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Revision 1.6 by root, Wed Apr 3 08:47:44 2013 UTC vs.
Revision 1.30 by root, Sat Apr 6 09:28:45 2013 UTC

3AnyEvent::Fork - everything you wanted to use fork() for, but couldn't 3AnyEvent::Fork - everything you wanted to use fork() for, but couldn't
4 4
5=head1 SYNOPSIS 5=head1 SYNOPSIS
6 6
7 use AnyEvent::Fork; 7 use AnyEvent::Fork;
8
9 AnyEvent::Fork
10 ->new
11 ->require ("MyModule")
12 ->run ("MyModule::server", my $cv = AE::cv);
13
14 my $fh = $cv->recv;
8 15
9=head1 DESCRIPTION 16=head1 DESCRIPTION
10 17
11This module allows you to create new processes, without actually forking 18This module allows you to create new processes, without actually forking
12them from your current process (avoiding the problems of forking), but 19them from your current process (avoiding the problems of forking), but
13preserving most of the advantages of fork. 20preserving most of the advantages of fork.
14 21
15It can be used to create new worker processes or new independent 22It can be used to create new worker processes or new independent
16subprocesses for short- and long-running jobs, process pools (e.g. for use 23subprocesses for short- and long-running jobs, process pools (e.g. for use
17in pre-forked servers) but also to spawn new external processes (such as 24in pre-forked servers) but also to spawn new external processes (such as
18CGI scripts from a webserver), which can be faster (and more well behaved) 25CGI scripts from a web server), which can be faster (and more well behaved)
19than using fork+exec in big processes. 26than using fork+exec in big processes.
20 27
21Special care has been taken to make this module useful from other modules, 28Special care has been taken to make this module useful from other modules,
22while still supporting specialised environments such as L<App::Staticperl> 29while still supporting specialised environments such as L<App::Staticperl>
23or L<PAR::Packer>. 30or L<PAR::Packer>.
24 31
32=head1 WHAT THIS MODULE IS NOT
33
34This module only creates processes and lets you pass file handles and
35strings to it, and run perl code. It does not implement any kind of RPC -
36there is no back channel from the process back to you, and there is no RPC
37or message passing going on.
38
39If you need some form of RPC, you can either implement it yourself
40in whatever way you like, use some message-passing module such
41as L<AnyEvent::MP>, some pipe such as L<AnyEvent::ZeroMQ>, use
42L<AnyEvent::Handle> on both sides to send e.g. JSON or Storable messages,
43and so on.
44
25=head1 PROBLEM STATEMENT 45=head1 PROBLEM STATEMENT
26 46
27There are two ways to implement parallel processing on UNIX like operating 47There are two traditional ways to implement parallel processing on UNIX
28systems - fork and process, and fork+exec and process. They have different 48like operating systems - fork and process, and fork+exec and process. They
29advantages and disadvantages that I describe below, together with how this 49have different advantages and disadvantages that I describe below,
30module tries to mitigate the disadvantages. 50together with how this module tries to mitigate the disadvantages.
31 51
32=over 4 52=over 4
33 53
34=item Forking from a big process can be very slow (a 5GB process needs 54=item Forking from a big process can be very slow.
350.05s to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead 55
56A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box. This
36is often shared with exec (because you have to fork first), but in some 57overhead is often shared with exec (because you have to fork first), but
37circumstances (e.g. when vfork is used), fork+exec can be much faster. 58in some circumstances (e.g. when vfork is used), fork+exec can be much
59faster.
38 60
39This module can help here by telling a small(er) helper process to fork, 61This module can help here by telling a small(er) helper process to fork,
40or fork+exec instead. 62which is faster then forking the main process, and also uses vfork where
63possible. This gives the speed of vfork, with the flexibility of fork.
41 64
42=item Forking usually creates a copy-on-write copy of the parent 65=item Forking usually creates a copy-on-write copy of the parent
43process. Memory (for example, modules or data files that have been 66process.
44will not take additional memory). When exec'ing a new process, modules 67
45and data files might need to be loaded again, at extra cpu and memory 68For example, modules or data files that are loaded will not use additional
46cost. Likewise when forking, all data structures are copied as well - if 69memory after a fork. When exec'ing a new process, modules and data files
70might need to be loaded again, at extra CPU and memory cost. But when
71forking, literally all data structures are copied - if the program frees
47the program frees them and replaces them by new data, the child processes 72them and replaces them by new data, the child processes will retain the
48will retain the memory even if it isn't used. 73old version even if it isn't used, which can suddenly and unexpectedly
74increase memory usage when freeing memory.
75
76The trade-off is between more sharing with fork (which can be good or
77bad), and no sharing with exec.
