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Re: [PATCH 1/2] copy-file-range: new module

From: Paul Eggert
Subject: Re: [PATCH 1/2] copy-file-range: new module
Date: Fri, 7 Jun 2019 01:04:43 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0

Although I agree with both of Bruno's reasons, on further thought it makes sense for Gnulib to provide a stub instead of an emulation for this particular function.

The primary use for copy_file_range is to copy files more efficiently than a read+write loop. However, the read+write loop will typically still be needed anyway, to handle non-regular files, copying across file system boundaries, or even copying regular input files whose size shrinks while copying as copy_file_range (mistakenly, in my view) reports an error if asked to copy past end of file.

Because of all these special cases, a reliable use of copy_file_ranges will need to be followed by a read+write loop that handles the special cases where copy_file_ranges fails. And this read+write loop will work even if copy_file_range is a stub that always fails. So there's no practical need to emulate copy_file_range.

For an example of the code I'm thinking of, see the call to copy_file_range that I recently introduced into GNU Emacs:


Because of the above, I simplified Gnulib's implementation of copy_file_range by installing the attached. If we find a good use for copy_file_range that could take advantage of an emulator rather than a stub, we can revert this.

Attachment: 0001-copy-file-range-simplify-into-a-stub.txt
Description: Text document

Attachment: 0002-copy-file-fix-typo.txt
Description: Text document

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