Pillow has type stubs

Had to make some more invasive changes to our media code. I replaced our
naughty calls to `_getdecoder` with calls to [check_codec], introduced
about seven years ago.

[check_codec]: https://github.com/python-pillow/Pillow/commit/799e8312cb6ede52d83d96f9ae414fb8ba457154

mypy didn't like our use of `_getexif`. The [Pillow 3.1.0 release
notes][pillow-3.1] describe this as

> private, experimental, but generally widely used

There's a public `getexif()` without an underscore, but that's only
available since [Pillow 6.0.0][pillow-6]

[pillow-3.1]: https://pillow.readthedocs.io/en/stable/releasenotes/3.1.0.html#jpegimageplugin-getexif
[pillow-6]: https://pillow.readthedocs.io/en/stable/releasenotes/6.0.0.html#added-exif-class

2 files changed, 25 insertions, 29 deletions

diff --git a/synapse/rest/media/v1/__init__.py b/synapse/rest/media/v1/__init__.py
index 3dd16d4bb5..d5b74cddf1 100644
--- a/synapse/rest/media/v1/__init__.py
+++ b/synapse/rest/media/v1/__init__.py
@@ -12,33 +12,21 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import PIL.Image
+from PIL.features import check_codec
 
 # check for JPEG support.
-try:
-    PIL.Image._getdecoder("rgb", "jpeg", None)
-except OSError as e:
-    if str(e).startswith("decoder jpeg not available"):
-        raise Exception(
-            "FATAL: jpeg codec not supported. Install pillow correctly! "
-            " 'sudo apt-get install libjpeg-dev' then 'pip uninstall pillow &&"
-            " pip install pillow --user'"
-        )
-except Exception:
-    # any other exception is fine
-    pass
+if not check_codec("jpg"):
+    raise Exception(
+        "FATAL: jpeg codec not supported. Install pillow correctly! "
+        " 'sudo apt-get install libjpeg-dev' then 'pip uninstall pillow &&"
+        " pip install pillow --user'"
+    )
 
 
 # check for PNG support.
-try:
-    PIL.Image._getdecoder("rgb", "zip", None)
-except OSError as e:
-    if str(e).startswith("decoder zip not available"):
-        raise Exception(
-            "FATAL: zip codec not supported. Install pillow correctly! "
-            " 'sudo apt-get install libjpeg-dev' then 'pip uninstall pillow &&"
-            " pip install pillow --user'"
-        )
-except Exception:
-    # any other exception is fine
-    pass
+if not check_codec("zlib"):
+    raise Exception(
+        "FATAL: zip codec not supported. Install pillow correctly! "
+        " 'sudo apt-get install libjpeg-dev' then 'pip uninstall pillow &&"
+        " pip install pillow --user'"
+    )
diff --git a/synapse/rest/media/v1/thumbnailer.py b/synapse/rest/media/v1/thumbnailer.py
index df54a40649..0955e08970 100644
--- a/synapse/rest/media/v1/thumbnailer.py
+++ b/synapse/rest/media/v1/thumbnailer.py
@@ -61,9 +61,14 @@ class Thumbnailer:
         self.transpose_method = None
         try:
             # We don't use ImageOps.exif_transpose since it crashes with big EXIF
-            image_exif = self.image._getexif()
+            # 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?
+            image_exif = self.image._getexif()  # type: ignore
             if image_exif is not None:
                 image_orientation = image_exif.get(EXIF_ORIENTATION_TAG)
+                assert isinstance(image_orientation, int)
                 self.transpose_method = EXIF_TRANSPOSE_MAPPINGS.get(image_orientation)
         except Exception as e:
             # A lot of parsing errors can happen when parsing EXIF
@@ -76,7 +81,10 @@ class Thumbnailer:
             A tuple containing the new image size in pixels as (width, height).
         """
         if self.transpose_method is not None:
-            self.image = self.image.transpose(self.transpose_method)
+            # 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.
+            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
@@ -101,7 +109,7 @@ class Thumbnailer:
         else:
             return (max_height * self.width) // self.height, max_height
 
-    def _resize(self, width: int, height: int) -> Image:
+    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.
@@ -151,7 +159,7 @@ class Thumbnailer:
             cropped = scaled_image.crop((crop_left, 0, crop_right, height))
         return self._encode_image(cropped, output_type)
 
-    def _encode_image(self, output_image: Image, output_type: str) -> BytesIO:
+    def _encode_image(self, output_image: Image.Image, output_type: str) -> BytesIO:
         output_bytes_io = BytesIO()
         fmt = self.FORMATS[output_type]
         if fmt == "JPEG":