49 78
50This module allows the main program to do a controlled fork, and allows 79This module allows the main program to do a controlled fork, and allows
51modules to exec processes safely at any time. When creating a custom 80modules to exec processes safely at any time. When creating a custom
52process pool you can take advantage of data sharing via fork without 81process pool you can take advantage of data sharing via fork without
53risking to share large dynamic data structures that will blow up child 82risking to share large dynamic data structures that will blow up child
54memory usage. 83memory usage.
55 84
85In other words, this module puts you into control over what is being
86shared and what isn't, at all times.
87
56=item Exec'ing a new perl process might be difficult and slow. For 88=item Exec'ing a new perl process might be difficult.
89
57example, it is not easy to find the correct path to the perl interpreter, 90For example, it is not easy to find the correct path to the perl
58and all modules have to be loaded from disk again. Long running processes 91interpreter - C<$^X> might not be a perl interpreter at all.
59might run into problems when perl is upgraded for example.
60 92
61This module supports creating pre-initialised perl processes to be used
62as template, and also tries hard to identify the correct path to the perl 93This module tries hard to identify the correct path to the perl
63interpreter. With a cooperative main program, exec'ing the interpreter 94interpreter. With a cooperative main program, exec'ing the interpreter
64might not even be necessary. 95might not even be necessary, but even without help from the main program,
96it will still work when used from a module.
65 97
98=item Exec'ing a new perl process might be slow, as all necessary modules
99have to be loaded from disk again, with no guarantees of success.
100
101Long running processes might run into problems when perl is upgraded
102and modules are no longer loadable because they refer to a different
103perl version, or parts of a distribution are newer than the ones already
104loaded.
105
106This module supports creating pre-initialised perl processes to be used as
107a template for new processes.
108
66=item Forking might be impossible when a program is running. For example, 109=item Forking might be impossible when a program is running.
67POSIX makes it almost impossible to fork from a multithreaded program and
68do anything useful in the child - strictly speaking, if your perl program
69uses posix threads (even indirectly via e.g. L<IO::AIO> or L<threads>),
70you cannot call fork on the perl level anymore, at all.
71 110
111For example, POSIX makes it almost impossible to fork from a
112multi-threaded program while doing anything useful in the child - in
113fact, if your perl program uses POSIX threads (even indirectly via
114e.g. L<IO::AIO> or L<threads>), you cannot call fork on the perl level
115anymore without risking corruption issues on a number of operating
116systems.
117
72This module can safely fork helper processes at any time, by caling 118This module can safely fork helper processes at any time, by calling
73fork+exec in C, in a POSIX-compatible way. 119fork+exec in C, in a POSIX-compatible way (via L<Proc::FastSpawn>).
74 120
75=item Parallel processing with fork might be inconvenient or difficult 121=item Parallel processing with fork might be inconvenient or difficult
122to implement. Modules might not work in both parent and child.
123
76to implement. For example, when a program uses an event loop and creates 124For example, when a program uses an event loop and creates watchers it
77watchers it becomes very hard to use the event loop from a child 125becomes very hard to use the event loop from a child program, as the
78program, as the watchers already exist but are only meaningful in the 126watchers already exist but are only meaningful in the parent. Worse, a
79parent. Worse, a module might want to use such a system, not knowing 127module might want to use such a module, not knowing whether another module
80whether another module or the main program also does, leading to problems. 128or the main program also does, leading to problems.
81 129
82This module only lets the main program create pools by forking (because 130Apart from event loops, graphical toolkits also commonly fall into the
83only the main program can know when it is still safe to do so) - all other 131"unsafe module" category, or just about anything that communicates with
84pools are created by fork+exec, after which such modules can again be 132the external world, such as network libraries and file I/O modules, which
85loaded. 133usually don't like being copied and then allowed to continue in two
134processes.
135
136With this module only the main program is allowed to create new processes
137by forking (because only the main program can know when it is still safe
138to do so) - all other processes are created via fork+exec, which makes it
139possible to use modules such as event loops or window interfaces safely.
86 140
87=back 141=back
142
143=head1 EXAMPLES
144
145=head2 Create a single new process, tell it to run your worker function.
146
147 AnyEvent::Fork
148 ->new
149 ->require ("MyModule")
150 ->run ("MyModule::worker, sub {
151 my ($master_filehandle) = @_;
152
153 # now $master_filehandle is connected to the
154 # $slave_filehandle in the new process.
155 });
156
157MyModule::worker might look like this:
158
159 sub MyModule::worker {
160 my ($slave_filehandle) = @_;
161
162 # now $slave_filehandle is connected to the $master_filehandle
163 # in the original prorcess. have fun!
164 }
165
166=head2 Create a pool of server processes all accepting on the same socket.
