upload_fileobj sliently closes the passed file

[Foxicution]

Describe the bug

When passing a file to boto3.client("s3").upload_fileobj the file gets closed silently. This causes, bugs (or requires reopening the file), to continue working on it.

Expected Behavior

File remains open after passing it to upload_fileobj.

Current Behavior

File gets closed and any further operation on the file produces and exception:
ValueError: I/O operation on closed file.

Reproduction Steps

To produce the exception because of upload_fileobj silently closing the file:

from io import BytesIO
import boto3
client = boto3.client("s3")
text = "Some text to upload"
with BytesIO(text.encode("utf-8")) as f:
    client.upload_fileobj(f, "bucket", "path/filename.txt")
    f.seek(0, 2) # or any other operation

If we swap the the operations around the exception goes away:

from io import BytesIO
import boto3
client = boto3.client("s3")
text = "Some text to upload"
with BytesIO(text.encode("utf-8")) as f:
    f.seek(0, 2) # or any other operation
    client.upload_fileobj(f, "bucket", "path/filename.txt")

Possible Solution

No response

Additional Information/Context

No response

SDK version used

1.28.38

Environment details (OS name and version, etc.)

NixOS 23.05

[tim-finnigan]

Hi @Foxicution thanks for reaching out. We have received a few issues in the past related to this. It looks like the most recent one was mistakenly auto-closed: #3150. I just reopened it for tracking. The last comment I made there is still relevant:

...I brought this up for discussion with the team and they agreed that a short reference to this behavior could be added to the documentation if you want to create a PR. There is still some debate over whether the s3transfer issue should be considered a bug. But someone did share a workaround here in the comments.

I'll try raising this issue again with the team because I do see a lot of frustration in that related s3transfer issue (boto/s3transfer#80). But I'm going to close this as a duplicate and we can continue tracking the linked issues.

[github-actions]

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