gh-150075: tar.addfile() doesn't set member offsets

[grantlouisherman]

Original bug report:

Bug report

Bug description:

It appears that tarfile.TarFile.addfile() sets neither offset nor offset_data attributes in the TarInfo added to self.members, when it's done adding the file. Members in a reopened TAR have correct values.

Reproducer

from io import BytesIO
from tarfile import TarFile, TarInfo

with TarFile("test.tar", "w") as tar:
    data = b"data"

    tarinfo = TarInfo("test1.txt")
    tarinfo.size = len(data)
    tar.addfile(tarinfo, BytesIO(data))

    tarinfo = TarInfo("test2.txt")
    tarinfo.size = len(data)
    tar.addfile(tarinfo, BytesIO(data))

    for member in tar.getmembers():
        print(member.name, member.offset, member.offset_data)

with TarFile("test.tar", "r") as tar:
    for member in tar.getmembers():
        print(member.name, member.offset, member.offset_data)

Expected output

test1.txt 0 512
test2.txt 1024 1536
test1.txt 0 512
test2.txt 1024 1536

Actual output

test1.txt 0 0
test2.txt 0 0
test1.txt 0 512
test2.txt 1024 1536

CPython versions tested on:

3.15

Operating systems tested on:

Linux

---

Fix
I added the offset info for a given file based on the current offset of the file and then after the block size is calculated. I also included a test to validate the behavior

• Issue: tarfile: addfile() doesn't set member offsets #150075

[bedevere-app]

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[python-cla-bot]

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[grantlouisherman]

I dont think the news is necessary it seems like a pretty minor addition

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

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[grantlouisherman]

@ethanfurman should I squash commits?

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.