| 1 | NAME |
1 | NAME |
| 2 | AnyEvent::Fork - everything you wanted to use fork() for, but couldn't |
2 | AnyEvent::Fork - everything you wanted to use fork() for, but couldn't |
| 3 | |
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| 4 | ATTENTION, this is a very early release, and very untested. Consider it |
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| 5 | a technology preview. |
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| 6 | |
3 | |
| 7 | SYNOPSIS |
4 | SYNOPSIS |
| 8 | use AnyEvent::Fork; |
5 | use AnyEvent::Fork; |
| 9 | |
6 | |
| 10 | ################################################################## |
7 | ################################################################## |
| … | |
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| 357 | my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
354 | my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
| 358 | |
355 | |
| 359 | print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" |
356 | print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" |
| 360 | } |
357 | } |
| 361 | |
358 | |
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359 | TYPICAL PROBLEMS |
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360 | This section lists typical problems that remain. I hope by recognising |
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361 | them, most can be avoided. |
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362 | |
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363 | "leaked" file descriptors for exec'ed processes |
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364 | POSIX systems inherit file descriptors by default when exec'ing a |
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365 | new process. While perl itself laudably sets the close-on-exec flags |
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366 | on new file handles, most C libraries don't care, and even if all |
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367 | cared, it's often not possible to set the flag in a race-free |
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368 | manner. |
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369 | |
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370 | That means some file descriptors can leak through. And since it |
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371 | isn't possible to know which file descriptors are "good" and |
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372 | "neccessary" (or even to know which file descreiptors are open), |
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373 | there is no good way to close the ones that might harm. |
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374 | |
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375 | As an example of what "harm" can be done consider a web server that |
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376 | accepts connections and afterwards some module uses AnyEvent::Fork |
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377 | for the first time, causing it to fork and exec a new process, which |
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378 | might inherit the network socket. When the server closes the socket, |
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379 | it is still open in the child (which doesn't even know that) and the |
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380 | client might conclude that the connection is still fine. |
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381 | |
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382 | For the main program, there are multiple remedies available - |
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383 | AnyEvent::Fork::Early is one, creating a process early and not using |
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384 | "new_exec" is another, as in both cases, the first process can be |
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385 | exec'ed well before many random file descriptors are open. |
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386 | |
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387 | In general, the solution for these kind of problems is to fix the |
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388 | libraries or the code that leaks those file descriptors. |
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389 | |
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390 | Fortunately, most of these lekaed descriptors do no harm, other than |
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391 | sitting on some resources. |
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392 | |
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393 | "leaked" file descriptors for fork'ed processes |
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394 | Normally, AnyEvent::Fork does start new processes by exec'ing them, |
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395 | which closes file descriptors not marked for being inherited. |
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396 | |
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397 | However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a |
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398 | way to create these processes by forking, and this leaks more file |
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399 | descriptors than exec'ing them, as there is no way to mark |
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400 | descriptors as "close on fork". |
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401 | |
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402 | An example would be modules like EV, IO::AIO or Gtk2. Both create |
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403 | pipes for internal uses, and Gtk2 might open a connection to the X |
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404 | server. EV and IO::AIO can deal with fork, but Gtk2 might have |
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405 | trouble with a fork. |
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406 | |
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407 | The solution is to either not load these modules before use'ing |
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408 | AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay |
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409 | initialising them, for example, by calling "init Gtk2" manually. |
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410 | |
| 362 | PORTABILITY NOTES |
411 | PORTABILITY NOTES |
| 363 | Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
412 | Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
| 364 | nop, and ::Template is not going to work), and it cost a lot of blood |
413 | nop, and ::Template is not going to work), and it cost a lot of blood |
| 365 | and sweat to make it so, mostly due to the bloody broken perl that |
414 | and sweat to make it so, mostly due to the bloody broken perl that |
| 366 | nobody seems to care about. The fork emulation is a bad joke - I have |
415 | nobody seems to care about. The fork emulation is a bad joke - I have |
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| 369 | |
418 | |
| 370 | Cygwin perl is not supported at the moment, as it should implement fd |
419 | Cygwin perl is not supported at the moment, as it should implement fd |
| 371 | passing, but doesn't, and rolling my own is hard, as cygwin doesn't |
420 | passing, but doesn't, and rolling my own is hard, as cygwin doesn't |
| 372 | support enough functionality to do it. |
421 | support enough functionality to do it. |
| 373 | |
422 | |
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423 | SEE ALSO |
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424 | AnyEvent::Fork::Early (to avoid executing a perl interpreter), |
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425 | AnyEvent::Fork::Template (to create a process by forking the main |
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426 | program at a convenient time). |
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427 | |
| 374 | AUTHOR |
428 | AUTHOR |
| 375 | Marc Lehmann <schmorp@schmorp.de> |
429 | Marc Lehmann <schmorp@schmorp.de> |
| 376 | http://home.schmorp.de/ |
430 | http://home.schmorp.de/ |
| 377 | |
431 | |
