Provide more info why we don't have any thumbnails to serve (#13038)

Fix https://github.com/matrix-org/synapse/issues/13016

## New error code and status

### Before

Previously, we returned a `404` for `/thumbnail` which isn't even in the spec.

```json
{
  "errcode": "M_NOT_FOUND",
  "error": "Not found [b'hs1', b'tefQeZhmVxoiBfuFQUKRzJxc']"
}
```

### After

What does the spec say?

> 400: The request does not make sense to the server, or the server cannot thumbnail the content. For example, the client requested non-integer dimensions or asked for negatively-sized images.
>
> *-- https://spec.matrix.org/v1.1/client-server-api/#get_matrixmediav3thumbnailservernamemediaid*

Now with this PR, we respond with a `400` when we don't have thumbnails to serve and we explain why we might not have any thumbnails.

```json
{
    "errcode": "M_UNKNOWN",
    "error": "Cannot find any thumbnails for the requested media ([b'example.com', b'12345']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)",
}
```

> Cannot find any thumbnails for the requested media ([b'example.com', b'12345']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)


---

We still respond with a 404 in many other places. But we can iterate on those later and maybe keep some in some specific places after spec updates/clarification: https://github.com/matrix-org/matrix-spec/issues/1122

We can also iterate on the bugs where Synapse doesn't thumbnail when it should in other issues/PRs.

changelog.d/13038.feature

@@ -0,0 +1 @@
+Provide more info why we don't have any thumbnails to serve.

synapse/config/repository.py

@@ -42,6 +42,18 @@ THUMBNAIL_SIZE_YAML = """\
         #    method: %(method)s
 """
 
+# A map from the given media type to the type of thumbnail we should generate
+# for it.
+THUMBNAIL_SUPPORTED_MEDIA_FORMAT_MAP = {
+    "image/jpeg": "jpeg",
+    "image/jpg": "jpeg",
+    "image/webp": "jpeg",
+    # Thumbnails can only be jpeg or png. We choose png thumbnails for gif
+    # because it can have transparency.
+    "image/gif": "png",
+    "image/png": "png",
+}
+
 HTTP_PROXY_SET_WARNING = """\
 The Synapse config url_preview_ip_range_blacklist will be ignored as an HTTP(s) proxy is configured."""
 
@@ -79,13 +91,22 @@ def parse_thumbnail_requirements(
         width = size["width"]
         height = size["height"]
         method = size["method"]
-        jpeg_thumbnail = ThumbnailRequirement(width, height, method, "image/jpeg")
-        png_thumbnail = ThumbnailRequirement(width, height, method, "image/png")
-        requirements.setdefault("image/jpeg", []).append(jpeg_thumbnail)
-        requirements.setdefault("image/jpg", []).append(jpeg_thumbnail)
-        requirements.setdefault("image/webp", []).append(jpeg_thumbnail)
-        requirements.setdefault("image/gif", []).append(png_thumbnail)
-        requirements.setdefault("image/png", []).append(png_thumbnail)
+
+        for format, thumbnail_format in THUMBNAIL_SUPPORTED_MEDIA_FORMAT_MAP.items():
+            requirement = requirements.setdefault(format, [])
+            if thumbnail_format == "jpeg":
+                requirement.append(
+                    ThumbnailRequirement(width, height, method, "image/jpeg")
+                )
+            elif thumbnail_format == "png":
+                requirement.append(
+                    ThumbnailRequirement(width, height, method, "image/png")
+                )
+            else:
+                raise Exception(
+                    "Unknown thumbnail mapping from %s to %s. This is a Synapse problem, please report!"
+                    % (format, thumbnail_format)
+                )
     return {
         media_type: tuple(thumbnails) for media_type, thumbnails in requirements.items()
     }

synapse/rest/media/v1/thumbnail_resource.py

@@ -17,9 +17,11 @@
 import logging
 from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple
 
-from synapse.api.errors import SynapseError
+from synapse.api.errors import Codes, SynapseError, cs_error
+from synapse.config.repository import THUMBNAIL_SUPPORTED_MEDIA_FORMAT_MAP
 from synapse.http.server import (
     DirectServeJsonResource,
+    respond_with_json,
     set_corp_headers,
     set_cors_headers,
 )
@@ -309,6 +311,19 @@ class ThumbnailResource(DirectServeJsonResource):
             url_cache: True if this is from a URL cache.
             server_name: The server name, if this is a remote thumbnail.
         """
+        logger.debug(
+            "_select_and_respond_with_thumbnail: media_id=%s desired=%sx%s (%s) thumbnail_infos=%s",
+            media_id,
+            desired_width,
+            desired_height,
+            desired_method,
+            thumbnail_infos,
+        )
+
+        # If `dynamic_thumbnails` is enabled, we expect Synapse to go down a
+        # different code path to handle it.
+        assert not self.dynamic_thumbnails
+
         if thumbnail_infos:
             file_info = self._select_thumbnail(
                 desired_width,
@@ -384,8 +399,29 @@ class ThumbnailResource(DirectServeJsonResource):
                 file_info.thumbnail.length,
             )
         else:
+            # This might be because:
+            # 1. We can't create thumbnails for the given media (corrupted or
+            #    unsupported file type), or
+            # 2. The thumbnailing process never ran or errored out initially
+            #    when the media was first uploaded (these bugs should be
+            #    reported and fixed).
+            # Note that we don't attempt to generate a thumbnail now because
+            # `dynamic_thumbnails` is disabled.
             logger.info("Failed to find any generated thumbnails")
-            respond_404(request)
+
+            respond_with_json(
+                request,
+                400,
+                cs_error(
+                    "Cannot find any thumbnails for the requested media (%r). This might mean the media is not a supported_media_format=(%s) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)"
+                    % (
+                        request.postpath,
+                        ", ".join(THUMBNAIL_SUPPORTED_MEDIA_FORMAT_MAP.keys()),
+                    ),
+                    code=Codes.UNKNOWN,
+                ),
+                send_cors=True,
+            )
 
