How to debug Xdebug... or any other weird bug in PHP

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How to Debug Bizarre PHP Bugs Photo by Dmitry Bukhantsov

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Kévin Dunglas ➔ Co-founder of Les-Tilleuls.coop ➔ Creator of FrankenPHP and API Platform ➔ Co-maintainer of Caddy and Symfony ➔ Contributor to PHP, and by necessity, to Xdebug 🐞 @dunglas

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PHP, Web and Cloud Experts ➔ Development: PHP, JS, Go, Rust, C... ➔ DevOps and SRE ➔ Consultancy and maintenance ➔ Agile management, UX and UI design… ➔ contact@les-tilleuls.coop 💌

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Where's the Bug?

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As a user: is the bug in my code or in the library I'm using? There might be a bug in the lib (but could be in your code) The bug is probably in your code (but could be in the lib) Is this lib popular? Has a new version been published recently? Does downgrading solve this issue? Are you using the lib in a weird or advanced way? Have others users reported this bug? YES NO YES NO YES NO YES NO NO YES

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As a maintainer: is the bug in my lib or in user’s code? The bug is likely in their code It is a regular contributor? How many users reported the bug? SOME MANY A SINGLE USER NO YES

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Bizarre Bugs!

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A bizarre bug

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More Bizarre Bugs

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🔥🔥

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Reminder The bug is likely in their code It is a regular contributor? How many users reported the bug? SOME MANY A SINGLE USER NO YES W e’re here!

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But What Is Xdebug Anyway?

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A must-have for PHP development: ➔ Step debugging ➔ Better error reporting ➔ Function calls tracing ➔ Proﬁling ➔ Code coverage ➔ Installed by default with Symfony Docker, API Platform and Laravel Herd ➔ Supported by PhpStorm, VSCode and (Neo)vim Xdebug

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Asking For Help

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Let’s Try To Find the Issue Together!

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Step 1: Reproducing

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A Minimal Reproducer

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Step 2: Gathering Information

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➔ Debug symbols ● Make stack traces readable ● Available as extra packages for some (but not all) distributions ○ Debian: "How To Get A Backtrace" ● Not available (yet?) for oﬃcial Docker images: docker-library/php#1538 ➔ The GNU Debugger (or lldb on Mac) Let Me Introduce You To:

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My Reco: Compile PHP with Debug Symbols From Source gh repo clone php/php-src cd php-src/ ./buildconf -f ./configure \ --enable-embed \ --enable-zts \ --disable-zend-signals \ --enable-debug make sudo make install

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Compile FrankenPHP to Use Our libphp gh repo clone dunglas/frankenphp cd frankenphp/caddy/frankenphp export PATH="/usr/local/bin/:$PATH" export CGO_CFLAGS=$(php-config --includes) export CGO_LDFLAGS="$(php-config --ldflags) $(php-config --libs)" go build

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Compile Xdebug gh repo clone xdebug/xdebug cd xdebug git checkout 3.3.1 phpize ./configure make #sudo make install

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Load Xdebug ; php.ini, in the root directory of the project zend_extension = /path/to/xdebug.so xdebug_mode = debug xdebug.start_with_request = yes

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Run GDB to Gather a Stack Trace gdb --args ./frankenphp run # Or on Apple Silicon lldb -- ./frankenphp run # “run” to start or resume execution # “bt” to show the back trace # “break example.c:17” to set a breakpoint # “p” to print variable values

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When You Can’t Use GDB… Sometimes (e.g. in prod) you can’t install GDB. ➔ Enable verbose logs ➔ In case of segfault, enable core dumps: ulimit -S -c unlimited ➔ lsof: list open ﬁles and network sockets ➔ strace: list system calls (I/O…) ➔ top / htop: inspect CPU and memory usage ➔ …

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Step 3: Doing Our Research

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➔ Symfony Docker and Laravel Octane uses FrankenPHP ➔ FrankenPHP uses POSIX threads and GoRoutines (GoRoutines also use threads) ➔ Using threads requires a thread-safe (ZTS) build of PHP and of PHP extensions ➔ PHP extensions must be compatible with non-thread-safe (NTS) and thread-safe (ZTS) builds of PHP Some Context

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The PHP Parallelism Model (NTS)

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The PHP Parallelism Model (ZTS)

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From The PHP Internals Book

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Step 4: Develop Theories

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grep -r error_reporting xdebug static void xdebug_base_overloaded_functions_restore(void) { // ... if (XG_BASE(orig_error_reporting_func)) { orig = zend_hash_str_find_ptr( EG(function_table), "error_reporting", sizeof("error_reporting") - 1 ); if (orig) { orig->internal_function.handler = XG_BASE(orig_error_reporting_func); // ...

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Step 5: Write and Submit a Patch

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🎉

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Summary ➔ Step 0: COMMUNICATE ➔ Step 1: gather evidences to create a reproducer ➔ Step 2: use the right tools ➔ Step 3: use a search engine (and read books) ➔ Step 4: develop theories ➔ Step 5: contribute!

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Thank you! Come to our booth or call us if you have weird bugs you can’t ﬁx! les-tilleuls.coop contact@les-tilleuls.coop @coopTilleuls