167
168 # create listener socket
169 my $listener = ...;
170
171 # create a pool template, initialise it and give it the socket
172 my $pool = AnyEvent::Fork
173 ->new
174 ->require ("Some::Stuff", "My::Server")
175 ->send_fh ($listener);
176
177 # now create 10 identical workers
178 for my $id (1..10) {
179 $pool
180 ->fork
181 ->send_arg ($id)
182 ->run ("My::Server::run");
183 }
184
185 # now do other things - maybe use the filehandle provided by run
186 # to wait for the processes to die. or whatever.
187
188My::Server::run might look like this:
189
190 sub My::Server::run {
191 my ($slave, $listener, $id) = @_;
192
193 close $slave; # we do not use the socket, so close it to save resources
194
195 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
196 # or anything we usually couldn't do in a process forked normally.
197 while (my $socket = $listener->accept) {
198 # do sth. with new socket
199 }
200 }
201
202=head2 use AnyEvent::Fork as a faster fork+exec
203
204This runs /bin/echo hi, with stdout redirected to /tmp/log and stderr to
205the communications socket. It is usually faster than fork+exec, but still
206let's you prepare the environment.
207
208 open my $output, ">/tmp/log" or die "$!";
209
210 AnyEvent::Fork
211 ->new
212 ->eval ('
213 sub run {
214 my ($fh, $output, @cmd) = @_;
215
216 # perl will clear close-on-exec on STDOUT/STDERR
217 open STDOUT, ">&", $output or die;
218 open STDERR, ">&", $fh or die;
219
220 exec @cmd;
221 }
222 ')
223 ->send_fh ($output)
224 ->send_arg ("/bin/echo", "hi")
225 ->run ("run", my $cv = AE::cv);
226
227 my $stderr = $cv->recv;
88 228
89=head1 CONCEPTS 229=head1 CONCEPTS
90 230
91This module can create new processes either by executing a new perl 231This module can create new processes either by executing a new perl
92process, or by forking from an existing "template" process. 232process, or by forking from an existing "template" process.
109needed the first time. Forking from this process shares the memory used 249needed the first time. Forking from this process shares the memory used
110for the perl interpreter with the new process, but loading modules takes 250for the perl interpreter with the new process, but loading modules takes
111time, and the memory is not shared with anything else. 251time, and the memory is not shared with anything else.
112 252
113This is ideal for when you only need one extra process of a kind, with the 253This is ideal for when you only need one extra process of a kind, with the
114option of starting and stipping it on demand. 254option of starting and stopping it on demand.
255
256Example:
257
258 AnyEvent::Fork
259 ->new
260 ->require ("Some::Module")
261 ->run ("Some::Module::run", sub {
262 my ($fork_fh) = @_;
263 });
115 264
116=item fork a new template process, load code, then fork processes off of 265=item fork a new template process, load code, then fork processes off of
117it and run the code 266it and run the code
118 267
119When you need to have a bunch of processes that all execute the same (or 268When you need to have a bunch of processes that all execute the same (or
125modules you loaded) is shared between the processes, and each new process 274modules you loaded) is shared between the processes, and each new process
126consumes relatively little memory of its own. 275consumes relatively little memory of its own.
127 276
128The disadvantage of this approach is that you need to create a template 277The disadvantage of this approach is that you need to create a template
129process for the sole purpose of forking new processes from it, but if you 278process for the sole purpose of forking new processes from it, but if you
130only need a fixed number of proceses you can create them, and then destroy 279only need a fixed number of processes you can create them, and then destroy
131the template process. 280the template process.
281
282Example:
283
284 my $template = AnyEvent::Fork->new->require ("Some::Module");
285
286 for (1..10) {
287 $template->fork->run ("Some::Module::run", sub {
288 my ($fork_fh) = @_;
289 });
290 }
291
292 # at this point, you can keep $template around to fork new processes
293 # later, or you can destroy it, which causes it to vanish.
132 294
133=item execute a new perl interpreter, load some code, run it 295=item execute a new perl interpreter, load some code, run it
134 296
135This is relatively slow, and doesn't allow you to share memory between 297This is relatively slow, and doesn't allow you to share memory between
136multiple processes. 298multiple processes.
138The only advantage is that you don't have to have a template process 300The only advantage is that you don't have to have a template process
139hanging around all the time to fork off some new processes, which might be 301hanging around all the time to fork off some new processes, which might be
140an advantage when there are long time spans where no extra processes are 302an advantage when there are long time spans where no extra processes are
141needed. 303needed.
142 304
305Example:
306
307 AnyEvent::Fork
308 ->new_exec
309 ->require ("Some::Module")
310 ->run ("Some::Module::run", sub {
311 my ($fork_fh) = @_;
312 });
313
143=back 314=back
144 315
145=head1 FUNCTIONS 316=head1 THE C<AnyEvent::Fork> CLASS
317
318This module exports nothing, and only implements a single class -
319C<AnyEvent::Fork>.
320
321There are two class constructors that both create new processes - C<new>
322and C<new_exec>. The C<fork> method creates a new process by forking an
323existing one and could be considered a third constructor.
