cp -a on Cygwin/MSYS2 spams xattr errors and fails directory merges (File exists os error 17)

uutils/coreutils issue

Filed: https://github.com/uutils/coreutils/issues/12958

---

Component

cp

Environment

• OS: Cygwin (MSYS2 MSYSTEM=CYGWIN)
• Package: uutils-coreutils (provides coreutils)
• Observed while bootstrapping a Cygwin package tree (MSYS2-packages fork)

Description

On Cygwin/MSYS2, cp -a / cp -arf attempts to preserve extended attributes via the Rust xattr crate. Cygwin does not support xattrs the way Linux does, so every file copied with archive mode produces stderr noise:

cp: setting attributes for '/usr/bin/cygwin1.dll': unsupported platform, please file a bug at `https://github.com/Stebalien/xattr'
cp: setting attributes for '/usr/include/aio.h': unsupported platform, please file a bug at `https://github.com/Stebalien/xattr'
...

GNU cp -a on the same system does not emit these messages and completes successfully.

This is related to #9704 (cp -p / -a preserving xattrs when GNU does not), but the Cygwin case is worse because:

1. Volume: thousands of warnings when copying a full /usr tree.
2. Hard failure: merging an existing directory tree can abort with a non-GNU error:

cp -arf dest-install/usr dest/
cp: File exists (os error 17)

GNU cp -arf merges usr/ into an existing dest/usr/ without error.

Steps to reproduce

1. xattr warnings

# MSYS2 Cygwin environment with uutils-coreutils installed
mkdir -p /tmp/cp-xattr-test/{src,dest}
echo test > /tmp/cp-xattr-test/src/foo.txt
cp -arf /tmp/cp-xattr-test/src/* /tmp/cp-xattr-test/dest/

On affected systems, copying real package trees (hundreds of headers/DLLs) floods stderr with unsupported platform xattr messages.

Minimal bootstrap example:

cp -arf ./tmp/usr/ /

2. directory merge failure (EEXIST)

mkdir -p /tmp/cp-merge/{dest-install/usr/bin,dest/usr/bin}
echo new > /tmp/cp-merge/dest-install/usr/bin/gcc.exe
echo old > /tmp/cp-merge/dest/usr/bin/gcc.exe
echo old > /tmp/cp-merge/dest/usr/bin/cpp.exe

cp -arf /tmp/cp-merge/dest-install/* /tmp/cp-merge/dest/
# uutils: cp: File exists (os error 17)
# GNU cp: succeeds, overwrites files inside dest/usr/

Expected behavior

Match GNU coreutils on Cygwin:

• cp -a should not attempt xattr preservation unless --preserve=xattr is given (see #9704).
• If xattr operations are unsupported on the platform, failures should be silent (similar to PR #10083 / ENOTSUP suppression on other Unix targets).
• cp -arf src/usr dest/ should merge into existing dest/usr/ like GNU cp, not fail with File exists (os error 17).

Actual behavior

• cp -a tries to set xattrs via the xattr crate on every file.
• Cygwin returns "unsupported platform" for each attempt (stderr spam).
• Directory-tree merges that GNU handles can fail with File exists (os error 17).

Impact

Breaks POSIX shell scripts that use cp -arf to overlay package trees onto existing directories -- common in bootstrap/build scripts. Workaround is to use tar pipes instead of cp.

Suggested fix

1. Do not include xattr in the default preserve set for -a on any platform (align with GNU; #9704).
2. On Cygwin (target_os = "cygwin"), treat xattr as unsupported: skip preservation entirely unless explicitly requested.
3. Suppress or downgrade xattr crate errors on unsupported platforms (no stderr per file).
4. Fix recursive directory merge so cp -arf foo/bar dest/ works when dest/bar/ already exists (GNU semantics).

References

• Related: https://github.com/uutils/coreutils/issues/9704
• xattr crate: https://github.com/Stebalien/xattr
• Cygwin support: https://github.com/uutils/coreutils/pull/9535

Start with the cp component and reproduce both commands in a Cygwin/MSYS2 environment, then trace its archive-mode xattr handling through the xattr crate and its recursive directory merge path. Compare the results with GNU cp and related issue #9704 and PR #10083. Done means unsupported xattrs produce no per-file errors and cp -arf merges existing directories without EEXIST failure.