it the socket
35			my $pool = AnyEvent::Fork
36			->new
37			->require ("Some::Stuff", "My::Server")
38			->send_fh ($listener);
39
40			# now create 10 identical workers
41			for my $id (1..10) {
42			$pool
43			->fork
44			->send_arg ($id)
45			->run ("My::Server::run");
46			}
47
48			# now do other things - maybe use the filehandle provided by run
49			# to wait for the processes to die. or whatever.
50
51			# My::Server::run might look like this
52			sub My::Server::run {
53			my ($slave, $listener, $id) = @_;
54
55			close $slave; # we do not use the socket, so close it to save resources
56
57			# we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
58			# or anything we usually couldn't do in a process forked normally.
59			while (my $socket = $listener->accept) {
60			# do sth. with new socket
61			}
62			}
63
64			DESCRIPTION
65			This module allows you to create new processes, without actually forking
66			them from your current process (avoiding the problems of forking), but
67			preserving most of the advantages of fork.
68
69			It can be used to create new worker processes or new independent
70			subprocesses for short- and long-running jobs, process pools (e.g. for
71			use in pre-forked servers) but also to spawn new external processes
72			(such as CGI scripts from a webserver), which can be faster (and more
73			well behaved) than using fork+exec in big processes.
74
75			Special care has been taken to make this module useful from other
76			modules, while still supporting specialised environments such as
77			App::Staticperl or PAR::Packer.
78
79			PROBLEM STATEMENT
80			There are two ways to implement parallel processing on UNIX like
81			operating systems - fork and process, and fork+exec and process. They
82			have different advantages and disadvantages that I describe below,
83			together with how this module tries to mitigate the disadvantages.
84
85			Forking from a big process can be very slow (a 5GB process needs 0.05s
86			to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is
87			often shared with exec (because you have to fork first), but in some
88			circumstances (e.g. when vfork is used), fork+exec can be much faster.
89			This module can help here by telling a small(er) helper process to
90			fork, or fork+exec instead.
91
92			Forking usually creates a copy-on-write copy of the parent process.
93			Memory (for example, modules or data files that have been will not take
94			additional memory). When exec'ing a new process, modules and data files
95			might need to be loaded again, at extra cpu and memory cost. Likewise
96			when forking, all data structures are copied as well - if the program
97			frees them and replaces them by new data, the child processes will
98			retain the memory even if it isn't used.
99			This module allows the main program to do a controlled fork, and
100			allows modules to exec processes safely at any time. When creating a
101			custom process pool you can take advantage of data sharing via fork
102			without risking to share large dynamic data structures that will
103			blow up child memory usage.
104
105			Exec'ing a new perl process might be difficult and slow. For example, it
106			is not easy to find the correct path to the perl interpreter, and all
107			modules have to be loaded from disk again. Long running processes might
108			run into problems when perl is upgraded for example.
109			This module supports creating pre-initialised perl processes to be
110			used as template, and also tries hard to identify the correct path
111			to the perl interpreter. With a cooperative main program, exec'ing
112			the interpreter might not even be necessary.
113
114			Forking might be impossible when a program is running. For example,
115			POSIX makes it almost impossible to fork from a multithreaded program
116			and do anything useful in the child - strictly speaking, if your perl
117			program uses posix threads (even indirectly via e.g. IO::AIO or
118			threads), you cannot call fork on the perl level anymore, at all.
119			This module can safely fork helper processes at any time, by caling
120			fork+exec in C, in a POSIX-compatible way.
121
122			Parallel processing with fork might be inconvenient or difficult to
123			implement. For example, when a program uses an event loop and creates
124			watchers it becomes very hard to use the event loop from a child
125			program, as the watchers already exist but are only meaningful in the
126			parent. Worse, a module might want to use such a system, not knowing
127			whether another module or the main program also does, leading to
128			problems.
129			This module only lets the main program create pools by forking
130			(because only the main program can know when it is still safe to do
131			so) - all other pools are created by fork+exec, after which such
132			modules can again be loaded.
133
134			CONCEPTS
135			This module can create new processes either by executing a new perl
136			process, or by forking from an existing "template" process.
137
138			Each such process comes with its own file handle that can be used to
139			communicate with it (it's actually a socket - one end in the new
140			process, one end in the main process), and among the things you can do
141			in it are load modules, fork new processes, send file handles to it, and
142			execute functions.
143
144			There are multiple ways to create additional processes to execute some
145			jobs:
146
147			fork a new process from the "default" template process, load code, run
148			it
149			This module has a "default" template process which it executes when
150			it is needed the first time. Forking from this process shares the
151			memory used for the perl interpreter with the new process, but
152			loading modules takes time, and the memory is not shared with
153			anything else.
154
155			This is ideal for when you only need one extra process of a kind,
156			with the option of starting and stipping it on demand.