324
325Most of the remaining methods deal with preparing the new process, by
326loading code, evaluating code and sending data to the new process. They
327usually return the process object, so you can chain method calls.
328
329If a process object is destroyed before calling its C<run> method, then
330the process simply exits. After C<run> is called, all responsibility is
331passed to the specified function.
332
333As long as there is any outstanding work to be done, process objects
334resist being destroyed, so there is no reason to store them unless you
335need them later - configure and forget works just fine.
146 336
147=over 4 337=over 4
148 338
149=cut 339=cut
150 340
151package AnyEvent::Fork; 341package AnyEvent::Fork;
152 342
153use common::sense; 343use common::sense;
154 344
155use Socket (); 345use Errno ();
156 346
157use AnyEvent; 347use AnyEvent;
158use AnyEvent::Fork::Util;
159use AnyEvent::Util (); 348use AnyEvent::Util ();
160 349
350use IO::FDPass;
351
352our $VERSION = 0.5;
353
161our $PERL; # the path to the perl interpreter, deduces with various forms of magic 354our $PERL; # the path to the perl interpreter, deduces with various forms of magic
162
163=item my $pool = new AnyEvent::Fork key => value...
164
165Create a new process pool. The following named parameters are supported:
166 355
167=over 4 356=over 4
168 357
169=back 358=back
170 359
177our $TEMPLATE; 366our $TEMPLATE;
178 367
179sub _cmd { 368sub _cmd {
180 my $self = shift; 369 my $self = shift;
181 370
182 # ideally, we would want to use "a (w/a)*" as format string, but perl versions 371 # ideally, we would want to use "a (w/a)*" as format string, but perl
183 # from at least 5.8.9 to 5.16.3 are all buggy and can't unpack it. 372 # versions from at least 5.8.9 to 5.16.3 are all buggy and can't unpack
184 push @{ $self->[2] }, pack "N/a", pack "(w/a)*", @_; 373 # it.
374 push @{ $self->[2] }, pack "a L/a*", $_[0], $_[1];
185 375
186 $self->[3] ||= AE::io $self->[1], 1, sub { 376 $self->[3] ||= AE::io $self->[1], 1, sub {
377 do {
378 # send the next "thing" in the queue - either a reference to an fh,
379 # or a plain string.
380
187 if (ref $self->[2][0]) { 381 if (ref $self->[2][0]) {
382 # send fh
188 AnyEvent::Fork::Util::fd_send fileno $self->[1], fileno ${ $self->[2][0] } 383 unless (IO::FDPass::send fileno $self->[1], fileno ${ $self->[2][0] }) {
384 return if $! == Errno::EAGAIN || $! == Errno::EWOULDBLOCK;
385 undef $self->[3];
386 die "AnyEvent::Fork: file descriptor send failure: $!";
387 }
388
189 and shift @{ $self->[2] }; 389 shift @{ $self->[2] };
190 390
191 } else { 391 } else {
392 # send string
192 my $len = syswrite $self->[1], $self->[2][0] 393 my $len = syswrite $self->[1], $self->[2][0];
394
395 unless ($len) {
396 return if $! == Errno::EAGAIN || $! == Errno::EWOULDBLOCK;
397 undef $self->[3];
193 or do { undef $self->[3]; die "AnyEvent::Fork: command write failure: $!" }; 398 die "AnyEvent::Fork: command write failure: $!";
399 }
194 400
195 substr $self->[2][0], 0, $len, ""; 401 substr $self->[2][0], 0, $len, "";
196 shift @{ $self->[2] } unless length $self->[2][0]; 402 shift @{ $self->[2] } unless length $self->[2][0];
197 } 403 }
404 } while @{ $self->[2] };
198 405
199 unless (@{ $self->[2] }) { 406 # everything written
200 undef $self->[3]; 407 undef $self->[3];
408
409 # invoke run callback, if any
201 $self->[0]->($self->[1]) if $self->[0]; 410 $self->[4]->($self->[1]) if $self->[4];
202 }
203 }; 411 };
412
413 () # make sure we don't leak the watcher
204} 414}
205 415
206sub _new { 416sub _new {
207 my ($self, $fh) = @_; 417 my ($self, $fh, $pid) = @_;
208 418
209 AnyEvent::Util::fh_nonblocking $fh, 1; 419 AnyEvent::Util::fh_nonblocking $fh, 1;
210 420
211 $self = bless [ 421 $self = bless [
212 undef, # run callback 422 $pid,
213 $fh, 423 $fh,
214 [], # write queue - strings or fd's 424 [], # write queue - strings or fd's
215 undef, # AE watcher 425 undef, # AE watcher
216 ], $self; 426 ], $self;
217 427
218# my ($a, $b) = AnyEvent::Util::portable_socketpair;
219
220# queue_cmd $template, "Iabc";
221# push @{ $template->[2] }, \$b;
222
223# use Coro::AnyEvent; Coro::AnyEvent::sleep 1;
224# undef $b;
