Date parsing helpers in email module incorrectly raise IndexError for some malformed inputs

[wbolster]

BPO	45001
Nosy	@warsaw, @ned-deily, @bitdancer, @ambv, @wbolster, @miss-islington
PRs	bpo-45001: Make email date parsing more robust against malformed input #27946 [3.10] bpo-45001: Make email date parsing more robust against malformed input (GH-27946) #27972 [3.9] bpo-45001: Make email date parsing more robust against malformed input (GH-27946) #27973 [3.8] bpo-45001: Make email date parsing more robust against malformed input (GH-27946) #27974 [3.7] bpo-45001: Make email date parsing more robust against malformed input (GH-27946) #27975 [3.6] bpo-45001: Make email date parsing more robust against malformed input (GH-27946) #27976

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GitHub fields:

assignee = None
closed_at = <Date 2021-08-30.18:58:53.311>
created_at = <Date 2021-08-25.13:23:27.104>
labels = ['3.8', 'expert-email', '3.10', '3.11', '3.7', 'type-crash', '3.9']
title = 'Date parsing helpers in email module incorrectly raise IndexError for some malformed inputs'
updated_at = <Date 2021-08-30.18:58:53.311>
user = 'https://github.com/wbolster'

bugs.python.org fields:

activity = <Date 2021-08-30.18:58:53.311>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-30.18:58:53.311>
closer = 'ned.deily'
components = ['email']
creation = <Date 2021-08-25.13:23:27.104>
creator = 'wbolster'
dependencies = []
files = []
hgrepos = []
issue_num = 45001
keywords = ['patch']
message_count = 9.0
messages = ['400261', '400262', '400351', '400356', '400357', '400358', '400655', '400657', '400658']
nosy_count = 6.0
nosy_names = ['barry', 'ned.deily', 'r.david.murray', 'lukasz.langa', 'wbolster', 'miss-islington']
pr_nums = ['27946', '27972', '27973', '27974', '27975', '27976']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue45001'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[wbolster]

Various date parsing utilities in the email module, such as email.utils.parsedate(), are supposed to gracefully handle invalid input, typically by raising an appropriate exception or by returning None.

The internal email._parseaddr._parsedate_tz() helper used by some of these date parsing routines tries to be robust against malformed input, but unfortunately it can still crash ungracefully when a non-empty but whitespace-only input is passed. This manifests as an unexpected IndexError.

In practice, this can happen when parsing an email with only a newline inside a ‘Date:’ header, which unfortunately happens occasionally in the real world.

Here's a minimal example:

$ python
Python 3.9.6 (default, Jun 30 2021, 10:22:16) 
[GCC 11.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import email.utils
>>> email.utils.parsedate('foo')
>>> email.utils.parsedate(' ')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.9/email/_parseaddr.py", line 176, in parsedate
    t = parsedate_tz(data)
  File "/usr/lib/python3.9/email/_parseaddr.py", line 50, in parsedate_tz
    res = _parsedate_tz(data)
  File "/usr/lib/python3.9/email/_parseaddr.py", line 72, in _parsedate_tz
    if data[0].endswith(',') or data[0].lower() in _daynames:
IndexError: list index out of range

The fix is rather straight-forward; will open a pull request shortly.

[wbolster]

pull request with fix at #27946

[ambv]

New changeset 989f6a3 by wouter bolsterlee in branch 'main':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946)
989f6a3

[miss-islington]

New changeset 9a79242 by Miss Islington (bot) in branch '3.10':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946)
9a79242

[ambv]

New changeset 2cdbd3b by Miss Islington (bot) in branch '3.9':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946) (GH-27973)
2cdbd3b