gdk-pixbuf: fix memory leaks and API misuse in pixbuf_cons_fuzzer

[OwenSanzas]

Summary

This is not a bug in gdk-pixbuf. It is a bug in the fuzz harness pixbuf_cons_fuzzer that causes memory leaks (1704 bytes in 8 allocations per iteration) detectable by LeakSanitizer, and an API contract violation (gdk_pixbuf_scale with same src and dest).

False Positive Impact

If left unfixed, any leak report from this fuzzer will be a false positive — the leaks originate in the harness, not in gdk-pixbuf. This can lead to:

• Wasted developer time: Maintainers investigating leak reports that are not real gdk-pixbuf bugs
• Noise in OSS-Fuzz dashboards: Persistent unfixed "bugs" that are actually harness defects
• Amplified impact in the AI era: AI-assisted fuzzing tools increasingly reference existing OSS-Fuzz harnesses as ground truth. A buggy harness pattern can be copied and propagated by LLM-based harness generators, multiplying false positives across downstream projects

Bugs Fixed

P1 (Harness Logic): Multiple intermediate GdkPixbuf objects leaked — tmp is overwritten repeatedly without freeing previous values:

tmp = gdk_pixbuf_rotate_simple(pixbuf, rot_amount * 90);  // rotated leaked
tmp = gdk_pixbuf_flip(pixbuf, TRUE);                       // flipped leaked
tmp = gdk_pixbuf_composite_color_simple(pixbuf, ...);      // composite leaked

Also, GBytes from g_bytes_new_static() is never freed.

P2 (API Protocol): gdk_pixbuf_scale(pixbuf, pixbuf, ...) uses the same pixbuf as both source and destination. The API documentation requires they be different.

LSan Report

==14==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 96 byte(s) in 1 object(s) allocated from:
    #0 calloc
    #1 g_malloc0 (gmem.c:133)
    ...
    #5 gdk_pixbuf_new_from_data (gdk-pixbuf-data.c:79)
    #6 LLVMFuzzerTestOneInput (pixbuf_cons_fuzzer.c:65)

SUMMARY: AddressSanitizer: 1704 byte(s) leaked in 8 allocation(s).

The leak triggers on the first seed corpus input and is 100% reproducible.

Fix

1. Free each intermediate tmp with g_object_unref() before reassignment
2. Add g_bytes_unref(bytes) on all paths
3. Replace gdk_pixbuf_scale(src==dest) with gdk_pixbuf_scale_simple()

[github-actions]

OwenSanzas is a new contributor to projects/gdk-pixbuf. The PR must be approved by known contributors before it can be merged. The past contributors are: arthurscchan, hunsche, correctmost, tysmith, DonggeLiu, inferno-chromium, Google-Autofuzz, ecalp-tps