225# die "x" . <$a>;
226
227 $self 428 $self
228} 429}
229 430
230# fork template from current process, used by AnyEvent::Fork::Early/Template 431# fork template from current process, used by AnyEvent::Fork::Early/Template
231sub _new_fork { 432sub _new_fork {
232 my ($fh, $slave) = AnyEvent::Util::portable_socketpair; 433 my ($fh, $slave) = AnyEvent::Util::portable_socketpair;
434 my $parent = $$;
435
233 my $pid = fork; 436 my $pid = fork;
234 437
235 if ($pid eq 0) { 438 if ($pid eq 0) {
236 require AnyEvent::Fork::Serve; 439 require AnyEvent::Fork::Serve;
440 $AnyEvent::Fork::Serve::OWNER = $parent;
237 close $fh; 441 close $fh;
442 $0 = "$_[1] of $parent";
443 $SIG{CHLD} = 'IGNORE';
238 AnyEvent::Fork::Serve::serve ($slave); 444 AnyEvent::Fork::Serve::serve ($slave);
239 AnyEvent::Fork::Util::_exit 0; 445 exit 0;
240 } elsif (!$pid) { 446 } elsif (!$pid) {
241 die "AnyEvent::Fork::Early/Template: unable to fork template process: $!"; 447 die "AnyEvent::Fork::Early/Template: unable to fork template process: $!";
242 } 448 }
243 449
244 AnyEvent::Fork->_new ($fh) 450 AnyEvent::Fork->_new ($fh, $pid)
245} 451}
246 452
247=item my $proc = new AnyEvent::Fork 453=item my $proc = new AnyEvent::Fork
248 454
249Create a new "empty" perl interpreter process and returns its process 455Create a new "empty" perl interpreter process and returns its process
250object for further manipulation. 456object for further manipulation.
251 457
252The new process is forked from a template process that is kept around 458The new process is forked from a template process that is kept around
253for this purpose. When it doesn't exist yet, it is created by a call to 459for this purpose. When it doesn't exist yet, it is created by a call to
254C<new_exec> and kept around for future calls. 460C<new_exec> first and then stays around for future calls.
255 461
256=cut 462=cut
257 463
258sub new { 464sub new {
259 my $class = shift; 465 my $class = shift;
295reduces the amount of memory sharing that is possible, and is also slower. 501reduces the amount of memory sharing that is possible, and is also slower.
296 502
297You should use C<new> whenever possible, except when having a template 503You should use C<new> whenever possible, except when having a template
298process around is unacceptable. 504process around is unacceptable.
299 505
300The path to the perl interpreter is divined usign various methods - first 506The path to the perl interpreter is divined using various methods - first
301C<$^X> is investigated to see if the path ends with something that sounds 507C<$^X> is investigated to see if the path ends with something that sounds
302as if it were the perl interpreter. Failing this, the module falls back to 508as if it were the perl interpreter. Failing this, the module falls back to
303using C<$Config::Config{perlpath}>. 509using C<$Config::Config{perlpath}>.
304 510
305=cut 511=cut
314 my $perl = $; 520 my $perl = $;
315 521
316 # first we try $^X, but the path must be absolute (always on win32), and end in sth. 522 # first we try $^X, but the path must be absolute (always on win32), and end in sth.
317 # that looks like perl. this obviously only works for posix and win32 523 # that looks like perl. this obviously only works for posix and win32
318 unless ( 524 unless (
319 (AnyEvent::Fork::Util::WIN32 || $perl =~ m%^/%) 525 ($^O eq "MSWin32" || $perl =~ m%^/%)
320 && $perl =~ m%[/\\]perl(?:[0-9]+(\.[0-9]+)+)?(\.exe)?$%i 526 && $perl =~ m%[/\\]perl(?:[0-9]+(\.[0-9]+)+)?(\.exe)?$%i
321 ) { 527 ) {
322 # if it doesn't look perlish enough, try Config 528 # if it doesn't look perlish enough, try Config
323 require Config; 529 require Config;
324 $perl = $Config::Config{perlpath}; 530 $perl = $Config::Config{perlpath};
328 require Proc::FastSpawn; 534 require Proc::FastSpawn;
329 535
330 my ($fh, $slave) = AnyEvent::Util::portable_socketpair; 536 my ($fh, $slave) = AnyEvent::Util::portable_socketpair;
331 Proc::FastSpawn::fd_inherit (fileno $slave); 537 Proc::FastSpawn::fd_inherit (fileno $slave);
332 538
539 # new fh's should always be set cloexec (due to $^F),
540 # but hey, not on win32, so we always clear the inherit flag.
