Improve WebVTT parser with robust error handling and input validation

Enhances the WebVTT partial parser by adding comprehensive error handling, type validation, and defensive checks to prevent unexpected failures during parsing. Specifically, input types are validated in _MatchParser and parse_fragment, ensuring only valid strings or bytes are accepted. Timestamp parsing now raises clear errors for invalid matches, while regex operations are guarded to avoid NoneType attribute errors. The .decode() step in parse_fragment uses safe fallback to handle invalid byte sequences gracefully.

yt_dlp/webvtt.py

@@ -22,7 +22,7 @@ class _MatchParser:
 
     def __init__(self, string):
         if not isinstance(string, str):
-            raise TypeError("Expected string input to _MatchParser")
+            raise TypeError('Expected string input to _MatchParser')
         self._data = string
         self._pos = 0
 
@@ -33,7 +33,7 @@ def match(self, r):
             if self._data.startswith(r, self._pos):
                 return len(r)
             return None
-        raise ValueError(f"Expected regex or string, got {type(r).__name__}")
+        raise ValueError(f'Expected regex or string, got {type(r).__name__}')
 
     def advance(self, by):
         if by is None:
@@ -45,7 +45,7 @@ def advance(self, by):
         elif isinstance(by, int):
             amt = by
         else:
-            raise ValueError(f"Unsupported advance type: {type(by).__name__}")
+            raise ValueError(f'Unsupported advance type: {type(by).__name__}')
         self._pos += amt
         return by
 
@@ -105,7 +105,7 @@ def _parse_ts(ts):
     into an MPEG PES timestamp: a tick counter at 90 kHz resolution.
     """
     if ts is None or not isinstance(ts, re.Match):
-        raise ValueError("Invalid timestamp match for _parse_ts")
+        raise ValueError('Invalid timestamp match for _parse_ts')
     return 90 * sum(
         int(part or 0) * mult for part, mult in zip(ts.groups(), (3600_000, 60_000, 1000, 1)))
 
@@ -300,9 +300,9 @@ def parse_fragment(frag_content):
     a bytes object containing the raw contents of a WebVTT file.
     """
     if not isinstance(frag_content, (bytes, bytearray)):
-        raise TypeError("Expected bytes for frag_content")
+        raise TypeError('Expected bytes for frag_content')
 
-    parser = _MatchParser(frag_content.decode(errors="replace"))
+    parser = _MatchParser(frag_content.decode(errors='replace'))
 
     yield Magic.parse(parser)