Description:
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I'm using a PHP script to convert an image (png in my testcase) into a 32-bit png and write it to disk again (I stripped out all transformations and security checks), see code below. When called with "/thumbnail.php?file=/path/to/image.png" I get an error when trying to write the file to disk with IMagick::writeImage(). When I echo the image with IMagick::getImageBlob() I get the expected result. The target directory is chmod'd to 777 to ensure there are no write restrictions.

Reproduce code:
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<?php

$thumb = new Imagick($_GET['file']);
$thumb->setImageFormat('PNG32');

// dir '/srv/temp/thumbnails is 777 and owner:group is www-data:www-data (the apache user)
$thumb->writeImage('/srv/temp/thumbnails/test.png');


Expected result:
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I expect the extension to write the image to disk.

Actual result:
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This error message is thrown:

Fatal error: Uncaught exception 'ImagickException' with message 'WriteBlob Failed `/srv/temp/thumbnails/test.png' @ error/png.c/PNGErrorHandler/1455' in /srv/htdocs/thumbnail.php:7 Stack trace: #0 /srv/htdocs/thumbnail.php(7): Imagick->writeimage('/srv/temp/thumb...') #1 {main} thrown in /srv/htdocs/thumbnail.php on line 7