Artwork fetch: release the DNS slot if its thread can't start; stricter GIF control blocks

Final review follow-up. A failed Thread.start() leaked a resolver slot (four
failures disabled artwork lookups). GIF graphic-control blocks must have the
fixed 4-byte payload (otherwise dropped) and an image with no pixel data is
rejected.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/test_frame_android_library.py

@@ -232,6 +232,17 @@ class ArtworkTests(unittest.TestCase):
             time.sleep(0.01)
         _images._resolvers.release()  # the stuck lookups finished and gave their slots back
 
+    def test_resolver_slot_released_when_thread_cannot_start(self):
+        from apk_sources import _images
+        with patch.object(_images.threading.Thread, 'start', side_effect=RuntimeError("can't start new thread")):
+            for _ in range(6):
+                with self.assertRaises(RuntimeError):
+                    _images.get('https://example.org/a.png', deadline=time.monotonic() + 1)
+        for _ in range(4):  # every slot came back
+            self.assertTrue(_images._resolvers.acquire(blocking=False))
+        for _ in range(4):
+            _images._resolvers.release()
+
     def test_deadline_covers_name_resolution(self):
         import threading
         from apk_sources import _images
@@ -287,6 +298,17 @@ class ArtworkTests(unittest.TestCase):
             with self.subTest(cut=cut), self.assertRaises(ValueError):
                 art.gif_frame(one[:cut])
 
+    def test_gif_control_block_and_empty_image(self):
+        image = self.FRAME[8:]  # the image without its graphic control block
+        head = b'GIF89a' + struct.pack('<HHBBB', 1, 1, 0x80, 0, 0) + b'\xff\xff\xff\x00\x00\x00'
+        good = b'\x21\xf9\x04\x00\x00\x00\x00\x00'
+        self.assertEqual(art.gif_frame(head + good + image + b'\x3b'), head + good + image + b'\x3b')
+        bad = b'\x21\xf9\x02\x00\x00\x00'  # wrong payload size: dropped, not passed on
+        self.assertEqual(art.gif_frame(head + bad + image + b'\x3b'), head + image + b'\x3b')
+        empty = b'\x2c' + struct.pack('<HHHHB', 0, 0, 1, 1, 0) + b'\x02\x00'
+        with self.assertRaises(ValueError):
+            art.gif_frame(head + empty + b'\x3b')
+
     def test_png_variants_left_to_chromium_and_limits(self):
         def png(w, h, depth, color, interlace):
             return art.PNG + art.chunk(b'IHDR', struct.pack('>IIBBBBB', w, h, depth, color, 0, 0, interlace)) + \

ui/apk_sources/_images.py

@@ -86,8 +86,12 @@ def _resolve(host, port, timeout):
             found['error'] = e
         finally:
             _resolvers.release()
-    worker = threading.Thread(target=run, daemon=True)
-    worker.start()
+    try:
+        worker = threading.Thread(target=run, daemon=True)
+        worker.start()
+    except BaseException:
+        _resolvers.release()  # the worker never ran, so it can't release its slot
+        raise
     worker.join(timeout)
     if 'error' in found:
         raise found['error']

ui/frame_artwork.py

@@ -56,7 +56,7 @@ def gif_frame(data):
             raise ValueError('truncated GIF')
         if data[pos] == 0x21 and pos + 1 < len(data):  # extension: keep the frame's graphic control
             end = _gif_blocks(data, pos + 2)
-            if data[pos + 1] == 0xf9:
+            if data[pos + 1] == 0xf9 and end - pos == 8 and data[pos + 2] == 4:  # GIF89a: fixed 4-byte payload
                 control = data[pos:end]
             pos = end
         elif data[pos] == 0x2c and pos + 10 <= len(data):  # the first image
@@ -70,6 +70,8 @@ def gif_frame(data):
             if pos >= len(data) or not 2 <= data[pos] <= 8:  # the LZW minimum code size
                 raise ValueError('invalid GIF image data')
             end = _gif_blocks(data, pos + 1)
+            if end - pos <= 2:  # code size then the terminator: no pixels at all
+                raise ValueError('invalid GIF image data')
             return head + control + data[start:end] + b'\x3b'
         else:
             raise ValueError('invalid GIF block')