Contributing Advent 23: Reproducing issues

Many users that find flaws in software will just shrug and find a workaround, some will provide a cryptic bug report, but very few will actually write a coherent bug report that makes it very clear as to how to reproduce an issue. Without a reproducible case, it can be very tricky to find the bug, and then subsequently fix it.

If people encounter bugs in Xdebug's remote debugging I very often ask them to produce a remote log file. This log file will contain the communication packets between Xdebug and the IDE. In some cases, this highlights that an IDE did something wrong, or perhaps there is a misconfiguration (often with path mappings!). Together with the script that was used to create a log it is often very simple for me to reproduce the issue, and then fix it.

However, sometimes I get a bug report that goes along "the massive Symfony application causes an issue". I can not do anything with that. Even though it might be easy to reproduce (1. install Symfony, 2. install application, 3. run massive PHPUnit test suite), I do not have the time (and knowledge) to debug any random PHP application. As a user of the Open Source software that would like a bug fixed, you should spend some time making it easy for the developers of said software to figure out the problem. That means that you need to dissect your massive Symfony application and produce a small script that shows the behaviour. If possible, this script should also never rely on external resources such as a database, or some SOAP service.

A reproducible issue is also one that you can reliably trigger. That means hitting a single URL or executing a single script on the command line. With PHP or PHP extensions such as Xdebug there are sometimes issues that only show up once in a while, because memory corruption do not always result in crashes. A few tools are available to more easily highlight those issues.

In almost every case, you would want to set the environment variable export USE_ZEND_ALLOC=0. Zend's memory allocator can sometimes hide issues because of "clever" memory management, and by making PHP use the standard OS memory management routines issues can show up earlier. Setting this environment variable is especially important if you also use the second tool: Valgrind.

Valgrind is a memory debugging tool which helps me find things like using uninitialised memory, memory that is already freed, writing out of bounds or memory leaks. It is an unmissable tool for me in very many cases. Providing a Valgrind log with a bug report is very helpful.

Valgrind is only useful if you have PHP and Xdebug compiled in debug mode. If you use PHP from a Linux distribution's package you should install something like php5-dev or php5-devel to also have the debugging symbols available. For Xdebug you would need to compile it yourself (and not use PECL, or a distribution's package).

In some cases, memory leaks and errors show up only when PHP or an extension is shutdown. If an extension is already unloaded, then Valgrind can not pinpoint exactly where in the code the issue occurred. If this happens, you see lines with ??? in the output. By setting yet another environment variable, you prevent PHP from unloading the module. This environment variable is: export ZEND_DONT_UNLOAD_MODULES=1. With both USE_ZEND_ALLOC=0 and ZEND_DONT_UNLOAD_MODULES=1 set, Valgrind gives the best results. And a bug where Valgrind shows that Xdebug (or something else) does something wrong is the best kind you can file!

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Comments

nice blog post, this applies not only to xdebug but also to symfony and many other php repos out there, thanks for sharing.

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