Apache's private /tmp/ on ArchLinux
Today I figured something out that really confused me yesterday. I was trying to use up xdebug's function tracing, so I set up my config like this:
- /etc/php/conf.d/xdebug.ini
zend_extension=/usr/lib/php/modules/xdebug.so xdebug.remote_enable=off xdebug.remote_host=127.0.0.1 xdebug.remote_port=9000 xdebug.remote_handler=dbgp xdebug.trace_enable_trigger=1 xdebug.trace_output_dir=/tmp/ xdebug.trace_output_name=xdebug.%t
But sending the XDEBUG_TRACE trigger never did produce any output in /tmp/. I then tried to call xdebug_start_trace() and xdebug_stop_trace() directly. But still no output in /tmp/.
Next I made a simple test script doing nothing more than starting tracing, echoing “test” and stop tracing. I ran it from the command line and I got a trace file in /tmp/! So I ran the same file through my webserver… and got nothing.
That's when it dawned on me and I changed my test file to this:
<?php xdebug_start_trace(); echo "test"; xdebug_stop_trace(); print_r(glob('/tmp/*'));
I ran it from my webserver and there they were! All the traces I produced earlier. But doing an ls /tmp/ did not show them!?
A bit googling finally brought the answer. In ArchLinux, Apache is started from systemd with the option PrivateTmp=true. Apparently that creates a virtual /tmp/ using the Kernel's file system namespace feature – here's a blog post describing it a bit more.
So the solution is to reconfigure the xdebug.trace_output_dir to point to a different directory.
TIL: /tmp/ might not be what you think it is.