diff --git a/synapse/media/thumbnailer.py b/synapse/media/thumbnailer.py
new file mode 100644
index 0000000000..f909a4fb9a
--- /dev/null
+++ b/synapse/media/thumbnailer.py
@@ -0,0 +1,221 @@
+# Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2020-2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+from io import BytesIO
+from types import TracebackType
+from typing import Optional, Tuple, Type
+
+from PIL import Image
+
+logger = logging.getLogger(__name__)
+
+EXIF_ORIENTATION_TAG = 0x0112
+EXIF_TRANSPOSE_MAPPINGS = {
+ 2: Image.FLIP_LEFT_RIGHT,
+ 3: Image.ROTATE_180,
+ 4: Image.FLIP_TOP_BOTTOM,
+ 5: Image.TRANSPOSE,
+ 6: Image.ROTATE_270,
+ 7: Image.TRANSVERSE,
+ 8: Image.ROTATE_90,
+}
+
+
+class ThumbnailError(Exception):
+ """An error occurred generating a thumbnail."""
+
+
+class Thumbnailer:
+ FORMATS = {"image/jpeg": "JPEG", "image/png": "PNG"}
+
+ @staticmethod
+ def set_limits(max_image_pixels: int) -> None:
+ Image.MAX_IMAGE_PIXELS = max_image_pixels
+
+ def __init__(self, input_path: str):
+ # Have we closed the image?
+ self._closed = False
+
+ try:
+ self.image = Image.open(input_path)
+ except OSError as e:
+ # If an error occurs opening the image, a thumbnail won't be able to
+ # be generated.
+ raise ThumbnailError from e
+ except Image.DecompressionBombError as e:
+ # If an image decompression bomb error occurs opening the image,
+ # then the image exceeds the pixel limit and a thumbnail won't
+ # be able to be generated.
+ raise ThumbnailError from e
+
+ self.width, self.height = self.image.size
+ self.transpose_method = None
+ try:
+ # We don't use ImageOps.exif_transpose since it crashes with big EXIF
+ #
+ # Ignore safety: Pillow seems to acknowledge that this method is
+ # "private, experimental, but generally widely used". Pillow 6
+ # includes a public getexif() method (no underscore) that we might
+ # consider using instead when we can bump that dependency.
+ #
+ # At the time of writing, Debian buster (currently oldstable)
+ # provides version 5.4.1. It's expected to EOL in mid-2022, see
+ # https://wiki.debian.org/DebianReleases#Production_Releases
+ image_exif = self.image._getexif() # type: ignore
+ if image_exif is not None:
+ image_orientation = image_exif.get(EXIF_ORIENTATION_TAG)
+ assert type(image_orientation) is int
+ self.transpose_method = EXIF_TRANSPOSE_MAPPINGS.get(image_orientation)
+ except Exception as e:
+ # A lot of parsing errors can happen when parsing EXIF
+ logger.info("Error parsing image EXIF information: %s", e)
+
+ def transpose(self) -> Tuple[int, int]:
+ """Transpose the image using its EXIF Orientation tag
+
+ Returns:
+ A tuple containing the new image size in pixels as (width, height).
+ """
+ if self.transpose_method is not None:
+ # Safety: `transpose` takes an int rather than e.g. an IntEnum.
+ # self.transpose_method is set above to be a value in
+ # EXIF_TRANSPOSE_MAPPINGS, and that only contains correct values.
+ with self.image:
+ self.image = self.image.transpose(self.transpose_method) # type: ignore[arg-type]
+ self.width, self.height = self.image.size
+ self.transpose_method = None
+ # We don't need EXIF any more
+ self.image.info["exif"] = None
+ return self.image.size
+
+ def aspect(self, max_width: int, max_height: int) -> Tuple[int, int]:
+ """Calculate the largest size that preserves aspect ratio which
+ fits within the given rectangle::
+
+ (w_in / h_in) = (w_out / h_out)
+ w_out = max(min(w_max, h_max * (w_in / h_in)), 1)
+ h_out = max(min(h_max, w_max * (h_in / w_in)), 1)
+
+ Args:
+ max_width: The largest possible width.