541 Proc::FastSpawn::fd_inherit (fileno $fh, 0);
542
333 # quick. also doesn't work in win32. of course. what did you expect 543 # quick. also doesn't work in win32. of course. what did you expect
334 #local $ENV{PERL5LIB} = join ":", grep !ref, @INC; 544 #local $ENV{PERL5LIB} = join ":", grep !ref, @INC;
335 my %env = %ENV; 545 my %env = %ENV;
336 $env{PERL5LIB} = join ":", grep !ref, @INC; 546 $env{PERL5LIB} = join +($^O eq "MSWin32" ? ";" : ":"), grep !ref, @INC;
337 547
338 Proc::FastSpawn::spawn ( 548 my $pid = Proc::FastSpawn::spawn (
339 $perl, 549 $perl,
340 ["perl", "-MAnyEvent::Fork::Serve", "-e", "AnyEvent::Fork::Serve::me", fileno $slave], 550 ["perl", "-MAnyEvent::Fork::Serve", "-e", "AnyEvent::Fork::Serve::me", fileno $slave, $$],
341 [map "$_=$env{$_}", keys %env], 551 [map "$_=$env{$_}", keys %env],
342 ) or die "unable to spawn AnyEvent::Fork server: $!"; 552 ) or die "unable to spawn AnyEvent::Fork server: $!";
343 553
344 $self->_new ($fh) 554 $self->_new ($fh, $pid)
555}
556
557=item $pid = $proc->pid
558
559Returns the process id of the process I<iff it is a direct child of the
560process> running AnyEvent::Fork, and C<undef> otherwise.
561
562Normally, only processes created via C<< AnyEvent::Fork->new_exec >> and
563L<AnyEvent::Fork::Template> are direct children, and you are responsible
564to clean up their zombies when they die.
565
566All other processes are not direct children, and will be cleaned up by
567AnyEvent::Fork itself.
568
569=cut
570
571sub pid {
572 $_[0][0]
573}
574
575=item $proc = $proc->eval ($perlcode, @args)
576
577Evaluates the given C<$perlcode> as ... perl code, while setting C<@_> to
578the strings specified by C<@args>, in the "main" package.
579
580This call is meant to do any custom initialisation that might be required
581(for example, the C<require> method uses it). It's not supposed to be used
582to completely take over the process, use C<run> for that.
583
584The code will usually be executed after this call returns, and there is no
585way to pass anything back to the calling process. Any evaluation errors
586will be reported to stderr and cause the process to exit.
587
588If you want to execute some code to take over the process (see the
589"fork+exec" example in the SYNOPSIS), you should compile a function via
590C<eval> first, and then call it via C<run>. This also gives you access to
591any arguments passed via the C<send_xxx> methods, such as file handles.
592
593Returns the process object for easy chaining of method calls.
594
595=cut
596
597sub eval {
598 my ($self, $code, @args) = @_;
599
600 $self->_cmd (e => pack "(w/a*)*", $code, @args);
601
602 $self
345} 603}
346 604
347=item $proc = $proc->require ($module, ...) 605=item $proc = $proc->require ($module, ...)
348 606
349Tries to load the given modules into the process 607Tries to load the given module(s) into the process
350 608
351Returns the process object for easy chaining of method calls. 609Returns the process object for easy chaining of method calls.
610
611=cut
612
613sub require {
614 my ($self, @modules) = @_;
615
616 s%::%/%g for @modules;
617 $self->eval ('require "$_.pm" for @_', @modules);
618
619 $self
620}
352 621
353=item $proc = $proc->send_fh ($handle, ...) 622=item $proc = $proc->send_fh ($handle, ...)
354 623
355Send one or more file handles (I<not> file descriptors) to the process, 624Send one or more file handles (I<not> file descriptors) to the process,
356to prepare a call to C<run>. 625to prepare a call to C<run>.
360accomplished by simply not storing the file handles anywhere after passing 629accomplished by simply not storing the file handles anywhere after passing
361them to this method. 630them to this method.
362 631
363Returns the process object for easy chaining of method calls. 632Returns the process object for easy chaining of method calls.
364 633
634Example: pass a file handle to a process, and release it without
635closing. It will be closed automatically when it is no longer used.
636
637 $proc->send_fh ($my_fh);
638 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
639
365=cut 640=cut
366 641
367sub send_fh { 642sub send_fh {
368 my ($self, @fh) = @_; 643 my ($self, @fh) = @_;
369 644
378=item $proc = $proc->send_arg ($string, ...) 653=item $proc = $proc->send_arg ($string, ...)
379 654
380Send one or more argument strings to the process, to prepare a call to 655Send one or more argument strings to the process, to prepare a call to
381C<run>. The strings can be any octet string. 656C<run>. The strings can be any octet string.
