imagecopy fails when source image is corrupt - php

Question

I wrote a simple PHP script to add additional information to a webcam JPG file. It adds header and footer with some text. To do so I create a new image with this information and then copy the original JPG inside using imagecopy. And it all works fine.
The webcam has a poor wifi connection to the internet, so it seldom happens that the JPG file uploaded via FTP is partial or corrupt: I can open it using GIMP or other image software, and I see it has some missing information to the bottom. When this happens the above imagecopy seems to fail in copying image data, and the destination image remains blank (for the image area).
I tried everything I found to check the original JPG is valid:
// Check image
if (exif_imagetype($last) != IMAGETYPE_JPEG) // Not a valid jpeg
continue;
$details = getimagesize($last);
if ($details === FALSE) // Not a valid mage
continue;
$im = imagecreatefromjpeg($last);
but all tests pass. I also added:
if (imagecopy($dest_image, $im, 0, $top_banner_height+1, 0, 0, $img_width, $img_heigth) === FALSE) {
imagedestroy($im);
imagedestroy($dest_image);
continue;
}
but I still cannot catch an unterminated upload. How can I check if the image is valid for GD processing?
edit: this is how the source image appears in GIMP:
This is a portion of the original uploaded file.
As requested I added how I open the file, using imagecreatefromjpeg. It cannot be a permission problem because the script works fine 90% of the times, it's just when it encounters such images which fails.
edit2: I originally thought it could be a concurrency problem, being that I run the script via cron every minute, but the FTP upload is out of server control, so they run asynchronously. So perhaps the script was accessing the file exactly while being uploaded, but I checked and it's not the case, as I wrote above the uploaded file is corrupt in the beginning.
edit3: the suggested imagecolorat is not a solution (at least not at for all cases): I just found a messed up picture that would pass that test. jpeginfo says: Corrupt JPEG data: 10839 extraneous bytes before marker 0xd9

Answer 1

Thanks for all the suggestions, but in the end I found another possible way.
As said above and in other answers the command line tool jpeginfo -c filename checks if the image is a valid jpeg, printing a warning or and error (and a return code != 0). So it's the most precise solution so far, but involves an external command.
On the PHP side there doesn't seem to be a perfect check, as also the counterpart Imagick::valid doesn't perform a validity check on the full image.
At that very same page there's another proposed solution:
// check for the existence of the EOI segment header at the end of the file
$file = fopen($image_file, "r");
if (0 !== fseek($file, -2, SEEK_END) || "\xFF\xD9" !== fread($file, 2)) {
// jpeg is not valid
fclose($file);
return FALSE;
}
This works great, but only for unfinished images (first case). The example posted in edit 3 still passes this test.