157
158			Example:
159
160			AnyEvent::Fork
161			->new
162			->require ("Some::Module")
163			->run ("Some::Module::run", sub {
164			my ($fork_fh) = @_;
165			});
166
167			fork a new template process, load code, then fork processes off of it
168			and run the code
169			When you need to have a bunch of processes that all execute the same
170			(or very similar) tasks, then a good way is to create a new template
171			process for them, loading all the modules you need, and then create
172			your worker processes from this new template process.
173
174			This way, all code (and data structures) that can be shared (e.g.
175			the modules you loaded) is shared between the processes, and each
176			new process consumes relatively little memory of its own.
177
178			The disadvantage of this approach is that you need to create a
179			template process for the sole purpose of forking new processes from
180			it, but if you only need a fixed number of proceses you can create
181			them, and then destroy the template process.
182
183			Example:
184
185			my $template = AnyEvent::Fork->new->require ("Some::Module");
186
187			for (1..10) {
188			$template->fork->run ("Some::Module::run", sub {
189			my ($fork_fh) = @_;
190			});
191			}
192
193			# at this point, you can keep $template around to fork new processes
194			# later, or you can destroy it, which causes it to vanish.
195
196			execute a new perl interpreter, load some code, run it
197			This is relatively slow, and doesn't allow you to share memory
198			between multiple processes.
199
200			The only advantage is that you don't have to have a template process
201			hanging around all the time to fork off some new processes, which
202			might be an advantage when there are long time spans where no extra
203			processes are needed.
204
205			Example:
206
207			AnyEvent::Fork
208			->new_exec
209			->require ("Some::Module")
210			->run ("Some::Module::run", sub {
211			my ($fork_fh) = @_;
212			});
213
214			FUNCTIONS
215			my $pool = new AnyEvent::Fork key => value...
216			Create a new process pool. The following named parameters are
217			supported:
218
219			my $proc = new AnyEvent::Fork
220			Create a new "empty" perl interpreter process and returns its
221			process object for further manipulation.
222
223			The new process is forked from a template process that is kept
224			around for this purpose. When it doesn't exist yet, it is created by
225			a call to "new_exec" and kept around for future calls.
226
227			When the process object is destroyed, it will release the file
228			handle that connects it with the new process. When the new process
229			has not yet called "run", then the process will exit. Otherwise,
230			what happens depends entirely on the code that is executed.
231
232			$new_proc = $proc->fork
233			Forks $proc, creating a new process, and returns the process object
234			of the new process.
235
236			If any of the "send_" functions have been called before fork, then
237			they will be cloned in the child. For example, in a pre-forked
238			server, you might "send_fh" the listening socket into the template
239			process, and then keep calling "fork" and "run".
240
241			my $proc = new_exec AnyEvent::Fork
242			Create a new "empty" perl interpreter process and returns its
243			process object for further manipulation.
244
245			Unlike the "new" method, this method always spawns a new perl
246			process (except in some cases, see AnyEvent::Fork::Early for
247			details). This reduces the amount of memory sharing that is
248			possible, and is also slower.
249
250			You should use "new" whenever possible, except when having a
251			template process around is unacceptable.
252
253			The path to the perl interpreter is divined usign various methods -
254			first $^X is investigated to see if the path ends with something
255			that sounds as if it were the perl interpreter. Failing this, the
256			module falls back to using $Config::Config{perlpath}.
257
258			$proc = $proc->eval ($perlcode, @args)
259			Evaluates the given $perlcode as ... perl code, while setting @_ to
260			the strings specified by @args.
261
262			This call is meant to do any custom initialisation that might be
263			required (for example, the "require" method uses it). It's not
264			supposed to be used to completely take over the process, use "run"
265			for that.
266
267			The code will usually be executed after this call returns, and there
268			is no way to pass anything back to the calling process. Any
269			evaluation errors will be reported to stderr and cause the process
270			to exit.
271
272			Returns the process object for easy chaining of method calls.
273
274			$proc = $proc->require ($module, ...)
275			Tries to load the given module(s) into the process
276
277			Returns the process object for easy chaining of method calls.
278
279			$proc = $proc->send_fh ($handle, ...)
280			Send one or more file handles (not file descriptors) to the
281			process, to prepare a call to "run".
282
283			The process object keeps a reference to the handles until this is
284			done, so you must not explicitly close the handles. This is most
285			easily accomplished by simply not storing the file handles anywhere
286			after passing them to this method.
287
288			Returns the process object for easy chaining of method calls.
289
290			Example: pass an fh to a process, and release it without closing. it
291			will be closed automatically when it is no longer used.
292
293			$proc->send_fh ($my_fh);
294			undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
295
296			$proc = $proc->send_arg ($string, ...)
297			Send one or more argument strings to the process, to prepare a call
298			to "run". The strings can be any octet string.
