Patched code sample

# The following conceptual code demonstrates the logic used to fix the vulnerability.
# The original code would loop indefinitely if an 'endcase' tag was missing.
# The fix involves explicitly checking for the end of the token stream inside
# the parsing loop, raising an error instead of hanging.

# Assume the existence of:
# - A `stream` object that provides `current` and `peek` tokens.
# - A `TOKEN_EOF` constant representing the end-of-file token type.
# - A `TemplateSyntaxError` exception class.

# In the context of a tag parsing function:
while not stream.current.istag("endcase"):
    if stream.current.istag("when") or stream.current.istag("else"):
        # Logic to handle 'when' and 'else' blocks would be here.
        stream.next()
        continue

    # THE FIX:
    # Check if the next token is the end of the stream. If so, it means
    # the 'case' block is unclosed, so we must raise an error to prevent
    # an infinite loop.
    if stream.peek.type == TOKEN_EOF:
        raise TemplateSyntaxError(
            f"expected 'endcase', but found {stream.current.value!r}",
            linenum=stream.current.linenum,
        )

    # In the real fix, this part raises an error for any other unexpected token.
    # For simplicity, we just advance the stream.
    stream.next()