tarfile stream read performance regression

[hajoscher]

BPO	34010
Nosy	@vstinner, @methane, @miss-islington, @hajoscher
PRs	bpo-34010: Fix tarfile read performance regression #8020 [3.7] bpo-34010: Fix tarfile read performance regression (GH-8020) #8082 [3.6] bpo-34010: Fix tarfile read performance regression (GH-8020) #8083

^{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 2018-07-04.08:25:19.312>
created_at = <Date 2018-06-30.09:27:00.578>
labels = ['3.8', '3.7', 'library', 'performance']
title = 'tarfile stream read performance regression'
updated_at = <Date 2018-07-04.12:27:59.474>
user = 'https://github.com/hajoscher'

bugs.python.org fields:

activity = <Date 2018-07-04.12:27:59.474>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-04.08:25:19.312>
closer = 'methane'
components = ['Library (Lib)']
creation = <Date 2018-06-30.09:27:00.578>
creator = 'hajoscher'
dependencies = []
files = []
hgrepos = []
issue_num = 34010
keywords = ['patch', '3.2regression']
message_count = 10.0
messages = ['320763', '320848', '321011', '321018', '321020', '321021', '321022', '321023', '321040', '321041']
nosy_count = 4.0
nosy_names = ['vstinner', 'methane', 'miss-islington', 'hajoscher']
pr_nums = ['8020', '8082', '8083']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue34010'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[hajoscher]

Buffer read of large files in a compressed tarfile stream performs poorly.

The buffered read in tarfile _Stream is extending a bytes object.
It is much more efficient to use a list followed by a join.
Using a list can mean seconds instead of minutes.

This performance regression was introduced in b506dc3.

How to test:
# create random tarfile 50Mb
dd if=/dev/urandom of=test.bin count=50 bs=1M
tar czvf test.tgz test.bin

# read with tarfile as stream (note pipe symbol in 'r|gz')
import tarfile
tfile = tarfile.open("test.tgz", 'r|gz')
for t in tfile:
    file = tfile.extractfile(t)
    if file:
        print(len(file.read()))

[methane]

Nice catch.

I confirmed this is a hard regression of performance.
Decompressing a file must be O(n) when n=filesize, but O(n^2) now.

While we live with this regression for a long time, I feel it's worth enough to backport.
This can be DoS vulnerability.

Can you write NEWS entry for it?

[hajoscher]

Yes, it performance is really bad for large files, and memory consumption as well. I will write something for NEWS.

[methane]

New changeset 12a08c4 by INADA Naoki (hajoscher) in branch 'master':
bpo-34010: Fix tarfile read performance regression (GH-8020)
12a08c4

[methane]

thanks

[vstinner]

https://github.com/python/cpython/pull/8020/files/77a54a39aace1a38794884218abe801b85b54e62#diff-ef64d8b610dda67977a63a9837f46349

•        buf = "".join(t)
  

+ buf = b"".join(t)

@hajoscher: "It never caused a problem, since this line is never called; size is never None in the function call. But still, should be fixed, I guess."

Would it be possible to have an unit test for this modified line? Untested code is broken, as you showed :-)

[miss-islington]

New changeset c1b75b5 by Miss Islington (bot) in branch '3.7':
bpo-34010: Fix tarfile read performance regression (GH-8020)
c1b75b5

[miss-islington]

New changeset d7a0ad7 by Miss Islington (bot) in branch '3.6':
bpo-34010: Fix tarfile read performance regression (GH-8020)
d7a0ad7

[methane]

@victor I think removing unused code is better than adding test for it and maintain it.

So I removed that unused code block in #52336.