'_Stream' object has no attribute 'seekable'

Bug report

Bug description:

From a recent application, I have found tarfile._Stream doesn't have a seekable() method. A quick reproduction of the problem is as follows:

import tarfile
import io

data = b"Test data."
buf = io.BytesIO()
with tarfile.open(fileobj=buf, mode="w") as tf:
    ti = tarfile.TarInfo(name="test.txt")
    ti.size = len(data)
    tf.addfile(tarinfo=ti, fileobj=io.BytesIO(data))

buf.seek(0)
tf = tarfile.open(fileobj=buf, mode="r|")
for entry in tf:
    f = tf.extractfile(entry)  # `f` is in type of `class ExFileObject(io.BufferedReader)`
    f.seekable()  # AttributeError
    f.seek(0)  # AttributeError

Output:

Traceback (most recent call last):
  File "a.py", line 17, in <module>
    print(f.seekable())
  File "AppData\Local\Programs\Python\Python310\lib\tarfile.py", line 641, in seekable
    return self.fileobj.seekable()
AttributeError: '_Stream' object has no attribute 'seekable'

When opening tarfile with r| mode, ExFileObject.fileobj would be a _Stream object. So when calling ExFileObject.seekable(), as _Stream object dun have seekable() method, it will raise AttributeError.

Also, as BufferedReader.seek() will also check its raw.seekable() (i.e. in Modules/_io/bufferedio.c, func _io__Buffered_seek_impl(), there is a call to _PyIOBase_check_seekable(state, self->raw, Py_True)), f.seek() in the above example will also raise AttributeError.

I have thought of if we can simply define seekable() in _Stream object, but as it is only "forward-seekable", I am not sure if directly returning a true/false in _Stream.seekable() is appropriate or not. However, this issue does break the IOBase interface and there should be a solution.

Thanks for everyone's time and effort for looking into this.

CPython versions tested on:

3.10, 3.11, 3.12, 3.13, CPython main branch

Operating systems tested on:

Linux, Windows

まず、提供された tarfile の再現を r| mode で実行し、次に tarfile.py の tarfile._Stream と ExFileObject、および Modules/_io/bufferedio.c の _io__Buffered_seek_impl() を調べます。前方向のみのストリームに対する適切な seek の動作を判断し、報告されている seekable() と seek() の AttributeErrors がストリーミングのセマンティクスを壊すことなく解消されることを確認します。