有没有安全的方法可以将非公共方法用作信号处理程序？

Question

I am implementing a pre-fork tcp socket server in PHP, working with multiple child processes, shared memory, semaphores, and various signals.

I am currently using protected methods for my signal handlers.

Unfortunately, this occasionally fails with "cannot access protected method" errors.

I am pretty sure that this happens when the signal is delivered while the code is in a method in some other class (the shared memory class, for example).

I would prefer to use protected methods because these methods really shouldn't be called from any other user land code.

My question is: Is there any safe way to use non-public methods as signal handlers?

Answer 1

Short answer: No. It is not safe to use a non-public method as a signal handler.

Longer answer (why): Although I am no PHP internals expert, I have written a number of custom PHP extensions over the years and have some familiarity with the internals so I decided to research this question by looking at the actual PHP functions involved.

User land signal handlers are not actually called by the operating system.

The pcntl_signal() function adds a structure representing the user land signal handler to an internal list and then (through some other calls), sets up the pcntl_signal_handler() internal function as the actual handler for the specified signal.

When the signal is delivered, the operating system calls pcntl_signal_handler() which modifies the internal signal handler table and sets the global pending_signals flag to one. If asynchronous signals are enabled it also sets the global vm_interrupt flag to one.

At some point after that, PHP will call the internal version of pcntl_signal_dispatch() which does some sanity checking and house keeping and eventually calls the call_user_function() internal function.

As far as I can tell, call_user_function() is used whenever a user land callable is to be called from within PHP (well, call_user_function() is actually a macro that calls _call_user_function_ex() ).

The _call_user_function_ex() function sets up a structure and then calls zend_call_function() .

z end_call_function() eventually calls zend_is_callable_ex() which makes a number of checks to see if the user land function is callable, including the current scope, and that check fails if the signal is delivered while the application is executing in any scope other than the class in which the signal handler was delivered (a user land database abstraction method or logger, for example).

So no, it is not safe to use a non-public method as a signal handler.