     def _select_thumbnail(
         self,

tests/rest/media/v1/test_media_storage.py

@@ -126,7 +126,9 @@ class _TestImage:
         expected_scaled: The expected bytes from scaled thumbnailing, or None if
             test should just check for a valid image returned.
         expected_found: True if the file should exist on the server, or False if
-            a 404 is expected.
+            a 404/400 is expected.
+        unable_to_thumbnail: True if we expect the thumbnailing to fail (400), or
+            False if the thumbnailing should succeed or a normal 404 is expected.
     """
 
     data: bytes
@@ -135,6 +137,7 @@ class _TestImage:
     expected_cropped: Optional[bytes] = None
     expected_scaled: Optional[bytes] = None
     expected_found: bool = True
+    unable_to_thumbnail: bool = False
 
 
 @parameterized_class(
@@ -192,6 +195,7 @@ class _TestImage:
                 b"image/gif",
                 b".gif",
                 expected_found=False,
+                unable_to_thumbnail=True,
             ),
         ),
     ],
@@ -366,18 +370,29 @@ class MediaRepoTests(unittest.HomeserverTestCase):
     def test_thumbnail_crop(self) -> None:
         """Test that a cropped remote thumbnail is available."""
         self._test_thumbnail(
-            "crop", self.test_image.expected_cropped, self.test_image.expected_found
+            "crop",
+            self.test_image.expected_cropped,
+            expected_found=self.test_image.expected_found,
+            unable_to_thumbnail=self.test_image.unable_to_thumbnail,
         )
 
     def test_thumbnail_scale(self) -> None:
         """Test that a scaled remote thumbnail is available."""
         self._test_thumbnail(
-            "scale", self.test_image.expected_scaled, self.test_image.expected_found
+            "scale",
+            self.test_image.expected_scaled,
+            expected_found=self.test_image.expected_found,
+            unable_to_thumbnail=self.test_image.unable_to_thumbnail,
         )
 
     def test_invalid_type(self) -> None:
         """An invalid thumbnail type is never available."""
-        self._test_thumbnail("invalid", None, False)
+        self._test_thumbnail(
+            "invalid",
+            None,
+            expected_found=False,
+            unable_to_thumbnail=self.test_image.unable_to_thumbnail,
+        )
 
     @unittest.override_config(
         {"thumbnail_sizes": [{"width": 32, "height": 32, "method": "scale"}]}
@@ -386,7 +401,12 @@ class MediaRepoTests(unittest.HomeserverTestCase):
         """
         Override the config to generate only scaled thumbnails, but request a cropped one.
         """
-        self._test_thumbnail("crop", None, False)
+        self._test_thumbnail(
+            "crop",
+            None,
+            expected_found=False,
+            unable_to_thumbnail=self.test_image.unable_to_thumbnail,
+        )
 
     @unittest.override_config(
         {"thumbnail_sizes": [{"width": 32, "height": 32, "method": "crop"}]}
@@ -395,14 +415,22 @@ class MediaRepoTests(unittest.HomeserverTestCase):
         """
         Override the config to generate only cropped thumbnails, but request a scaled one.
         """
-        self._test_thumbnail("scale", None, False)
+        self._test_thumbnail(
+            "scale",
+            None,
+            expected_found=False,
+            unable_to_thumbnail=self.test_image.unable_to_thumbnail,
+        )
 
     def test_thumbnail_repeated_thumbnail(self) -> None:
         """Test that fetching the same thumbnail works, and deleting the on disk
         thumbnail regenerates it.
         """
         self._test_thumbnail(
-            "scale", self.test_image.expected_scaled, self.test_image.expected_found
+            "scale",
+            self.test_image.expected_scaled,
+            expected_found=self.test_image.expected_found,
+            unable_to_thumbnail=self.test_image.unable_to_thumbnail,
         )
 
         if not self.test_image.expected_found:
@@ -459,8 +487,24 @@ class MediaRepoTests(unittest.HomeserverTestCase):
             )
 
     def _test_thumbnail(
-        self, method: str, expected_body: Optional[bytes], expected_found: bool
+        self,
+        method: str,
+        expected_body: Optional[bytes],
+        expected_found: bool,
+        unable_to_thumbnail: bool = False,
     ) -> None:
+        """Test the given thumbnailing method works as expected.
+
+        Args:
+            method: The thumbnailing method to use (crop, scale).
+            expected_body: The expected bytes from thumbnailing, or None if
+                test should just check for a valid image.
+            expected_found: True if the file should exist on the server, or False if
+                a 404/400 is expected.
+            unable_to_thumbnail: True if we expect the thumbnailing to fail (400), or
+                False if the thumbnailing should succeed or a normal 404 is expected.
+        """
+
         params = "?width=32&height=32&method=" + method
         channel = make_request(
             self.reactor,
@@ -496,6 +540,16 @@ class MediaRepoTests(unittest.HomeserverTestCase):
             else:
                 # ensure that the result is at least some valid image
                 Image.open(BytesIO(channel.result["body"]))
+        elif unable_to_thumbnail:
+            # A 400 with a JSON body.
+            self.assertEqual(channel.code, 400)
+            self.assertEqual(
+                channel.json_body,
+                {
+                    "errcode": "M_UNKNOWN",
+                    "error": "Cannot find any thumbnails for the requested media ([b'example.com', b'12345']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)",
+                },
+            )
         else:
             # A 404 with a JSON body.
             self.assertEqual(channel.code, 404)