382 657
658The protocol is optimised to pass a moderate number of relatively short
659strings - while you can pass up to 4GB of data in one go, this is more
660meant to pass some ID information or other startup info, not big chunks of
661data.
662
383Returns the process object for easy chaining of emthod calls. 663Returns the process object for easy chaining of method calls.
384 664
385=cut 665=cut
386 666
387sub send_arg { 667sub send_arg {
388 my ($self, @arg) = @_; 668 my ($self, @arg) = @_;
389 669
390 $self->_cmd (a => @arg); 670 $self->_cmd (a => pack "(w/a*)*", @arg);
391 671
392 $self 672 $self
393} 673}
394 674
395=item $proc->run ($func, $cb->($fh)) 675=item $proc->run ($func, $cb->($fh))
396 676
397Enter the function specified by the fully qualified name in C<$func> in 677Enter the function specified by the function name in C<$func> in the
398the process. The function is called with the communication socket as first 678process. The function is called with the communication socket as first
399argument, followed by all file handles and string arguments sent earlier 679argument, followed by all file handles and string arguments sent earlier
400via C<send_fh> and C<send_arg> methods, in the order they were called. 680via C<send_fh> and C<send_arg> methods, in the order they were called.
401 681
402If the called function returns, the process exits. 682The function name should be fully qualified, but if it isn't, it will be
683looked up in the main package.
403 684
404Preparing the process can take time - when the process is ready, the 685If the called function returns, doesn't exist, or any error occurs, the
686process exits.
687
688Preparing the process is done in the background - when all commands have
405callback is invoked with the local communications socket as argument. 689been sent, the callback is invoked with the local communications socket
690as argument. At this point you can start using the socket in any way you
691like.
406 692
407The process object becomes unusable on return from this function. 693The process object becomes unusable on return from this function - any
694further method calls result in undefined behaviour.
408 695
409If the communication socket isn't used, it should be closed on both sides, 696If the communication socket isn't used, it should be closed on both sides,
410to save on kernel memory. 697to save on kernel memory.
411 698
412The socket is non-blocking in the parent, and blocking in the newly 699The socket is non-blocking in the parent, and blocking in the newly
413created process. The close-on-exec flag is set on both. Even if not used 700created process. The close-on-exec flag is set in both.
701
414otherwise, the socket can be a good indicator for the existance of the 702Even if not used otherwise, the socket can be a good indicator for the
415process - if the othe rprocess exits, you get a readable event on it, 703existence of the process - if the other process exits, you get a readable
416because exiting the process closes the socket (if it didn't create any 704event on it, because exiting the process closes the socket (if it didn't
417children using fork). 705create any children using fork).
706
707Example: create a template for a process pool, pass a few strings, some
708file handles, then fork, pass one more string, and run some code.
709
710 my $pool = AnyEvent::Fork
711 ->new
712 ->send_arg ("str1", "str2")
713 ->send_fh ($fh1, $fh2);
714
715 for (1..2) {
716 $pool
717 ->fork
718 ->send_arg ("str3")
719 ->run ("Some::function", sub {
720 my ($fh) = @_;
721
722 # fh is nonblocking, but we trust that the OS can accept these
723 # few octets anyway.
724 syswrite $fh, "hi #$_\n";
725
726 # $fh is being closed here, as we don't store it anywhere
727 });
728 }
729
730 # Some::function might look like this - all parameters passed before fork
731 # and after will be passed, in order, after the communications socket.
732 sub Some::function {
733 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
734
735 print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
736 }
418 737
419=cut 738=cut
420 739
421sub run { 740sub run {
422 my ($self, $func, $cb) = @_; 741 my ($self, $func, $cb) = @_;
423 742
424 $self->[0] = $cb; 743 $self->[4] = $cb;
425 $self->_cmd ("r", $func); 744 $self->_cmd (r => $func);
426} 745}
427 746
428=back 747=back
748
749=head1 PERFORMANCE
750
751Now for some unscientific benchmark numbers (all done on an amd64
752GNU/Linux box). These are intended to give you an idea of the relative
753performance you can expect, they are not meant to be absolute performance
754numbers.
755
756OK, so, I ran a simple benchmark that creates a socket pair, forks, calls
757exit in the child and waits for the socket to close in the parent. I did
758load AnyEvent, EV and AnyEvent::Fork, for a total process size of 5100kB.
759
760 2079 new processes per second, using manual socketpair + fork
761
762Then I did the same thing, but instead of calling fork, I called
763AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the
764socket form the child to close on exit. This does the same thing as manual
765socket pair + fork, except that what is forked is the template process
766(2440kB), and the socket needs to be passed to the server at the other end
767of the socket first.
768
769 2307 new processes per second, using AnyEvent::Fork->new
770
771And finally, using C<new_exec> instead C<new>, using vforks+execs to exec
772a new perl interpreter and compile the small server each time, I get:
773
774 479 vfork+execs per second, using AnyEvent::Fork->new_exec
775
776So how can C<< AnyEvent->new >> be faster than a standard fork, even
777though it uses the same operations, but adds a lot of overhead?