299
300			Returns the process object for easy chaining of emthod calls.
301
302			$proc->run ($func, $cb->($fh))
303			Enter the function specified by the fully qualified name in $func in
304			the process. The function is called with the communication socket as
305			first argument, followed by all file handles and string arguments
306			sent earlier via "send_fh" and "send_arg" methods, in the order they
307			were called.
308
309			If the called function returns, the process exits.
310
311			Preparing the process can take time - when the process is ready, the
312			callback is invoked with the local communications socket as
313			argument.
314
315			The process object becomes unusable on return from this function.
316
317			If the communication socket isn't used, it should be closed on both
318			sides, to save on kernel memory.
319
320			The socket is non-blocking in the parent, and blocking in the newly
321			created process. The close-on-exec flag is set on both. Even if not
322			used otherwise, the socket can be a good indicator for the existance
323			of the process - if the other process exits, you get a readable
324			event on it, because exiting the process closes the socket (if it
325			didn't create any children using fork).
326
327			Example: create a template for a process pool, pass a few strings,
328			some file handles, then fork, pass one more string, and run some
329			code.
330
331			my $pool = AnyEvent::Fork
332			->new
333			->send_arg ("str1", "str2")
334			->send_fh ($fh1, $fh2);
335
336			for (1..2) {
337			$pool
338			->fork
339			->send_arg ("str3")
340			->run ("Some::function", sub {
341			my ($fh) = @_;
342
343			# fh is nonblocking, but we trust that the OS can accept these
344			# extra 3 octets anyway.
345			syswrite $fh, "hi #$_\n";
346
347			# $fh is being closed here, as we don't store it anywhere
348			});
349			}
350
351			# Some::function might look like this - all parameters passed before fork
352			# and after will be passed, in order, after the communications socket.
353			sub Some::function {
354			my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
355
356			print scalar <$fh>; # prints "hi 1\n" and "hi 2\n"
357			}
358
359	root	1.3	TYPICAL PROBLEMS
360			This section lists typical problems that remain. I hope by recognising
361			them, most can be avoided.
362
363			"leaked" file descriptors for exec'ed processes
364			POSIX systems inherit file descriptors by default when exec'ing a
365			new process. While perl itself laudably sets the close-on-exec flags
366			on new file handles, most C libraries don't care, and even if all
367			cared, it's often not possible to set the flag in a race-free
368			manner.
369
370			That means some file descriptors can leak through. And since it
371			isn't possible to know which file descriptors are "good" and
372			"neccessary" (or even to know which file descreiptors are open),
373			there is no good way to close the ones that might harm.
374
375			As an example of what "harm" can be done consider a web server that
376			accepts connections and afterwards some module uses AnyEvent::Fork
377			for the first time, causing it to fork and exec a new process, which
378			might inherit the network socket. When the server closes the socket,
379			it is still open in the child (which doesn't even know that) and the
380			client might conclude that the connection is still fine.
381
382			For the main program, there are multiple remedies available -
383			AnyEvent::Fork::Early is one, creating a process early and not using
384			"new_exec" is another, as in both cases, the first process can be
385			exec'ed well before many random file descriptors are open.
386
387			In general, the solution for these kind of problems is to fix the
388			libraries or the code that leaks those file descriptors.
389
390			Fortunately, most of these lekaed descriptors do no harm, other than
391			sitting on some resources.
392
393			"leaked" file descriptors for fork'ed processes
394			Normally, AnyEvent::Fork does start new processes by exec'ing them,
395			which closes file descriptors not marked for being inherited.
396
397			However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
398			way to create these processes by forking, and this leaks more file
399			descriptors than exec'ing them, as there is no way to mark
400			descriptors as "close on fork".
401
402			An example would be modules like EV, IO::AIO or Gtk2. Both create
403			pipes for internal uses, and Gtk2 might open a connection to the X
404			server. EV and IO::AIO can deal with fork, but Gtk2 might have
405			trouble with a fork.
406
407			The solution is to either not load these modules before use'ing
408			AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
409			initialising them, for example, by calling "init Gtk2" manually.
410
411	root	1.2	PORTABILITY NOTES
412			Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a
413			nop, and ::Template is not going to work), and it cost a lot of blood
414			and sweat to make it so, mostly due to the bloody broken perl that
415			nobody seems to care about. The fork emulation is a bad joke - I have
416			yet to see something useful that you cna do with it without running into
417			memory corruption issues or other braindamage. Hrrrr.
418
419			Cygwin perl is not supported at the moment, as it should implement fd
420			passing, but doesn't, and rolling my own is hard, as cygwin doesn't
421			support enough functionality to do it.
422
423	root	1.3	SEE ALSO
424			AnyEvent::Fork::Early (to avoid executing a perl interpreter),
425			AnyEvent::Fork::Template (to create a process by forking the main
426			program at a convenient time).
427
428	root	1.2	AUTHOR
429			Marc Lehmann <schmorp@schmorp.de>
430			http://home.schmorp.de/
431