+ max_height: The largest possible height.
+ """
+
+ if max_width * self.height < max_height * self.width:
+ return max_width, max((max_width * self.height) // self.width, 1)
+ else:
+ return max((max_height * self.width) // self.height, 1), max_height
+
+ def _resize(self, width: int, height: int) -> Image.Image:
+ # 1-bit or 8-bit color palette images need converting to RGB
+ # otherwise they will be scaled using nearest neighbour which
+ # looks awful.
+ #
+ # If the image has transparency, use RGBA instead.
+ if self.image.mode in ["1", "L", "P"]:
+ if self.image.info.get("transparency", None) is not None:
+ with self.image:
+ self.image = self.image.convert("RGBA")
+ else:
+ with self.image:
+ self.image = self.image.convert("RGB")
+ return self.image.resize((width, height), Image.ANTIALIAS)
+
+ def scale(self, width: int, height: int, output_type: str) -> BytesIO:
+ """Rescales the image to the given dimensions.
+
+ Returns:
+ The bytes of the encoded image ready to be written to disk
+ """
+ with self._resize(width, height) as scaled:
+ return self._encode_image(scaled, output_type)
+
+ def crop(self, width: int, height: int, output_type: str) -> BytesIO:
+ """Rescales and crops the image to the given dimensions preserving
+ aspect::
+ (w_in / h_in) = (w_scaled / h_scaled)
+ w_scaled = max(w_out, h_out * (w_in / h_in))
+ h_scaled = max(h_out, w_out * (h_in / w_in))
+
+ Args:
+ max_width: The largest possible width.
+ max_height: The largest possible height.
+
+ Returns:
+ The bytes of the encoded image ready to be written to disk
+ """
+ if width * self.height > height * self.width:
+ scaled_width = width
+ scaled_height = (width * self.height) // self.width
+ crop_top = (scaled_height - height) // 2
+ crop_bottom = height + crop_top
+ crop = (0, crop_top, width, crop_bottom)
+ else:
+ scaled_width = (height * self.width) // self.height
+ scaled_height = height
+ crop_left = (scaled_width - width) // 2
+ crop_right = width + crop_left
+ crop = (crop_left, 0, crop_right, height)
+
+ with self._resize(scaled_width, scaled_height) as scaled_image:
+ with scaled_image.crop(crop) as cropped:
+ return self._encode_image(cropped, output_type)
+
+ def _encode_image(self, output_image: Image.Image, output_type: str) -> BytesIO:
+ output_bytes_io = BytesIO()
+ fmt = self.FORMATS[output_type]
+ if fmt == "JPEG":
+ output_image = output_image.convert("RGB")
+ output_image.save(output_bytes_io, fmt, quality=80)
+ return output_bytes_io
+
+ def close(self) -> None:
+ """Closes the underlying image file.
+
+ Once closed no other functions can be called.
+
+ Can be called multiple times.
+ """
+
+ if self._closed:
+ return
+
+ self._closed = True
+
+ # Since we run this on the finalizer then we need to handle `__init__`
+ # raising an exception before it can define `self.image`.
+ image = getattr(self, "image", None)
+ if image is None:
+ return
+
+ image.close()
+
+ def __enter__(self) -> "Thumbnailer":
+ """Make `Thumbnailer` a context manager that calls `close` on
+ `__exit__`.
+ """
+ return self
+
+ def __exit__(
+ self,
+ type: Optional[Type[BaseException]],
+ value: Optional[BaseException],
+ traceback: Optional[TracebackType],
+ ) -> None:
+ self.close()
+
+ def __del__(self) -> None:
+ # Make sure we actually do close the image, rather than leak data.
+ self.close()
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