Handle additional errors when previewing URLs. (#9333)

* Handle the case of lxml not finding a document tree.
* Parse the document encoding from the XML tag.

3 files changed, 145 insertions, 30 deletions

diff --git a/changelog.d/9333.bugfix b/changelog.d/9333.bugfix
new file mode 100644
index 0000000000..c34ba378c5
--- /dev/null
+++ b/changelog.d/9333.bugfix
@@ -0,0 +1 @@
+Fix additional errors when previewing URLs: "AttributeError 'NoneType' object has no attribute 'xpath'" and "ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.".
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index bf3be653aa..ae53b1d23f 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -58,7 +58,10 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
-_charset_match = re.compile(br"<\s*meta[^>]*charset\s*=\s*([a-z0-9-]+)", flags=re.I)
+_charset_match = re.compile(br'<\s*meta[^>]*charset\s*=\s*"?([a-z0-9-]+)"?', flags=re.I)
+_xml_encoding_match = re.compile(
+    br'\s*<\s*\?\s*xml[^>]*encoding="([a-z0-9-]+)"', flags=re.I
+)
 _content_type_match = re.compile(r'.*; *charset="?(.*?)"?(;|$)', flags=re.I)
 
 OG_TAG_NAME_MAXLEN = 50
@@ -300,24 +303,7 @@ class PreviewUrlResource(DirectServeJsonResource):
             with open(media_info["filename"], "rb") as file:
                 body = file.read()
 
-            encoding = None
-
-            # Let's try and figure out if it has an encoding set in a meta tag.
-            # Limit it to the first 1kb, since it ought to be in the meta tags
-            # at the top.
-            match = _charset_match.search(body[:1000])
-
-            # If we find a match, it should take precedence over the
-            # Content-Type header, so set it here.
-            if match:
-                encoding = match.group(1).decode("ascii")
-
-            # If we don't find a match, we'll look at the HTTP Content-Type, and
-            # if that doesn't exist, we'll fall back to UTF-8.
-            if not encoding:
-                content_match = _content_type_match.match(media_info["media_type"])
-                encoding = content_match.group(1) if content_match else "utf-8"
-
+            encoding = get_html_media_encoding(body, media_info["media_type"])
             og = decode_and_calc_og(body, media_info["uri"], encoding)
 
             # pre-cache the image for posterity
@@ -689,6 +675,48 @@ class PreviewUrlResource(DirectServeJsonResource):
             logger.debug("No media removed from url cache")
 
 
+def get_html_media_encoding(body: bytes, content_type: str) -> str:
+    """
+    Get the encoding of the body based on the (presumably) HTML body or media_type.
+
+    The precedence used for finding a character encoding is:
+
+    1. meta tag with a charset declared.
+    2. The XML document's character encoding attribute.
+    3. The Content-Type header.
+    4. Fallback to UTF-8.
+
+    Args:
+        body: The HTML document, as bytes.
+        content_type: The Content-Type header.
+
+    Returns:
+        The character encoding of the body, as a string.
+    """
+    # Limit searches to the first 1kb, since it ought to be at the top.
+    body_start = body[:1024]
+
+    # Let's try and figure out if it has an encoding set in a meta tag.
+    match = _charset_match.search(body_start)
+    if match:
+        return match.group(1).decode("ascii")
+
+    # TODO Support <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
+
+    # If we didn't find a match, see if it an XML document with an encoding.
+    match = _xml_encoding_match.match(body_start)
+    if match:
+        return match.group(1).decode("ascii")
+
+    # If we don't find a match, we'll look at the HTTP Content-Type, and
+    # if that doesn't exist, we'll fall back to UTF-8.
+    content_match = _content_type_match.match(content_type)
+    if content_match:
+        return content_match.group(1)
+
+    return "utf-8"
+
+
 def decode_and_calc_og(
     body: bytes, media_uri: str, request_encoding: Optional[str] = None
 ) -> Dict[str, Optional[str]]:
@@ -725,6 +753,11 @@ def decode_and_calc_og(
     def _attempt_calc_og(body_attempt: Union[bytes, str]) -> Dict[str, Optional[str]]:
         # Attempt to parse the body. If this fails, log and return no metadata.
         tree = etree.fromstring(body_attempt, parser)
+
+        # The data was successfully parsed, but no tree was found.
+        if tree is None:
+            return {}
+
         return _calc_og(tree, media_uri)
 
     # Attempt to parse the body. If this fails, log and return no metadata.
diff --git a/tests/test_preview.py b/tests/test_preview.py
index 0c6cbbd921..ea83299918 100644
--- a/tests/test_preview.py
+++ b/tests/test_preview.py
@@ -15,6 +15,7 @@
 
 from synapse.rest.media.v1.preview_url_resource import (
     decode_and_calc_og,
+    get_html_media_encoding,
     summarize_paragraphs,
 )
 
@@ -26,7 +27,7 @@ except ImportError:
     lxml = None
 
 
-class PreviewTestCase(unittest.TestCase):
+class SummarizeTestCase(unittest.TestCase):
     if not lxml:
         skip = "url preview feature requires lxml"
 
@@ -144,12 +145,12 @@ class PreviewTestCase(unittest.TestCase):
         )
 