778
779The difference is simply the process size: forking the 6MB process takes
780so much longer than forking the 2.5MB template process that the overhead
781introduced is canceled out.
782
783If the benchmark process grows, the normal fork becomes even slower:
784
785 1340 new processes, manual fork in a 20MB process
786 731 new processes, manual fork in a 200MB process
787 235 new processes, manual fork in a 2000MB process
788
789What that means (to me) is that I can use this module without having a
790very bad conscience because of the extra overhead required to start new
791processes.
792
793=head1 TYPICAL PROBLEMS
794
795This section lists typical problems that remain. I hope by recognising
796them, most can be avoided.
797
798=over 4
799
800=item "leaked" file descriptors for exec'ed processes
801
802POSIX systems inherit file descriptors by default when exec'ing a new
803process. While perl itself laudably sets the close-on-exec flags on new
804file handles, most C libraries don't care, and even if all cared, it's
805often not possible to set the flag in a race-free manner.
806
807That means some file descriptors can leak through. And since it isn't
808possible to know which file descriptors are "good" and "necessary" (or
809even to know which file descriptors are open), there is no good way to
810close the ones that might harm.
811
812As an example of what "harm" can be done consider a web server that
813accepts connections and afterwards some module uses AnyEvent::Fork for the
814first time, causing it to fork and exec a new process, which might inherit
815the network socket. When the server closes the socket, it is still open
816in the child (which doesn't even know that) and the client might conclude
817that the connection is still fine.
818
819For the main program, there are multiple remedies available -
820L<AnyEvent::Fork::Early> is one, creating a process early and not using
821C<new_exec> is another, as in both cases, the first process can be exec'ed
822well before many random file descriptors are open.
823
824In general, the solution for these kind of problems is to fix the
825libraries or the code that leaks those file descriptors.
826
827Fortunately, most of these leaked descriptors do no harm, other than
828sitting on some resources.
829
830=item "leaked" file descriptors for fork'ed processes
831
832Normally, L<AnyEvent::Fork> does start new processes by exec'ing them,
833which closes file descriptors not marked for being inherited.
834
835However, L<AnyEvent::Fork::Early> and L<AnyEvent::Fork::Template> offer
836a way to create these processes by forking, and this leaks more file
837descriptors than exec'ing them, as there is no way to mark descriptors as
838"close on fork".
839
840An example would be modules like L<EV>, L<IO::AIO> or L<Gtk2>. Both create
841pipes for internal uses, and L<Gtk2> might open a connection to the X
842server. L<EV> and L<IO::AIO> can deal with fork, but Gtk2 might have
843trouble with a fork.
844
845The solution is to either not load these modules before use'ing
846L<AnyEvent::Fork::Early> or L<AnyEvent::Fork::Template>, or to delay
847initialising them, for example, by calling C<init Gtk2> manually.
848
849=item exit runs destructors
850
851This only applies to users of Lc<AnyEvent::Fork:Early> and
852L<AnyEvent::Fork::Template>.
853
854When a process created by AnyEvent::Fork exits, it might do so by calling
855exit, or simply letting perl reach the end of the program. At which point
856Perl runs all destructors.
857
858Not all destructors are fork-safe - for example, an object that represents
859the connection to an X display might tell the X server to free resources,
860which is inconvenient when the "real" object in the parent still needs to
861use them.
862
863This is obviously not a problem for L<AnyEvent::Fork::Early>, as you used
864it as the very first thing, right?
865
866It is a problem for L<AnyEvent::Fork::Template> though - and the solution
867is to not create objects with nontrivial destructors that might have an
868effect outside of Perl.
869
870=back
871
872=head1 PORTABILITY NOTES
873
874Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a nop,
875and ::Template is not going to work), and it cost a lot of blood and sweat
876to make it so, mostly due to the bloody broken perl that nobody seems to
877care about. The fork emulation is a bad joke - I have yet to see something
878useful that you can do with it without running into memory corruption
879issues or other braindamage. Hrrrr.
880
881Cygwin perl is not supported at the moment, as it should implement fd
882passing, but doesn't, and rolling my own is hard, as cygwin doesn't
883support enough functionality to do it.
884
885=head1 SEE ALSO
886
887L<AnyEvent::Fork::Early> (to avoid executing a perl interpreter),
888L<AnyEvent::Fork::Template> (to create a process by forking the main
889program at a convenient time).
429 890
430=head1 AUTHOR 891=head1 AUTHOR
431 892
432 Marc Lehmann <schmorp@schmorp.de> 893 Marc Lehmann <schmorp@schmorp.de>
433 http://home.schmorp.de/ 894 http://home.schmorp.de/

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