[New-bugs-announce] [issue33529] Infinite loop on folding email if headers has no spaces

[radical164]

New submission from radical164 <tptlab at tuta.io>:

I just reported a bug about email folding at issue 33524, but this issue is more fatal in some languages like Chinese or Japanese, which does not insert spaces between each words.
Python 3.6.5 has this issue, while 3.6.4 does not.

Create an email with longer header than max_line_length set by its policy.  And the header contains non-ascii characters but no white spaces.
When try to fold it, python gets stuck and finally system hangs. There are no output unless I stop it with Ctrl-C.

^CTraceback (most recent call last):
  File "emailtest.py", line 7, in <module>
    policy.fold("Subject", msg["Subject"])
  File "/usr/lib/python3.6/email/policy.py", line 183, in fold
    return self._fold(name, value, refold_binary=True)
  File "/usr/lib/python3.6/email/policy.py", line 205, in _fold
    return value.fold(policy=self)
  File "/usr/lib/python3.6/email/headerregistry.py", line 258, in fold
    return header.fold(policy=policy)
  File "/usr/lib/python3.6/email/_header_value_parser.py", line 144, in fold
    return _refold_parse_tree(self, policy=policy)
  File "/usr/lib/python3.6/email/_header_value_parser.py", line 2651, in _refold_parse_tree
    part.ew_combine_allowed, charset)
  File "/usr/lib/python3.6/email/_header_value_parser.py", line 2735, in _fold_as_ew
    ew = _ew.encode(first_part)
  File "/usr/lib/python3.6/email/_encoded_words.py", line 215, in encode
    blen = _cte_encode_length['b'](bstring)
  File "/usr/lib/python3.6/email/_encoded_words.py", line 130, in len_b
    groups_of_3, leftover = divmod(len(bstring), 3)
KeyboardInterrupt


Code to reproduce:

from email.message import EmailMessage
from email.policy import default

policy = default # max_line_length = 78
msg = EmailMessage()
msg["Subject"] = "á"*100
policy.fold("Subject", msg["Subject"])


No problems in following cases:

1. If the header is shorter than max_line_length.
2. If the header can be split with spaces and the all chunk is shorter than max_line_length.
3. If the header is fully composed with ascii characters. In this case, there is no problem even if it is very long without spaces.

----------
components: email
messages: 316747
nosy: barry, r.david.murray, radical164
priority: normal
severity: normal
status: open
title: Infinite loop on folding email if headers has no spaces
type: behavior
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33529>
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