 
-class PreviewUrlTestCase(unittest.TestCase):
+class CalcOgTestCase(unittest.TestCase):
     if not lxml:
         skip = "url preview feature requires lxml"
 
     def test_simple(self):
-        html = """
+        html = b"""
         <html>
         <head><title>Foo</title></head>
         <body>
@@ -163,7 +164,7 @@ class PreviewUrlTestCase(unittest.TestCase):
         self.assertEqual(og, {"og:title": "Foo", "og:description": "Some text."})
 
     def test_comment(self):
-        html = """
+        html = b"""
         <html>
         <head><title>Foo</title></head>
         <body>
@@ -178,7 +179,7 @@ class PreviewUrlTestCase(unittest.TestCase):
         self.assertEqual(og, {"og:title": "Foo", "og:description": "Some text."})
 
     def test_comment2(self):
-        html = """
+        html = b"""
         <html>
         <head><title>Foo</title></head>
         <body>
@@ -202,7 +203,7 @@ class PreviewUrlTestCase(unittest.TestCase):
         )
 
     def test_script(self):
-        html = """
+        html = b"""
         <html>
         <head><title>Foo</title></head>
         <body>
@@ -217,7 +218,7 @@ class PreviewUrlTestCase(unittest.TestCase):
         self.assertEqual(og, {"og:title": "Foo", "og:description": "Some text."})
 
     def test_missing_title(self):
-        html = """
+        html = b"""
         <html>
         <body>
         Some text.
@@ -230,7 +231,7 @@ class PreviewUrlTestCase(unittest.TestCase):
         self.assertEqual(og, {"og:title": None, "og:description": "Some text."})
 
     def test_h1_as_title(self):
-        html = """
+        html = b"""
         <html>
         <meta property="og:description" content="Some text."/>
         <body>
@@ -244,7 +245,7 @@ class PreviewUrlTestCase(unittest.TestCase):
         self.assertEqual(og, {"og:title": "Title", "og:description": "Some text."})
 
     def test_missing_title_and_broken_h1(self):
-        html = """
+        html = b"""
         <html>
         <body>
         <h1><a href="foo"/></h1>
@@ -258,13 +259,20 @@ class PreviewUrlTestCase(unittest.TestCase):
         self.assertEqual(og, {"og:title": None, "og:description": "Some text."})
 
     def test_empty(self):
-        html = ""
+        """Test a body with no data in it."""
+        html = b""
+        og = decode_and_calc_og(html, "http://example.com/test.html")
+        self.assertEqual(og, {})
+
+    def test_no_tree(self):
+        """A valid body with no tree in it."""
+        html = b"\x00"
         og = decode_and_calc_og(html, "http://example.com/test.html")
         self.assertEqual(og, {})
 
     def test_invalid_encoding(self):
         """An invalid character encoding should be ignored and treated as UTF-8, if possible."""
-        html = """
+        html = b"""
         <html>
         <head><title>Foo</title></head>
         <body>
@@ -290,3 +298,76 @@ class PreviewUrlTestCase(unittest.TestCase):
         """
         og = decode_and_calc_og(html, "http://example.com/test.html")
         self.assertEqual(og, {"og:title": "ÿÿ Foo", "og:description": "Some text."})
+
+
+class MediaEncodingTestCase(unittest.TestCase):
+    def test_meta_charset(self):
+        """A character encoding is found via the meta tag."""
+        encoding = get_html_media_encoding(
+            b"""
+        <html>
+        <head><meta charset="ascii">
+        </head>
+        </html>
+        """,
+            "text/html",
+        )
+        self.assertEqual(encoding, "ascii")
+
+        # A less well-formed version.
+        encoding = get_html_media_encoding(
+            b"""
+        <html>
+        <head>< meta charset = ascii>
+        </head>
+        </html>
+        """,
+            "text/html",
+        )
+        self.assertEqual(encoding, "ascii")
+
+    def test_xml_encoding(self):
+        """A character encoding is found via the meta tag."""
+        encoding = get_html_media_encoding(
+            b"""
+        <?xml version="1.0" encoding="ascii"?>
+        <html>
+        </html>
+        """,
+            "text/html",
+        )
+        self.assertEqual(encoding, "ascii")
+
+    def test_meta_xml_encoding(self):
+        """Meta tags take precedence over XML encoding."""
+        encoding = get_html_media_encoding(
+            b"""
+        <?xml version="1.0" encoding="ascii"?>
+        <html>
+        <head><meta charset="UTF-16">
+        </head>
+        </html>
+        """,
+            "text/html",
+        )
+        self.assertEqual(encoding, "UTF-16")
+
+    def test_content_type(self):
+        """A character encoding is found via the Content-Type header."""
+        # Test a few variations of the header.
+        headers = (
+            'text/html; charset="ascii";',
+            "text/html;charset=ascii;",
+            'text/html;  charset="ascii"',
+            "text/html; charset=ascii",
+            'text/html; charset="ascii;',
+            'text/html; charset=ascii";',
+        )
+        for header in headers:
+            encoding = get_html_media_encoding(b"", header)
+            self.assertEqual(encoding, "ascii")
+
+    def test_fallback(self):
+        """A character encoding cannot be found in the body or header."""
+        encoding = get_html_media_encoding(b"", "text/html")
+        self.assertEqual(encoding, "utf-8")