Support writing LONG8 offsets in AppendingTiffWriter

Tests/test_file_tiff.py

@@ -108,10 +108,6 @@ def test_bigtiff(self, tmp_path: Path) -> None:
             assert_image_equal_tofile(im, "Tests/images/hopper.tif")
 
         with Image.open("Tests/images/hopper_bigtiff.tif") as im:
-            # The data type of this file's StripOffsets tag is LONG8,
-            # which is not yet supported for offset data when saving multiple frames.
-            del im.tag_v2[273]
-
             outfile = str(tmp_path / "temp.tif")
             im.save(outfile, save_all=True, append_images=[im], tiffinfo=im.tag_v2)
 
@@ -732,6 +728,20 @@ def im_generator(ims: list[Image.Image]) -> Generator[Image.Image, None, None]:
         with Image.open(mp) as reread:
             assert reread.n_frames == 3
 
+    def test_fixoffsets(self) -> None:
+        b = BytesIO(b"II\x2a\x00\x00\x00\x00\x00")
+        with TiffImagePlugin.AppendingTiffWriter(b) as a:
+            b.seek(0)
+            a.fixOffsets(1, isShort=True)
+
+            b.seek(0)
+            a.fixOffsets(1, isLong=True)
+
+            # Neither short nor long
+            b.seek(0)
+            with pytest.raises(RuntimeError):
+                a.fixOffsets(1)
+
     def test_saving_icc_profile(self, tmp_path: Path) -> None:
         # Tests saving TIFF with icc_profile set.
         # At the time of writing this will only work for non-compressed tiffs

src/PIL/TiffImagePlugin.py

@@ -2121,13 +2121,24 @@
     def write(self, data: Buffer, /) -> int:
         return self.f.write(data)
 
-    def readShort(self) -> int:
-        (value,) = struct.unpack(self.shortFmt, self.f.read(2))
+    def _fmt(self, field_size: int) -> str:
+        try:
+            return {2: "H", 4: "L", 8: "Q"}[field_size]
+        except KeyError:
+            msg = "offset is not supported"
+            raise RuntimeError(msg)
+
+    def _read(self, field_size: int) -> int:
+        (value,) = struct.unpack(
+            self.endian + self._fmt(field_size), self.f.read(field_size)
+        )
         return value
 
+    def readShort(self) -> int:
+        return self._read(2)
+
     def readLong(self) -> int:
-        (value,) = struct.unpack(self.longFmt, self.f.read(4))
-        return value
+        return self._read(4)
 
     @staticmethod
     def _verify_bytes_written(bytes_written: int | None, expected: int) -> None:
@@ -2140,15 +2151,18 @@
         bytes_written = self.f.write(struct.pack(self.longFmt, value))
         self._verify_bytes_written(bytes_written, 4)
 
+    def _rewriteLast(self, value: int, field_size: int) -> None:
+        self.f.seek(-field_size, os.SEEK_CUR)
+        bytes_written = self.f.write(
+            struct.pack(self.endian + self._fmt(field_size), value)
+        )
+        self._verify_bytes_written(bytes_written, field_size)
+
     def rewriteLastShort(self, value: int) -> None:
-        self.f.seek(-2, os.SEEK_CUR)
-        bytes_written = self.f.write(struct.pack(self.shortFmt, value))
-        self._verify_bytes_written(bytes_written, 2)
+        return self._rewriteLast(value, 2)
 
     def rewriteLastLong(self, value: int) -> None:
-        self.f.seek(-4, os.SEEK_CUR)
-        bytes_written = self.f.write(struct.pack(self.longFmt, value))
-        self._verify_bytes_written(bytes_written, 4)
+        return self._rewriteLast(value, 4)
 
     def writeShort(self, value: int) -> None:
         bytes_written = self.f.write(struct.pack(self.shortFmt, value))
@@ -2180,32 +2194,22 @@
                 cur_pos = self.f.tell()
 
                 if is_local:
-                    self.fixOffsets(
-                        count, isShort=(field_size == 2), isLong=(field_size == 4)
-                    )
+                    self._fixOffsets(count, field_size)
                     self.f.seek(cur_pos + 4)
                 else:
                     self.f.seek(offset)
-                    self.fixOffsets(
-                        count, isShort=(field_size == 2), isLong=(field_size == 4)
-                    )
+                    self._fixOffsets(count, field_size)
                     self.f.seek(cur_pos)
 
             elif is_local:
                 # skip the locally stored value that is not an offset
                 self.f.seek(4, os.SEEK_CUR)
 
-    def fixOffsets(
-        self, count: int, isShort: bool = False, isLong: bool = False
-    ) -> None:
-        if not isShort and not isLong:
-            msg = "offset is neither short nor long"
-            raise RuntimeError(msg)
-
+    def _fixOffsets(self, count: int, field_size: int) -> None:
         for i in range(count):
-            offset = self.readShort() if isShort else self.readLong()
+            offset = self._read(field_size)
             offset += self.offsetOfNewPage
-            if isShort and offset >= 65536:
+            if field_size == 2 and offset >= 65536:
                 # offset is now too large - we must convert shorts to longs
                 if count != 1:
                     msg = "not implemented"
@@ -2217,10 +2221,19 @@
                 self.f.seek(-10, os.SEEK_CUR)
                 self.writeShort(TiffTags.LONG)  # rewrite the type to LONG
                 self.f.seek(8, os.SEEK_CUR)
-            elif isShort:
-                self.rewriteLastShort(offset)
             else:
-                self.rewriteLastLong(offset)
+                self._rewriteLast(offset, field_size)
+
+    def fixOffsets(
+        self, count: int, isShort: bool = False, isLong: bool = False
+    ) -> None:
+        if isShort:
+            field_size = 2
+        elif isLong:
+            field_size = 4
+        else:
+            field_size = 0
+        return self._fixOffsets(count, field_size)
 
 
 def _save_all(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None: