truncate() changes current stream position

[andreymal]

BPO	32228
Nosy	@pitrou, @nitishch, @andreymal
PRs	bpo-32228: Reset raw_pos after unwinding the raw stream #4858 [3.6] bpo-32228: Reset raw_pos after unwinding the raw stream (GH-4858) #5389
Files	wtf1.py

^{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 2018-01-28.17:44:00.391>
created_at = <Date 2017-12-05.23:19:14.742>
labels = ['type-bug', '3.7', 'expert-IO']
title = 'truncate() changes current stream position'
updated_at = <Date 2018-01-28.17:44:00.388>
user = 'https://github.com/andreymal'

bugs.python.org fields:

activity = <Date 2018-01-28.17:44:00.388>
actor = 'pitrou'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-28.17:44:00.391>
closer = 'pitrou'
components = ['IO']
creation = <Date 2017-12-05.23:19:14.742>
creator = 'andreymal'
dependencies = []
files = ['47321']
hgrepos = []
issue_num = 32228
keywords = ['patch']
message_count = 4.0
messages = ['307698', '310962', '310973', '310974']
nosy_count = 3.0
nosy_names = ['pitrou', 'nitishch', 'andreymal']
pr_nums = ['4858', '5389']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32228'
versions = ['Python 3.6', 'Python 3.7']

[andreymal]

See attached file.

As documentation about "truncate" says, "The current stream position isn’t changed." This is true for most cases. But this is not true for "wtf1.py" example. If you run it with Linux (Tested Debian 8 and Arch; Windows was not tested by me) and Python 3.3+ (or with Python 2.7 using backported "io" module), then these lines:

print('tell:', fp.tell())
print('truncate:', fp.truncate())
print('tell again:', fp.tell())

prints this:

tell: 4098
truncate: 4098
tell again: 4

As you can see, "tell" before truncate and "tell" after truncate are different. Looks like bug.

This bug will not reproduce with Python 2.7 and builtin "open" function; it affects only "io.open".

[pitrou]

New changeset 059f58c by Antoine Pitrou (Nitish Chandra) in branch 'master':
bpo-32228: Reset raw_pos after unwinding the raw stream (bpo-4858)
059f58c

[pitrou]

New changeset 1d896ed by Antoine Pitrou in branch '3.6':
[3.6] bpo-32228: Reset raw_pos after unwinding the raw stream (GH-4858) (bpo-5389)
1d896ed

[pitrou]

I've now pushed Nitish's fix to 3.7 and 3.6. I have no interest in backporting to 2.7 at this point. Closing!