io/ioutil: Add CopyFile

[kelseyhightower]

Currently the standard library has the ioutil.ReadFile and ioutil.WriteFile functions,
but no ioutil.CopyFile function. As previouslly discussed in CL 1758041 -
https://golang.org/cl/1758041, the ioutil.CopyFile function should be simple.

[kelseyhightower]

Comment 1:

I've proposed the following CL for this issue:
https://golang.org/cl/152180043

[gopherbot]

Comment 2:

CL https://golang.org/cl/152180043 mentions this issue.

[minux]

Comment 3:

i don't think this need to be in the standard packages (no matter how easy it is to
implement it).
Besides, we are already in code freeze for 1.4.

Labels changed: added release-none, repo-main.

[kelseyhightower]

Comment 4:

Thanks for the feedback, but I think a CopyFile function adds value considering we have
ReadFile and WriteFile already.

[kelseyhightower]

Comment 5:

This request seemed to have the right amount of support in the past, but the previous CL
was never completed. The current CL at https://golang.org/cl/152180043
incorporates all of the feedback from the previous one at
https://golang.org/cl/1758041, but simplifies things a bit.

[ianlancetaylor]

Comment 6:

We should make a decision one way or another for 1.5.

Labels changed: added release-go1.5, removed release-none.

[adg]

Comment 7:

+1 for inclusion in 1.5.
I've implemented it a few times in various places. Its presence in ioutil would have
saved me some trouble. We can't write a version that will suit everyone, but we can
write one that is "correct". I don't see a downside.

[kelseyhightower]

New CL posted here: https://go-review.googlesource.com/#/c/1591

[bradfitz]

Like @adg, I would also like this function and find myself often re-implementing it, but there wasn't general agreement before the CL was proposed. (the code looks fine)

Assigning to @robpike for a decision.

/cc @rsc

[bradfitz]

(That is, assigning to @robpike to own finding a decision, not making it)

[minux]

What should be the correct order for the arguments? dst first or src first?
Normally, for Go APIs, the convention seems to be dst first, however, for
filename arguments, os.Rename, os.Symlink and os.Link uses the other
order. CopyFile is similar to those, so should it use the src then dst
order?

On a related note, do we need io/ioutil.MoveFile?
(it should first try os.Rename(src, dst), and if that fails, calls
CopyFile(dst, src),
and then os.Remove(src))

[mattn]

I hope ioutil.MoveFile use sys.MoveFile on windows.
And about order of arguments. We know memcpy(dst, src, size); so that we may think ioutil.CopyFile(dst, src). But src, dst seems general?

python

shutil.copyfile(src, dst)

java

Path.copy(src, dst, options)

perl

use File::Copy;
copy(src, dst);

ruby

File.copy(src, dst)

etc

sendfile(output_fd, input_fd, &offset, size)

shell

$ cp src.txt dst.txt

[minux]

Other issues worth considering for this API:
Some filesystems (HFS+ on OS X, NTFS) support additional data streams (resource forks and
alternative data streams, respectively).

File systems on Linux has the SELinux context, POSIX ACL and extended attributes.
I'm sure other systems have other additional attributes that ideally a file copying program should
try to preserve (or document that it doesn't care about those).

I don't mean to propose that ioutil.CopyFile should handle all those complexities, but we need to
determine the proper semantics of the API and document it.

[rjeczalik]

I'd also consider simplifying API by removing perm os.FileMode argument. Currently it's:

func CopyFile(src, dst string, perm os.FileMode) error

The rationale is that dst file eventually created by CopyFile should either inherit permissions from src file (the cp -a behaviour) or use default ones (0777&^umask). If user wants to have different perm, he/she needs to use os.Chmod explicitely. We'd have then one less thing that CopyFile could fail on.

[davecheney]

The rationale is that file created by CopyFile should either inherit
permissions from src file (the cp -a behaviour) or use default ones (0777 &
^umask). If user wants to have different perm, he/she needs to use os.Chmod
explicitely. We'd have then one less thing that CopyFile could fail on.

I agree

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[kelseyhightower]

@rjeczalik @davecheney The CopyFile function sets the perm to be consistent with WriteFile.

[kelseyhightower]

@mattn @minux The order of the args is consistent with io.Copy and io.CopyN

[rjeczalik]

@kelseyhightower

My take on that is the WriteFile requires perm, as it has not enough information to create a file for user. On the other hand CopyFile has already access to dst file's permissions, so it can proceed without requiring this information from the user. I'd rather not make those two consistent, in my opinion it feels artificial.

[kelseyhightower]

@rjeczalik Ok, I don't feel strongly about CopyFile setting permissions. So we can just remove it.

[kelseyhightower]

Based on the feedback from @robpike in https://go-review.googlesource.com/#/c/1591, it seems that we should not do this in the ioutil package. It's easy enough to write this bit of code when I need it so I'm happy to close this issue.

//cc @bradfitz @adg

[kelseyhightower]

Ok, I've abandoned this change here: https://go-review.googlesource.com/#/c/1591. Please feel free to close.