Upgrading to v13.0.0 orphans all existing thumbnails

[SupImDos]

Description

The v13.0.0 release changed ImageFile.serialize_storage() to return storage aliases (e.g., "default") instead of backend class paths (e.g., "django.core.files.storage.filesystem.FileSystemStorage"). While this fixes issues with cloud storage backends preserving OPTIONS, it introduces a breaking change that was not documented: all existing cached thumbnails may become orphaned after upgrading.

Explanation

The ImageFile.key property computes a unique key based on the image name and serialized storage:

@property
def key(self):
    return tokey(self.name, self.serialize_storage())

In v12.x, serialize_storage() returned the backend class path. In v13.0.0, it returns the storage alias when one is found:

def serialize_storage(self):
    # ... 
    backend = f"{cls.__module__}.{cls.__name__}"
    # Try our best to find and serialize the storage alias instead of the backend class.
    for alias, params in storages.backends.items():
        if params.get("BACKEND") == backend:
            return alias
    return backend

So this means:

• v12: serialize_storage() -> "django.core.files.storage.filesystem.FileSystemStorage" (or similar)
• v13: serialize_storage() -> "default"

Ultimately, different storage serialization -> different key -> cache miss -> regenerate thumbnail. The old thumbnails are left orphaned, i.e., they still have an entry in the cache/database and their file remains in storage.

Impact

• All existing thumbnails in the KVStore become unreachable (lookups use new key format, pre-existing cached entries use old format)
• All thumbnails regenerated on first request after deploy
• Old thumbnail files left orphaned in database/cache/storage

Reproduction

Minimal reproducible example: https://github.com/SupImDos/sorl-thumbnail-cache-key-mre

[AndreyObviously]

to avoid thumbnails regeneration I was forced did monkey patch of ImageFile.key

def monkey_patch_sorl():
    from django.utils.functional import LazyObject, empty
    from sorl.thumbnail.helpers import tokey
    from sorl.thumbnail.images import ImageFile as SorlImageFile

    def patched_image_file_key(self):
        if isinstance(self.storage, LazyObject):
            if self.storage._wrapped is empty:
                self.storage._setup()
            cls = self.storage._wrapped.__class__
        else:
            cls = self.storage.__class__
        backend = f"{cls.__module__}.{cls.__name__}"

        return tokey(self.name, backend)

    SorlImageFile.key = property(patched_image_file_key)


monkey_patch_sorl()

[AndreyObviously]

I think original code can be changed to something like next:

    def serialize_storage(self, some_marker=False):
        if isinstance(self.storage, LazyObject):
            # if storage is wrapped in a lazy object we need to get the real
            # thing.
            if self.storage._wrapped is empty:
                self.storage._setup()
            cls = self.storage._wrapped.__class__
        else:
            cls = self.storage.__class__
        backend = f"{cls.__module__}.{cls.__name__}"
        if some_marker:
            return backend
        # Try our best to find and serialize the storage alias instead of the backend class.
        for alias, params in storages.backends.items():
            if params.get("BACKEND") == backend:
                return alias
        return backend

    @property
    def key(self):
        return tokey(self.name, self.serialize_storage(some_marker=True))