Drupal 7 menu item called twice in one session - drupal-7

Question

Fellow Drupal developers,
I have a really strange issue. I have a small module that has a menu item, which outputs an image - in other words, it doesn't show any pages, any HTML or anything else, but simply sends a header('Content-Type: image/png'); and then outputs the PNG and exits with exit();
BUT... and this is really strange... sometimes it runs twice and goes through the function twice even though I only load the URL once. If I add a watchdog to the function and inspect the log afterwords, I can see that the function has been processed twice... sometimes. For no apparent reason it occasionally works as intended - one pass, one image output and then nothing, but at other times it runs twice.
If I add a counter that increments a number in the database, this number sometimes increments 1 and sometimes 2 in spite of me only loading the image once in the browser.
I have tested it on two servers (one Unix, one Windows)... same erratic behavior.
I have had my attention on headers and caching, but can't see that anything is wrong. The header for the image looks like this when I output a 1x1 PNG:
Date: Thu, 04 Oct 2012 09:21:51 GMT
Server: Apache/2.2.22 (Win32) PHP/5.2.17
X-Powered-By: PHP/5.2.17
Expires: Sun, 19 Nov 1978 05:00:00 GMT
Last-Modified: Thu, 04 Oct 2012 09:21:51 +0000
Cache-Control: no-cache, must-revalidate, post-check=0, pre-check=0
Etag: "1349342511"
Content-Length: 95
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: image/png
200 OK
If I add watchdogs here and there I can see that the module initializes more than one time, which is no surprise, but it really surprises me that my custom function is called more than once - and only sometimes. I have tried all kinds of magic, like adding a session variable that counts the number of passes and breaks after the first, but to no avail. The function runs more than once... most of the time.
It's critical for the purpose of the function that it ALWAYS runs once and only once.
Does anybody know what's happening?
Here's my basic code:
function my_image_menu() {
$items = array();
$items['image_1x1'] = array(
'title' => t('Create image'),
'description' => t('Output 1x1 PNG.'),
'page callback' => 'my_image_show',
'access arguments' => array('access content'),
);
return $items;
}
function my_image_show() {
watchdog('My Image', 'Image shown');
if (!headers_sent()) {
header('Content-Type: image/png');
echo base64_decode('iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEUAAACnej3aAAAAAXRSTlMAQObYZgAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=');
exit();
}
}
If I load http://mysite/image_1x1 I get one nice little 1x1 dot on the screen as expected, but most of the time (but not every time...) I get two "Image shown" entries in the log! In spite of the exit(), which should halt the script as far as I'm informed.
What voodoo might Drupal be doing on me?

Answer 1

Maybe this or maybe not! --- Both watchdog() and and exit() report to the log. Your watchdog() is not in a conditional - so it will always log. Your exit() is in a conditional so this will log only if the condition is met. This could explain the Voodoo.
Try die() instead of exit() for a cleaner log.

Answer 2

I have partly solved this problem by not outputting an image in the code but sending the browser on to a physical image file with a header command.
This seems to break any flow in Drupal and renders the image once as expected. The only disadvantage is that I can't just generate any image, but have to have it physically, but that's an obstacle I can overcome.
In other words replacing
if (!headers_sent()) {
header('Content-Type: image/png');
echo base64_decode('iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEUAAACnej3aAAAAAXRSTlMAQObYZgAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=');
exit(); }
with
if (!headers_sent()) {
header('location:/sites/default/files/1px.png');
}
in the code in my question and making sure that /sites/default/files/1px.png is on the server.
This will work for me now, but I'd still be glad to know what can stop what I guess is Drupal's handling of the exit() or die() commands.
Martin

Answer 3

May be you should try to check this on "clean" browser. Try Firefox (not Chrome!) without any extensions.
Add
watchdog('My Image debug info', print_r($_SERVER, true));
and analyze this output.