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2682. filesystem::copy() won't create a symlink to a directory

Section: 31.12.13.4 [fs.op.copy] Status: C++20 Submitter: Jonathan Wakely Opened: 2016-04-19 Last modified: 2021-02-25

Priority: 2

View all other issues in [fs.op.copy].

View all issues with C++20 status.

Discussion:

(First raised in c++std-lib-38544)

filesystem::copy doesn't create a symlink to a directory in this case:

copy("/", "root", copy_options::create_symlinks);

If the first path is a file then a symlink is created, but I think my implementation is correct to do nothing for a directory. We get to bullet 31.12.13.4 [fs.op.copy] (3.6) where is_directory(f) is true, but options == create_symlinks, so we go to the next bullet (3.7) which says "Otherwise, no effects."

I think the case above should either create a symlink, or should report an error. GNU cp -s gives an error in this case, printing "omitting directory '/'". An error seems reasonable, you can use create_symlink to create a symlink to a directory.

[2016-05 Issues Telecon]

This is related to 2681⁽ⁱ⁾; and should be considered together.

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Previous resolution [SUPERSEDED]:

Add a new bullet following (3.6) in 31.12.13.4 [fs.op.copy] as shown:

• If !exists(t), then create_directory(to, from).

• Then, iterate over the files in from, as if by for (directory_entry& x : directory_iterator(from)), and for each iteration copy(x.path(), to/x.path().filename(), options | copy_options::unspecified )

• Otherwise, if is_directory(f) && (options & copy_options::create_symlinks) != copy_options::none, then report an error with an error_code argument equal to make_error_code(errc::is_a_directory).

• Otherwise, no effects.

[2016-10-16, Eric reopens and provides improved wording]

The current PR makes using copy(...) to copy/create a directory symlink an error. For example, the following is now an error:

copy("/", "root", copy_options::create_symlinks);

However the current PR doesn't handle the case where both copy_options::create_symlinks and copy_options::recursive are specified. This case is still incorrectly handled by bullet (3.6) [fs.op.copy].

I suggest we move the PR before this bullet so that it catches the recursive copy case, since currently the conditions are ordered:

3.6 Otherwise if is_directory(f) && (bool(options & copy_options::recursive) || ...)
3.X Otherwise if is_directory(f) && bool(options & copy_options::create_symlinks)

So 3.6 catches create_symlinks | recursive but I believe we want 3.X to handle it instead.

[2018-01-26 issues processing telecon]

Status to 'Review'; we think this is OK but want some implementation experience before adopting it.

[2018-01-29 Jonathan says]

The proposed resolution for LWG 2682 has been in GCC's Filesystem TS implementation for more than a year. It's also in our std::filesystem implementation on the subversion trunk.

[2018-06; Rapperswil Wednesday evening]

JW: can we use the words we are shipping already since two years?
BO: what we got is better than what we had before
no objection to moving to Ready
ACTION move to Ready
ACTION link 2682 and LWG 3057⁽ⁱ⁾ and set a priority 2 and look at 3057 in San Diego

Daniel rebases wording to N4750.

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Add a new bullet before (4.8) in 31.12.13.4 [fs.op.copy] as shown:

   1. (4.7) — Otherwise, if is_regular_file(f), then:

      […]

   2. (4.?) — Otherwise, if

      is_directory(f) && 
      (options & copy_options::create_symlinks) != copy_options::none
      

      then report an error with an error_code argument equal to make_error_code(errc::is_a_directory).

   3. (4.8) — Otherwise, if

      is_directory(f) &&
      ((options & copy_options::recursive) != copy_options::none ||
      options == copy_options::none)
      

      then:

      1. (4.8.1) — If exists(t) is false, then create_directory(to, from).

      2. (4.8.2) — Then, iterate over the files in from, as if by

         for (const directory_entry& x : directory_iterator(from))
           copy(x.path(), to/x.path().filename(),
             options | copy_options::in-recursive-copy);
         

         where in-recursive-copy is a bitmask element of copy_options that is not one of the elements in 31.12.8.3 [fs.enum.copy.opts].

   4. (4.9) — Otherwise, for the signature with argument ec, ec.clear().

   5. (4.10) — Otherwise, no effects.