Ensure each charset is attempted only once during medai preview.

changelog.d/11089.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug when attempting to preview URLs which are in the `windows-1252` character encoding.

synapse/rest/media/v1/preview_url_resource.py

@@ -12,6 +12,7 @@
 # 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 codecs
 import datetime
 import errno
 import fnmatch
@@ -22,7 +23,7 @@
 import shutil
 import sys
 import traceback
-from typing import TYPE_CHECKING, Dict, Generator, Iterable, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Dict, Generator, Iterable, Optional, Set, Tuple, Union
 from urllib import parse as urlparse
 
 import attr
@@ -631,6 +632,11 @@ def try_remove_parent_dirs(dirs: Iterable[str]) -> None:
             logger.debug("No media removed from url cache")
 
 
+def _normalise_encoding(encoding: str) -> str:
+    """Use the Python codec's name as the normalised entry."""
+    return codecs.lookup(encoding).name
+
+
 def get_html_media_encodings(body: bytes, content_type: Optional[str]) -> Iterable[str]:
     """
     Get potential encoding of the body based on the (presumably) HTML body or the content-type header.
@@ -652,30 +658,42 @@ def get_html_media_encodings(body: bytes, content_type: Optional[str]) -> Iterab
     Returns:
         The character encoding of the body, as a string.
     """
+    # There's no point in returning an encoding more than once.
+    attempted_encodings: Set[str] = set()
+
     # Limit searches to the first 1kb, since it ought to be at the top.
     body_start = body[:1024]
 
     # Check if it has an encoding set in a meta tag.
     match = _charset_match.search(body_start)
     if match:
-        yield match.group(1).decode("ascii")
+        encoding = _normalise_encoding(match.group(1).decode("ascii"))
+        attempted_encodings.add(encoding)
+        yield encoding
 
     # TODO Support <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
 
     # Check if it has an XML document with an encoding.
     match = _xml_encoding_match.match(body_start)
     if match:
-        yield match.group(1).decode("ascii")
+        encoding = _normalise_encoding(match.group(1).decode("ascii"))
+        if encoding not in attempted_encodings:
+            attempted_encodings.add(encoding)
+            yield encoding
 
     # Check the HTTP Content-Type header for a character set.
     if content_type:
         content_match = _content_type_match.match(content_type)
         if content_match:
-            yield content_match.group(1)
+            encoding = _normalise_encoding(content_match.group(1))
+            if encoding not in attempted_encodings:
+                attempted_encodings.add(encoding)
+                yield encoding
 
     # Finally, fallback to UTF-8, then windows-1252.
-    yield "utf-8"
-    yield "windows-1252"
+    for fallback in ("utf-8", "windows-1252"):
+        if fallback not in attempted_encodings:
+            yield fallback
 
 
 def decode_body(

tests/test_preview.py

@@ -358,7 +358,7 @@ def test_meta_charset_underscores(self):
         """,
             "text/html",
         )
-        self.assertEqual(list(encodings), ["Shift_JIS", "utf-8", "windows-1252"])
+        self.assertEqual(list(encodings), ["shift_jis", "utf-8", "windows-1252"])
 
     def test_xml_encoding(self):
         """A character encoding is found via the meta tag."""
@@ -384,7 +384,7 @@ def test_meta_xml_encoding(self):
         """,
             "text/html",
         )
-        self.assertEqual(list(encodings), ["UTF-16", "ascii", "utf-8", "windows-1252"])
+        self.assertEqual(list(encodings), ["utf-16", "ascii", "utf-8", "windows-1252"])
 
     def test_content_type(self):
         """A character encoding is found via the Content-Type header."""
@@ -405,3 +405,17 @@ def test_fallback(self):
         """A character encoding cannot be found in the body or header."""
         encodings = get_html_media_encodings(b"", "text/html")
         self.assertEqual(list(encodings), ["utf-8", "windows-1252"])
+
+    def test_duplicates(self):
+        """Ensure each encoding is only attempted once."""
+        encodings = get_html_media_encodings(
+            b"""
+        <?xml version="1.0" encoding="utf8"?>
+        <html>
+        <head><meta charset="UTF-8">
+        </head>
+        </html>
+        """,
+            'text/html; charset="UTF_8"',
+        )
+        self.assertEqual(list(encodings), ["utf-8", "windows-1252"])