PHP Misleading Error: Maximum execution time of 0 seconds exceeded

December 2nd, 2016

Yesterday, on freenode #phpc, someone posted this curious error message:

PHP Fatal error: Maximum execution time of 0 seconds exceeded

Jokes aside of how, only eight hours into December, they had exceeded their monthly allotment of PHP time, this is a rather curious error message to receive: PHP’s set_time_limit() function and associated max_execution_time ini setting define a limit of 0 to mean no limit. A script with no limit should not be hitting a maximum execution time of zero seconds!

While several of us tried brainstorming possible causes, additional details were provided, including that the script, running on PHP 7 on unix, under php-fpm, was twice terminated after running for roughly two hours.

I set out to look at PHP’s source code, certain that there would be something that would shed light on this mystery.

Let’s investigate PHP internals

The first thing I did was search for “Maximum execution time” in the PHP sources, hoping to find exactly one hit. Ignoring test cases, that string appears exactly once in the source, in the zend_timeout() function in zend_execute_API.c.

When zend_timeout() is called, PHP calls a function specified by the SAPI (the interface between PHP and the web server) if one is defined, and then emits an error and exits.

Knowing now where the error message is generated (and that it only comes from one place), I needed to determine what calls zend_timeout(), so I searched the code for that function’s name. Excluding the function’s own definition, and its declaration in a header file, there were four results. Two of those results were specific to Windows support, and could be ignored.

The other two results were conveniently located next to each other in zend_set_timeout(), both used as the callback function to a signal handler.

This means that, in normal use, the execution time limit message can only be generated in response to receiving a signal.

Signals

In UNIX-like systems, signals provide for a limited form of inter-process communication, in the form of sending and receiving an interrupt with a numeric identifier. The Wikipedia page on signals provides additional detail.

There C standard defines six signals, but many operating systems define and use many additional signals.

Some signals sent to a process cause its unconditional termination (e.g. SIGKILL) or have other operating-system level effects (such as SIGSTOP and SIGCONT). Most other signals can be caught by the target process if it has installed a signal handler to receive that notification. (If a signal handler is not installed, a default handler is used, which often, but not always, results in the self-termination of the process.)

How PHP’s execution time limit works

So, then, what signal triggers zend_timeout()? A few lines up indicates the answer: SIGPROF. (Alternatively, SIGALRM is used when running under Cygwin on Windows.)

SIGPROF was a new one to me. Wikipedia says:

The SIGALRM, SIGVTALRM and SIGPROF signal is sent to a process when the time limit specified in a call to a preceding alarm setting function (such as setitimer) elapses. SIGALRM is sent when real or clock time elapses. SIGVTALRM is sent when CPU time used by the process elapses. SIGPROF is sent when CPU time used by the process and by the system on behalf of the process elapses.

And indeed, there is a call to setitimer() before PHP installs the signal handler on SIGPROF. The man page for setitimer() describes how to use it to set a timer that counts time the process spends executing, triggering a SIGPROF after the timer elapses.

Some more searching through PHP’s code makes clear how the entire process works: when PHP is loaded, set_time_limit() is called, or max_execution_time is changed, PHP clears the timer (if set), and re-sets the timer (if the timeout is non-zero). Additionally, during request start-up, the signal handler on SIGPROF is installed (regardless of whether a timeout is actually set).

When the timer set by setitimer fires, the SIGPROF handler, zend_timeout() runs, displays the error message with the number of seconds filled in from the current configuration, and exits.

Reading between the lines

That answers the question of how the execution time limit error was displayed. But it doesn’t answer why: one would naturally assume that since the timer isn’t set, the operating system won’t send SIGPROF, and the script wouldn’t be terminated.

The answer lies in one further detail of the UNIX signal mechanism: any process can send any other process any signal (except where disallowed by security policy, such as for processes owned by a different user).

This means that a script doesn’t actually need to hit the time limit to be killed. PHP just needs to be told that it has.

You can test this yourself by doing something similar to this:

$ php -r 'posix_kill(getmypid(), SIGPROF);'

Fatal error: Maximum execution time of 0 seconds exceeded in Command line code on line 1

So, what really happened?

Unfortunately, a signal does not come with the identity of its source. Were that the case, it would have been easy to determine what was killing the process and what configuration to change to stop it. In this case, the resolution was to modify the php code to “only” take 20 minutes to run, so the timeout was no longer an issue.

If we make the assumption that there wasn’t a malicious user on the server sending unwanted signals as a denial of service attack, the documentation for max_execution_time hints at one possibility:

Your web server can have other timeout configurations that may also interrupt PHP execution. Apache has a Timeout directive and IIS has a CGI timeout function. Both default to 300 seconds. See your web server documentation for specific details.

With some further research, I found that neither Apache nor nginx explicitly send SIGPROF, though. And while nginx does use setitimer, its use triggers a SIGALRM.

My best guess is that there likely was some sort of watchdog process on the server that killed off the process running php after it consumed too many resources (either memory or time).

It's probably a bug (or at least, undesirable confusion) in PHP that a SIGPROF that doesn’t arise strictly from a timer expiration displays the same message as if the timer did expire, but this looks to be correctable.

20 Years of PHP

June 8th, 2015

Today is PHP’s 20th birthday. Ben Ramsey has called on us to blog about our history with PHP, so here’s mine.

