HEIC preview fails with 512 MiB Imagick limit despite policy.xml override (fixed by newer ImageMagick/libheif)

TL;DR

Current included Imagick can't decode many larger heic due to bug, package update fixes bug.

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Description

HEIC images fail to generate previews in the stock Nextcloud Docker image due to an Imagick memory/security limit at exactly 512 MiB. Increasing ImageMagick policy limits and Nextcloud preview limits does not resolve the issue. Upgrading the ImageMagick/libheif stack inside the container immediately fixes the problem.

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Steps to reproduce

1. Deploy official Nextcloud Docker image
2. Upload HEIC image (modern phone photo, high resolution)
3. Trigger preview generation (web UI or background jobs)

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Actual behavior

Preview generation fails with:

ImagickException:` Memory allocation error:
Security limit exceeded: Allocating 599270415 bytes exceeds the security limit of 536870912 bytes
@ error/heic.c/IsHEIFSuccess/141

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Expected behavior

• ImageMagick should respect configured policy.xml limits
• HEIC previews should succeed when sufficient memory limits are configured

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What was tested

1. Nextcloud config

php 'preview_max_memory' => 1024,

→ No effect

2. ImageMagick policy override

/etc/ImageMagick-7/policy.xml modified:

<policy domain="resource" name="memory" value="1GiB"/>
<policy domain="resource" name="map" value="2GiB"/>
<policy domain="resource" name="disk" value="8GiB"/>
<policy domain="system" name="max-memory-request" value="1GiB"/>

→ Confirmed present inside running container
→ No effect (error still at exactly 536870912 bytes)

3. Direct PHP test (outside Nextcloud)

php -r 'try{
  $im=new Imagick();
  $im->readImage("/path/to/file.heic");
  echo "OK\n";
}catch(Throwable $e){
  echo $e->getMessage(),"\n";
}'

Result: Same 536870912-byte security limit error

→ Confirms issue is not Nextcloud-specific

4. Temporary upgrade inside container

Added newer packages (Debian sid):

apt-get update
apt-get install -y -t sid libheif1 libmagickcore-7.q16-10 libmagickwand-7.q16-10

Then reran same PHP test:

OK

→ Same HEIC file loads successfully

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Conclusion

• The failure is tied to the ImageMagick/libheif version shipped in the container
• The HEIC decode path appears to ignore or be constrained by a fixed 512 MiB limit in that build
• Policy changes (max-memory-request, etc.) do not affect this behavior
• Upgrading the stack resolves the issue immediately

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Environment

Nextcloud Docker image (official)
PHP imagick 3.8.1
ImageMagick 7.1.1-43 (container default)
libheif 1.19.8
Debian-based container

After manual upgrade:

• HEIC decoding works (with minor ABI warning)

Request

• Update ImageMagick/libheif packages in the base image

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Notes

This is reproducible via direct PHP Imagick usage (not specific to Nextcloud)
The error is deterministic and always occurs at 512 MiB allocation threshold
Behavior changes immediately with newer ImageMagick/libheif

Start by reproducing the failure with the direct PHP Imagick test and inspect the official Nextcloud Docker image's ImageMagick setup, including /etc/ImageMagick-7/policy.xml. Compare the shipped ImageMagick/libheif packages with the newer versions that were tested. Done means the updated image generates previews for the affected HEIC file without the 512 MiB error.