bug#49644: 27.2; copy-directory fails when Unix socket file is present in the source directory tree

bug#49644: 27.2; copy-directory fails when Unix socket file is present in the source directory tree

[Pierre Rouleau]

There is no Emacs crash involved here.  It's just a feature request.

Try `copy-directory' on a directory tree that contains one Unix socket.
Something like "~/elpa/gnupg" which often contains Unix socket files.
This will fail with an error stating that the Unix socket cannot be
copied.

Although its true that Unix sockets cannot be copied, it would be nice
to offer the users the ability to copy the directory tree anyway,
skipping the Unix socket files.

The function `copy-directory' does not provide a mechanism to skip the
copy of such files, so it becomes impossible to copy a directory tree
that contains such a file.  It would be useful to add argument to
provide the ability to skip such files.   Currently, the only way is to
override or advise the function `copy-file' when using `copy-directory'.

The cond clause at the end of `copy-directory' only supports
directories, symbolic links and normal files.  It does not support Unix
socket, which cannot be copied.  In some conditions users may want to
copy the directory tree even in the presence of Unix socket
files. Adding an optional argument for that would increase the
usefulness of the function while allowing user's code to document the
fact that they want (or don't want) to skip things like Unix sockets.

The latest version of files.el code in
https://github.com/emacs-mirror/emacs/blob/master/lisp/files.el#L6148 
still behaves like this.


In GNU Emacs 27.2 (build 1, x86_64-apple-darwin18.7.0)
of 2021-03-25 built on Mojave
System Description:  Mac OS X 10.14.6

Recent messages:
Generating autoloads for ztree-view.el...done
  INFO     Scraping files for pel-bundle-autoloads.el...100% 
Generating autoloads for ztree.el...done
  INFO     Scraping files for pel-bundle-autoloads.el...done
Wrote /Users/roup/.emacs.d/pel-bundle/pel-bundle-autoloads.el [2 times]
Restart Emacs to complete the process!
Type "q" in help window to delete it.
mouse-2, RET: find function's definition
\ is undefined
uncompressing files.el.gz...done

Configured using:
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bug#49644: 27.2; copy-directory fails when Unix socket file is present in the source directory tree

[Eli Zaretskii]

> From: Pierre Rouleau <prouleau001@gmail.com>
> Date: Mon, 19 Jul 2021 11:04:39 -0400
> 
> There is no Emacs crash involved here.  It's just a feature request.
> 
> Try `copy-directory' on a directory tree that contains one Unix socket.
> Something like "~/elpa/gnupg" which often contains Unix socket files.
> This will fail with an error stating that the Unix socket cannot be
> copied.
> 
> Although its true that Unix sockets cannot be copied, it would be nice
> to offer the users the ability to copy the directory tree anyway,
> skipping the Unix socket files.
> 
> The function `copy-directory' does not provide a mechanism to skip the
> copy of such files, so it becomes impossible to copy a directory tree
> that contains such a file.  It would be useful to add argument to
> provide the ability to skip such files.   Currently, the only way is to
> override or advise the function `copy-file' when using `copy-directory'.

I think it would be more useful to add an optional behavior whereby
any file that cannot be copied for some reason would not terminate the
copy.  There's no need to allow such tolerance only for sockets.  I
could imagine a huge file that cannot be copied because the target
directory doesn't have enough space, for example.  Perhaps ask the
user whether to continue, when a copy fails, or provide a user option

bug#49644: 27.2; copy-directory fails when Unix socket file is present in the source directory tree

[Pierre Rouleau]

> On Jul 19, 2021, at 12:10 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Pierre Rouleau <prouleau001@gmail.com>
>> Date: Mon, 19 Jul 2021 11:04:39 -0400
>> 
>> There is no Emacs crash involved here.  It's just a feature request.
>> 
>> Try `copy-directory' on a directory tree that contains one Unix socket.
>> Something like "~/elpa/gnupg" which often contains Unix socket files.
>> This will fail with an error stating that the Unix socket cannot be
>> copied.
>> 
>> Although its true that Unix sockets cannot be copied, it would be nice
>> to offer the users the ability to copy the directory tree anyway,
>> skipping the Unix socket files.
>> 
>> The function `copy-directory' does not provide a mechanism to skip the
>> copy of such files, so it becomes impossible to copy a directory tree
>> that contains such a file.  It would be useful to add argument to
>> provide the ability to skip such files.   Currently, the only way is to
>> override or advise the function `copy-file' when using `copy-directory'.
> 
> I think it would be more useful to add an optional behavior whereby
> any file that cannot be copied for some reason would not terminate the
> copy.  There's no need to allow such tolerance only for sockets.  I
> could imagine a huge file that cannot be copied because the target
> directory doesn't have enough space, for example.  Perhaps ask the
> user whether to continue, when a copy fails, or provide a user option

I agree that supporting more use cases would be beneficial. 
It would be nice that no prompting is required though or at least that it would be possible to use the function in a non-interactive way.

Providing user-options might be the way. 
That would leave the signature intact and yet would allow introducing several new behaviours, 
whether prompting is used or not, what type of condition is ignored, etc…. 

For my use case I would at least need to be able to set a copy directory user-option to prevent copy-file from stopping on a Unix socket.