PHP-FPM process management woes

Hi!

In the last few days I have been investigating Xdebug bug #2051:
"Segfault on fatal error when setting xdebug.mode via php-fpm pool
config" (https://bugs.xdebug.org/view.php?id=2051).

I have now tracked this down to an unexpectedness in how PHP-FPM handles
extension/module initialisation.

Literature since at least 2006 (Sara's Extending and Embedding PHP
book), as well as numerous presentations that I have seen and given, and
including the online PHP Internals Book
(https://www.phpinternalsbook.com/php7/extensions_design/php_lifecycle.html),
always expected that the MINIT (and MSHUTDOWN) functions are called once
per worker process. However, PHP-FPM does not do that. It only calls
extension's MINITs once in the main control process, but it does call
an MSHUTDOWN for each worker process.

In the past, this has already caused an issue where I couldn't create a
monitoring thread in MINIT, and then wait for it to end in MSHUTDOWN, as
MSHUTDOWN would wait on the same thread in each worker process, although
only one was created in MINIT.

The way how PHP-FPM handles this breaks the generally assumed "one
MINIT, one MSHUTDOWN call" per process approach.

In particular, in the case of Xdebug bug #2051 it creates a problem with
the following set-up:

1. php.ini has a xdebug.mode=off
2. the pool configuration has a php_admin_value[xdebug.mode] = debug
   directive

When PHP-FPM starts up, it calls Xdebug's MINIT, which checks the value
of the xdebug.mode INI setting. If it set to off it does
nothing, including setting up handles such as the zend_error_cb
handlers in xdebug_base_minit::

PHP_MINIT_FUNCTION(xdebug)
{
	…

	if (XDEBUG_MODE_IS_OFF()) {
		return SUCCESS;
	}

	…
	xdebug_base_minit(INIT_FUNC_ARGS_PASSTHRU);
}

MINIT sets the xdebug_old_error_cb and xdebug_new_error_cb handlers
to something else than NULL::

void xdebug_base_minit(INIT_FUNC_ARGS)
{
	/* Record Zend and Xdebug error callbacks, the actual setting is done in
	 * base on RINIT */
	xdebug_old_error_cb = zend_error_cb;
	xdebug_new_error_cb = xdebug_error_cb;

In each RINIT, which is called once per request, it also checks the INI
setting, and if set, uses some of the handlers that were set-up in
MINIT. Please note that PHP-FPM has changed the value of xdebug.mode
in this worker process to debug (ie, not off), (simplified)::

PHP_RINIT_FUNCTION(xdebug)
{
	…
	if (XDEBUG_MODE_IS_OFF()) {
		return SUCCESS;
	}
	…
	xdebug_base_rinit();
}

void xdebug_base_rinit()
{
	xdebug_base_use_xdebug_error_cb();
}

void xdebug_base_use_xdebug_error_cb(void)
{
	zend_error_cb = xdebug_new_error_cb;
}

When now an error occurs, PHP calls zend_error_cb, but this is now
unset (NULL) as when MINIT was called, the xdebug.mode setting was
still set to off, but during RINIT it was changed by PHP-FPM to
debug. Xdebug does not expect this setting to change, especially
because it is marked as PHP_INI_SYSTEM.

In my opinion this is a bug (or two) in PHP-FPM, as:

• it does not follow the expected one MINIT/MSHUTDOWN cycle, that for
  example Apache (1 and 2) use, and which is documented in books and
  online material (and my memory)
• it disrespects PHP_INI_SYSTEM

My suggestion for a fix would be to emulate what Apache always did:

One MINIT/MSHUTDOWN in the main control process (I think it needed that
to be able to implement php_admin_value and php_value), and then
additionally also in each worker process.

I did have a brief look at implementing this, but haven't managed to get
it to work yet — mainly because I am unfamiliar with the PHP-FPM code at
the moment.

cheers,
Derick

--
PHP 7.4 Release Manager
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