Fix /upload 500'ing when presented a very large image (#10029)

* Fix /upload 500'ing when presented a very large image

Catch DecompressionBombError and re-raise as ThumbnailErrors

* Set PIL's MAX_IMAGE_PIXELS to match homeserver.yaml

to get it to bomb out quicker, to load less into memory
in the case of super large images

* Add changelog entry for 10029

changelog.d/10029.bugfix

@@ -0,0 +1 @@
+Fixed a bug with very high resolution image uploads throwing internal server errors.

synapse/rest/media/v1/media_repository.py

@@ -76,6 +76,8 @@ def __init__(self, hs: "HomeServer"):
         self.max_upload_size = hs.config.max_upload_size
         self.max_image_pixels = hs.config.max_image_pixels
 
+        Thumbnailer.set_limits(self.max_image_pixels)
+
         self.primary_base_path = hs.config.media_store_path  # type: str
         self.filepaths = MediaFilePaths(self.primary_base_path)  # type: MediaFilePaths
 

synapse/rest/media/v1/thumbnailer.py

@@ -40,13 +40,22 @@ class Thumbnailer:
 
     FORMATS = {"image/jpeg": "JPEG", "image/png": "PNG"}
 
+    @staticmethod
+    def set_limits(max_image_pixels: int):
+        Image.MAX_IMAGE_PIXELS = max_image_pixels
+
     def __init__(self, input_path: str):
         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