Updating zip comment doesn't truncate the zip file

[yudilevi2]

BPO	40105
Nosy	@Yhg1s, @taleinat, @serhiy-storchaka, @miss-islington, @yudilevi2, @mzr
PRs	bpo-40105: fixed zip comment update #19333 bpo-40105: Truncating file if opened in append mode. #19337 [3.9] bpo-40105: ZipFile truncate in append mode with shorter comment (GH-19337) #22436 [3.8] bpo-40105: ZipFile truncate in append mode with shorter comment (GH-19337) #22437 [3.9] bpo-40105: ZipFile truncate in append mode with shorter comment (GH-19337) #22439

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-09-28.18:54:51.480>
created_at = <Date 2020-03-29.20:48:23.704>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = "Updating zip comment doesn't truncate the zip file"
updated_at = <Date 2020-09-28.20:18:50.333>
user = 'https://github.com/yudilevi2'

bugs.python.org fields:

activity = <Date 2020-09-28.20:18:50.333>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2020-09-28.18:54:51.480>
closer = 'taleinat'
components = ['Library (Lib)']
creation = <Date 2020-03-29.20:48:23.704>
creator = 'yudilevi'
dependencies = []
files = []
hgrepos = []
issue_num = 40105
keywords = ['patch']
message_count = 5.0
messages = ['365278', '377614', '377615', '377617', '377623']
nosy_count = 6.0
nosy_names = ['twouters', 'taleinat', 'serhiy.storchaka', 'miss-islington', 'yudilevi', 'mzr']
pr_nums = ['19333', '19337', '22436', '22437', '22439']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40105'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[yudilevi2]

Updating the zip file comment to a shorter comment should truncate the zip file at the end of the new comment.
Instead, the new comment is written to the zip file but the file length stays the same.

For example, for a zip file that has the following zip comment:
b'This is my old amazing comment, I bet you really like it!' # 57 character long

Executing the following code:

zipFile = ZipFile(filePath, 'a')
zipFile.comment = b'My new shorter comment' # 22 character long
zipFile.close()

Will actually update the comment length in the zip header to the correct new length (22), but the bytecode will still have the following data:
b'My new shorter comment comment, I bet you really like it!'

Python reads the comment correctly since it relies on the comment length from the metadata (as far as I can tell), but the file is corrupt.

This is similar to the following old issue -
https://bugs.python.org/issue9239

But I wasn't sure whether to try and re-open that old one or create a new one.

Tested on version 3.8.2 (Windows 10).

Thanks!

[taleinat]

New changeset ff9147d by Jan Mazur in branch 'master':
bpo-40105: ZipFile truncate in append mode with shorter comment (GH-19337)
ff9147d

[taleinat]

Thanks for the report and initial PR, Yudi!

Thanks for the second PR, Jan!