Re: Safe timeout handling

Hi Dmitry, ----- Original Message ----- From: "Dmitry Stogov" Sent: Wednesday, April 20, 2016

Hi, It's a well known PHP problem, that exceeding of execution time-out (max_execution_time) may lead to unexpected crashes. They occur because PHP may be interrupted in inconsistent state, and attempt to release allocated by request resources leads to failure. Almost any big site sees these crashes from time to time. I propose to delay actual request termination until a "safe" point in interpreter. Signal handler will just set EG(timed_out) flag. Interpreter will check this time from time to time (on jumps and calls that may make loops or recursion) and perform the actual termination. This approach already works in PHP for Windows.

I was thinking about this, checking for things like EG(exception) "constantly," a few months ago for another reason... What about instead of adding additional checks in the same place(s) in VM, we just limit it to 1 check, for multiple things? Just have EG(something_unexpected_to_check), and behind that (or in a function, I guess), the actual rare/unexpected thing gets checked: timed_out, exception, etc. It seems Bob had a similar idea in the PR comment, except literally using exceptions...

In addition I introduce hard_timeout (default value 2 seconds). In case the "soft" timeout wasn't handled "safely" in that 2 seconds (because of long running internal function), PHP process will be terminated without attempt to free any resources. ZTS build will ignore "hard_timeout" (in the same way as PHP on Windows do). The PR: https://github.com/php/php-src/pull/1876 It removes "exit_on_timeout" ini directive, and introduces "hard_timeout" instead. Additional checks in VM make 0.5-1% slowdown in term of instruction retired reported by callgrind.

A single check would save those additional instructions and branches, and would actually improve things on Windows (since this PR doesn't change anything there).

I think we don't need RFC for this. This is a long time desired fix. The same "interrupt" handling mechanism in the future may be reused for TICK and signal handling. Thanks. Dmitry.

- Matt