avoid reusing the image buffer across frames of a different mode

Tests/test_file_dcx.py

@@ -97,6 +97,37 @@ def test_seek_too_far() -> None:
         im.seek(frame)
 
 
+def test_seek_frame_with_different_mode() -> None:
+    # A later frame may use a different mode and pixel size than the first.
+    # The backing image must not be reused across such a change.
+    def pcx(bits: int, planes: int, w: int, h: int, body: bytes) -> bytes:
+        stride = (w * bits + 7) // 8
+        stride += stride % 2
+        header = bytes([10, 5, 1, bits])
+        header += o16(0) + o16(0) + o16(w - 1) + o16(h - 1) + o16(72) + o16(72)
+        header += bytes(48) + b"\0" + bytes([planes]) + o16(stride)
+        return header + bytes(128 - len(header)) + body
+
+    w = h = 128
+    first_frame = pcx(8, 1, w, h, b"\x41" * (w * h))
+    second_frame = pcx(8, 3, w, h, b"\x41" * (3 * w * h))
+    b = BytesIO(
+        o32(DcxImagePlugin.MAGIC)
+        + o32(16)
+        + o32(len(first_frame) + 16)
+        + o32(0)
+        + first_frame
+        + second_frame
+    )
+    with Image.open(b) as im:
+        im.load()
+        assert im.mode == "L"
+        im.seek(1)
+        im.load()
+        assert im.mode == "RGB"
+        assert im.getpixel((0, 0)) == (0x41, 0x41, 0x41)
+
+
 def test_seek_decompression_bomb() -> None:
     with open("Tests/images/pil184.pcx", "rb") as fp:
         first_frame = fp.read()

Tests/test_file_mpo.py

@@ -256,6 +256,22 @@ def test_seek(test_file: str) -> None:
         assert im.tell() == 0
 
 
+def test_seek_frame_with_different_mode() -> None:
+    # A frame may use a different mode and pixel size than the first,
+    # so the backing image must not be reused across the change.
+    out = BytesIO()
+    Image.new("L", (128, 128)).save(
+        out, "MPO", save_all=True, append_images=[Image.new("RGB", (128, 128))]
+    )
+    out.seek(0)
+    with Image.open(out) as im:
+        im.load()
+        assert im.mode == "L"
+        im.seek(1)
+        im.load()
+        assert im.mode == "RGB"
+
+
 def test_seek_decompression_bomb() -> None:
     with open("Tests/images/frozenpond.mpo", "rb") as fp:
         data = fp.read()

Tests/test_file_psd.py

@@ -112,6 +112,24 @@ def test_seek_tell() -> None:
     assert layer_number == 2
 
 
+def test_seek_layer_with_different_mode() -> None:
+    # A layer may use a different mode and pixel size than the composite image,
+    # so the backing image must not be reused across the change.
+    with open(test_file, "rb") as fp:
+        data = bytearray(fp.read())
+    # Patch the header to report a single grayscale channel for the composite,
+    # while the RGB/RGBA layer records are left untouched.
+    data[12:14] = b"\x00\x01"  # channels
+    data[24:26] = b"\x00\x01"  # grayscale mode
+
+    with Image.open(io.BytesIO(data)) as im:
+        im.load()
+        assert im.mode == "L"
+        im.seek(im.n_frames)
+        im.load()
+        assert im.mode == "RGBA"
+
+
 def test_seek_back_to_initial_frame() -> None:
     # Seeking away from the initial frame and then back should produce the same image
     with Image.open(test_file) as im:

src/PIL/DcxImagePlugin.py

@@ -77,6 +77,10 @@ def seek(self, frame: int) -> None:
         self.fp.seek(self._offset[frame])
         PcxImageFile._open(self)
         Image._decompression_bomb_check(self.size)
+        if self._im is not None and (
+            self.im.size != self.size or self.im.mode != self.mode
+        ):
+            self._im = None
 
     def tell(self) -> int:
         return self.frame

src/PIL/MicImagePlugin.py

@@ -81,6 +81,10 @@ def seek(self, frame: int) -> None:
 
         TiffImagePlugin.TiffImageFile._open(self)
         Image._decompression_bomb_check(self.size)
+        if self._im is not None and (
+            self.im.size != self.size or self.im.mode != self.mode
+        ):
+            self._im = None
 
         self.frame = frame
 

src/PIL/MpoImagePlugin.py

@@ -169,6 +169,10 @@ def seek(self, frame: int) -> None:
         self.tile = [
             ImageFile._Tile("jpeg", (0, 0, *self.size), self.offset, self.tile[0][-1])
         ]
+        if self._im is not None and (
+            self.im.size != self.size or self.im.mode != self.mode
+        ):
+            self._im = None
         self.__frame = frame
 
     def tell(self) -> int:

src/PIL/PsdImagePlugin.py

@@ -185,6 +185,10 @@ def seek(self, layer: int) -> None:
         _, self._mode, _, self.tile = self.layers[layer - 1]
         self.frame = layer
         self.fp = self._fp
+        if self._im is not None and (
+            self.im.size != self.size or self.im.mode != self.mode
+        ):
+            self._im = None
 
     def tell(self) -> int:
         # return layer number (0=image, 1..max=layers)