SIGTSTP not working

[domenicoangiulli]

Hi everyone, I'm trying to suspend a process (to send it in background/foreground on demand), but something is not working as expected.

This is my test code:

<?php

require __DIR__ . '/../vendor/autoload.php';

$loop = React\EventLoop\Factory::create();

$loop->addSignal(SIGTSTP, function () { echo 'SIGTSTP' . PHP_EOL; });

$loop->run();

And this is the result:

domenico@devtop:~$ php test.php
^ZSIGTSTP
^ZSIGTSTP
^ZSIGTSTP
^C

As you can see from the terminal output, the signal is intercepted, but nothing happens, the process is not suspended.

If I try to do the same action without a loop (for example with the following code) the process is suspended:

domenico@devtop:~$ php -r "sleep(5); echo 'hello'.PHP_EOL;"
^Z
[1]+ Stopped php -r "sleep(5); echo 'ciao'.PHP_EOL;"
domenico@devtop:~$ fg
php -r "sleep(5); echo 'ciao'.PHP_EOL;"
ciao

Why is this happening? Am I wrong?

I'm using the event-loop version 1.1.1 (2020-01-01).

domenico@devtop:~$ php -v                                                                                                                                                                                                                                                                                                                                      
PHP 7.2.24-0ubuntu0.18.04.3 (cli) (built: Feb 11 2020 15:55:52) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.2.24-0ubuntu0.18.04.3, Copyright (c) 1999-2018, by Zend Technologies

domenico@devtop:~$ php -m                                                                                                                                                                                                                                                                                                                                      
[PHP Modules]
bcmath
bz2
calendar
Core
ctype
curl
date
dom
exif
fileinfo
filter
ftp
gd
gettext
gnupg
hash
iconv
imagick
imap
intl
json
ldap
libxml
mbstring
mysqli
mysqlnd
OAuth
openssl
pcntl
pcre
PDO
pdo_mysql
pdo_sqlite
Phar
posix
readline
Reflection
session
shmop
SimpleXML
soap
sockets
sodium
SPL
sqlite3
ssh2
standard
sysvmsg
sysvsem
sysvshm
tokenizer
wddx
xml
xmlreader
xmlwriter
xsl
yaml
Zend OPcache
zip
zlib
zmq

[Zend Modules]
Zend OPcache

SIGTSTP can be ignored and since you're attaching a signal handler, the process will not be suspended. You'll need to use SIGSTOP to suspend the process (this signal cannot be ignored).

You could send yourself that signal in the signal handler to suspend the process:

posix_kill(getmypid(), SIGSTOP);

Modified working example:

<?php

require __DIR__ . '/vendor/autoload.php';

$loop = React\EventLoop\Factory::create();

$loop->addSignal(SIGTSTP, function () { echo 'SIGTSTP' . PHP_EOL; posix_kill(getmypid(), SIGSTOP); });

$loop->run();

Output:

$ php sigstop.php
^ZSIGTSTP

[1]+  Stopped                 php sigstop.php
$ fg
php sigstop.php

[domenicoangiulli]

Interesting! Is this a workaround or the usual way to do that?

I expected the process to be paused after the last statement of the handler.

A thing like that:

$loop->addSignal(SIGTSTP, function () {
	// 1) wait for the active task to complete
	// 2) save the state (for example, on disk)
	// 3) commit or rollback active transactions on DB
	// 4) gracefully close the connection with the DB server
}
// <== PHP engine pauses the script
);

...Similarly to what happens with a handler registered with register_shutdown_function()

That's the usual way. register_shutdown_function is entirely PHP engine internal. Signals are delivered by the OS and by attaching a signal handler through a system call, you've changed the default disposition of the signal, thus observing the behaviour of the process not getting suspended.

[domenicoangiulli]

All clear, thank you very much @CharlotteDunois!