HTMLParser truncates DOCTYPE declarations at '>' inside quoted system identifiers

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python Library Reference, html.parser, HTMLParser.handle_decl: "The decl parameter will be the entire contents of the declaration inside the <! ... > markup (e.g. 'DOCTYPE html')."

Expected: handle_decl called exactly once with 'DOCTYPE html SYSTEM "a>b"'.

Actual: handle_decl called with the truncated string 'DOCTYPE html SYSTEM "a'.

from html.parser import HTMLParser
from xml.parsers.expat import ParserCreate

s = '<!DOCTYPE html SYSTEM "' + 'a>b' + '">'
actual, declarations = [], []

try:
    validator = ParserCreate()
    validator.StartDoctypeDeclHandler = lambda *args: declarations.append(args)
    validator.Parse(s + '<html/>', True)
    if declarations != [('html', 'a>b', None, 0)]:
        raise ValueError('system identifier was not preserved')
except Exception as e:
    print('REFUTATION REJECTED:', 'invalid declaration:', str(e))
else:
    parser = HTMLParser()
    parser.handle_decl = actual.append
    parser.feed(s)
    parser.close()
    expected = [s[2:-1]]
    if actual != expected:
        print('REFUTATION CONFIRMED:', repr(s), 'actual:', actual, 'expected:', expected)
    else:
        print('REFUTATION REJECTED:', 'actual matches documented expectation')

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library html.parser:

REFUTATION CONFIRMED: '<!DOCTYPE html SYSTEM "a>b">' actual: ['DOCTYPE html SYSTEM "a'] expected: ['DOCTYPE html SYSTEM "a>b"']

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/html.parser-20261003-091556-c3

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).