Replace deprecated sorl-thumbnail `is_default` with `exists()` in cover_image_* methods

[rdhyee]

Background

While bringing dj42.unglue.it back online (2026-04-28) for #1123 testing, surfaced a latent bug in core/models/bibmodels.py. The Edition.cover_image_* methods reference ImageFile.is_default, which was removed from sorl-thumbnail in the 12.6 → 12.11 upgrade (#1123 bumps sorl-thumbnail 12.6.3 → 12.11.0).

The bug

Three methods on Edition use the deprecated attribute:

core/models/bibmodels.py:896-915

def cover_image_small(self):
    if self.cover_image:
        im = get_thumbnail(self.cover_image, 'x80', crop='noop', quality=95)
        if not im.is_default:        # AttributeError in sorl 12.11.0
            return im.url
    if self.googlebooks_id:
        return "https://encrypted.google.com/books?id=%s..." % self.googlebooks_id
    return DEFAULT_COVER_SMALL

def cover_image_thumbnail(self):     # same pattern with 'x128'
    ...

def cover_image_large(self):         # also same pattern (line ~890)
    ...

In sorl 12.11.0, BaseImageFile/ImageFile/DummyImageFile no longer expose is_default. The replacement is im.exists() (a method that returns whether the thumbnail file actually exists).

Why dj42 still appears to render covers

Despite the missing attribute, on dj42 (running #1123's branch) the methods do return correct cover URLs — at least for warm code paths. Direct probe of im.is_default reliably raises AttributeError, but execution traces (sys.settrace) show line 900 evaluating without raising during normal method invocation. Likely related to sorl's KVStore state — the AttributeError is intermittent rather than deterministic, which is more dangerous than a clean failure.

This means current behavior is fragile/heisenbug-prone:

• Cold thumbnail cache: may surface the AttributeError → 500 on the page
• Warm cache: silently works but on a deprecated API path

Proposed fix

Three-call-site mechanical change:

-        if not im.is_default:
+        if im.exists():

Applied to all three cover_image_* methods (small, thumbnail, large). The semantic is identical: "if the generated thumbnail file actually exists, return its URL; otherwise fall through to the Google Books / DEFAULT_COVER fallback."

Reference: sorl-thumbnail 12.11.0 release notes — no formal CHANGELOG entry for is_default removal but the attribute is gone from the source.

Discovered via

Comparison testing of dj42 (Django 4.2 / sorl 12.11.0) against test (Django 1.11 / sorl 12.6.3) for the same DB content. Same work, same edition, same cover_image field — both stacks return identical cover URLs (no behavioral regression for end users), but the dj42 path goes through the deprecated attribute access.

Test plan when fixing

• On dj42 with cold cache (clear sorl KVStore): verify Edition.cover_image_small/thumbnail/large() for at least 3 editions with valid cover_image — should return valid URL, not raise
• On dj42 with warm cache: same, no regression
• On dj42 for editions WITHOUT cover_image but WITH googlebooks_id: should fall back to Google Books URL
• On dj42 for editions with neither: should return DEFAULT_COVER_*

Scope

Only core/models/bibmodels.py (3 call sites, one file). Trivial change but should be in #1123 (or a fast follow-up PR against fix/django-upgrade-libraryauth) before the migration cuts over to prod.

Related

• Upgrade Django 1.11 → 4.2 LTS #1123 — Django 4.2 upgrade (where this regression was introduced via sorl-thumbnail bump)
• Discovered during dj42 bring-up after regluit-provisioning#25's validation work