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

[lygstate]

uutils/coreutils issue

Filed: #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 cp -p preserves extended attributes #9704).
• If xattr operations are unsupported on the platform, failures should be silent (similar to PR cp/mv: suppress xattr ENOTSUP errors for optional preservation #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; cp -p preserves extended attributes #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: cp -p preserves extended attributes #9704
• xattr crate: https://github.com/Stebalien/xattr
• Cygwin support: uucore: support cygwin #9535

[oech3]

Does it happen also with MinGW build of uu-cp.exe?

[lygstate]

don't know, I only test under cygwin. mingw do not need uutils.

[oech3]

There are 2 variants https://packages.msys2.org/search?t=pkg&q=uutils-coreutils

[lygstate]

https://packages.msys2.org/base/uutils-coreutils this one

[oech3]

I want to support cygpath on MinGW build and drop Cygwin one in the future. Then we can just use MinGW cp if it works.