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bug#61386: [PATCH] cp,mv,install: Disable sparse copy on macOS

From: Paul Eggert
Subject: bug#61386: [PATCH] cp,mv,install: Disable sparse copy on macOS
Date: Sat, 11 Feb 2023 13:53:43 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1

On 2023-02-10 17:21, George Valkov wrote:

remember to trace all code paths errors.

Yes, I've been doing that. However, I don't use macOS and the two of us are debugging remotely where I write the code and you debug it but neither of us know how macOS works. Of course it would be much more efficient if we weren't operating with these obstacles, but here we are.

Because the macOS behavior is poorly documented and we lack the source, if we can't figure this out fairly quickly coreutils should simply stop using fclonefileat because it's unreliable for users and a time sink for us. I'm hoping, though, we can get something working with another round or two.

With CLONE_ACL, I get 22 Invalid argument.

OK, this means that although Apple has documented CLONE_ACL and it's in your include files, it doesn't work in your version of macOS, because either it's not supported by the kernel, or by the filesystem code, or by the particular filesystem instance, or for some other reason. EINVAL hints that it's the kernel but that's by no means certain.

One possible way to defend against this macOS misbehavior is for cp to try to use CLONE_ACL, and if that fails with errno == EINVAL try again without CLONE_ACL. I attempted to implement this in the attached patch; please give it a try.

do we want to go the quick route of unlinking the destination and creating a 
fresh clone?

No, POSIX is clear that plain cp should not unlink and re-create the destination. Of course we can add a non-POSIX flag to do that, and we already have: cp --remove-destination.

Attachment: 0001-cp-improve-use-of-fclonefileat.patch
Description: Text Data

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