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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…
➔ [email protected] 💌
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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
[email protected]
@coopTilleuls