Way back in 1999, still in college, I got my first real software development job at a small company in DC, the predecessor to my current employer. My job was to write an Apache log analyzer, because the software package we were using at the time was very slow and produced inconsistent results between runs.

So I wrote it in C++, because that’s what I knew, and what I was using for my personal projects. But, we were a web services company, so why shouldn’t our log analyzer be accessible via the web?

We were using a couple of different web languages at the time. Some of our early stuff was in PERL, which I had tried before and didn’t like. We also had a site using this awful language called SQLWEB. But, it was suggested to me that I write the web interface using this scripting language called PHP. I had never heard of it before, but I quickly learned it (because, frankly with PHP 3, there wasn’t much to learn), and quickly became enamored with this language.

Sure, it didn’t have many of the features we’ve come to take for granted in modern PHP, such as OOP or closures, or even the foreach keyword (hooray for PHP 4!). But its key feature was that it didn’t need to be compiled. Up until then, every program I’d ever written had a slow write-compile-debug cycle, because compiling a new build and relaunching the app to test was always slow. But here, with PHP, all I needed to do was change my code and refresh the browser window, and the changes were immediately visible. PHP may have been slower than C, but I was way more productive.

We no longer use that log analyzer, but PHP is the foundation for every website we currently manage, and is the vast majority of the code I’ve written over my professional career. And since then, the PHP community has become so much bigger, with several different application frameworks, thousands of open source libraries made easily available through Composer and Packagist, more conferences every year than one person could possibly attend, and a great community that I’m happy to be a part of.

Happy birthday, PHP! Here’s to another 20 years of powering the web.

BowerBundle Released

November 24th, 2014

Last night, I released BowerBundle, a super-simple bundle for Symfony that enables running Bower update/install automatically after running Composer update/install. I’ve used this for several different Symfony apps, and now it’s available for your use as well.

You can download it via GitHub, or via jbafford/bower-bundle in Composer.

Slides for “Writing OOP Modules for Drupal 7” Posted

November 19th, 2014

The slides for “Writing OOP Modules for Drupal 7”, my first talk at last week’s php[world], have been posted to Speaker Deck.

Slides for “Stupid PHP Tricks” Posted

November 18th, 2014

The slides for “Stupid PHP Tricks”, my second talk at last week’s php[world], have been posted to Speaker Deck.

Automatic Build Versioning in Xcode with Subversion

November 17th, 2010

For awhile, I had wanted to include the svn revision number in my iOS app, and when I came across a blog post by Daniel Jalkut from a few years ago, I thought I had found an answer. No sooner than I implemented his script, though, did I discover that Apple doesn’t allow build numbers in iOS app packages.

I still really wanted this information in my dev builds so it would be easier to keep track of what version I was working with, so wound up I made a number of modifications to the script to make it suit my needs:
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GPSTrack 2.0 released

November 17th, 2010

Today, the first major update to GPSTrack, my iOS GPS tracking application, made its way to the App Store, now with support for the iPad (WiFi + 3G).

I didn’t make too big a fuss about the 1.0 version, knowing it would have the sort of annoying bugs that every 1.0 release has, and while 2.0 still doesn’t have all the features I wanted — I severely underestimated how much time adding iPad support was going to take, and some of which I hope to blog about soon — I’m pretty happy with the improvements I’ve made so far, but now that 2.0’s released, I can actually get back to adding new functionality.

Slides for CodeWorks talk posted

October 3rd, 2009

This morning, in the lovely “Sauna” room at CodeWorks DC, I presented my talk on “What Happens When a Website Crashes: A Case Study” on how The Bivings Group scaled the Pickens Plan website after it stopped working following a national advertising campaign after the first 2008 presidential debate drew thousands of people to the website.

The slides are now available from my talks page.

If you were in the audience, please feel feel free to rate the talk at joind.in. I welcome your comments.

Twitterslurp open source release

June 30th, 2009

Last month, I wrote about Twitterslurp, the twitter searching tool I developed at The Bivings Group, which displays a constantly-updating stream of tweets, as well as a leaderboard and stats graphs.

Today, we are very happy to release it as open source. You can download Twitterslurp from its Google Code project page at http://twitterslurp.googlecode.com/.

Since last month, I’ve made a lot of changes to improve the quality (and ease of configuration) of the Twitterslurp code. Twitterslurp’s error handling has been improved, and I added the ability to start and stop the tweet stream and show more than the most recent 20 tweets. Our graphics team also created a spiffy logo.

Yesterday and today, Twitterslurp has been driving a video wall of tweets at the Personal Democracy Forum conference in NYC. The conference, which just ended, had over 17,000 tweets in the last two days.

Previously, we ran test versions of Twitterslurp during mysqlconf and php|tek, and officially on behalf of the Dutch PHP Conference. Twitterslurp started as a project for a client to allow them to track tweets, and give members of their website rewards for tweeting with a particular hashtag.

We’ve also set up a copy of Twitterslurp tracking itself.

We’d love for you to check out Twitterslurp, and we’re open to any and all feedback.

Tracking php|tek Tweets With Twitterslurp

May 20th, 2009

For a client at work a few months ago, I created a Twitter search tool, Twitterslurp, that put all the tweets related to the client’s project on their webpage, updated in (close to) real-time via AJAX.

I’ve since added a lot of features, including a set of graphs, and we’ve set up a version of Twitterslurp for php|tek 2009.
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