syscall: preserve Cygwin UNC roots in ownership walks

[steadytao]

Fixes #1105

Summary

Preserve //server/share as the resolution anchor when the ownership-checked path walker opens an absolute path on Cygwin.

We treated every path beginning with / as POSIX-absolute, opened / and removed every leading slash. This changed //server/share/path into /server/share/path and caused UNC destinations to fail with ENOENT. The UNC share is now opened as the trusted root. Components below the share continue through the existing ownership-checked walk. Trusted symlinks which expand to UNC paths use the same handling.

[steadytao]

@sdbbs please test

[sdbbs]

Thanks @steadytao :

please test

I'm not sure exactly how, I'd need an executable; unfortunately I'm too busy right now to build from source.

I noticed on https://packages.msys2.org/packages/rsync there was a rsync build, newer than the one I was complaining about:

Version: 3.5.1-2
...
Build Date: 2026-09-30 18:32:03

... so I updated MSYS2 with pacman -Syu, I could see new rsync package get downloaded, however it still prints rsync version 3.5.1 protocol version 33 for --version, so I cannot tell if it packages the patches you've made. This new version I tried from pacman still does not work, unfortunately:

$ rsync -aP myfile.txt //192.168.1.10/MyShare/mydir/
sending incremental file list
rsync: [Receiver] change_dir#1 "//192.168.1.10/MyShare/mydir/" failed: No such file or directory (2)
rsync error: errors selecting input/output files, dirs (code 3) at main.c(781) [Receiver=3.5.1]
rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at rsync.c(879) [sender=3.5.1]

$ ls -lad //192.168.1.10/MyShare/mydir/
drwxr-xr-x 1 user None 0 Sep 28 14:51 //192.168.1.10/MyShare/mydir//

If you have a suggestion how can I obtain an executable quickly, I don't mind testing. Thanks again for the help!

[steadytao]

rsync.tar.gz

@sdbbs Just pulled the build from the Cygwin CI job. Let me know if it doesn't work.

[seks99x]

Chill mate, it happens. You could've easily installed all that missing dependencies with cygwin setup-x86_64.exe . When setting it up ,Select Packages section contain all the dependencies and is easily installed from the GUI setup (or setup-x86.exe -q -P package1,package2). I think also AI do know that clearly and should've said that.

[steadytao]

Eh I do more or less agree on the fact that AI does piss me off a lot of the time. But, yeah, preferably dont be overly doomer with things; we can strive to do better.

[steadytao]

Thank you for testing nonetheless.

[sdbbs]

Thanks for merging - and sorry about my burst (handling this issue was only the final thing in that day of mine, where also everything else went wrong); I have edited my comment, and uploaded the missing script. Thanks again!

[steadytao]

It is no worries at all. I completely understand where the frustration comes from and we all need time to get that out occasionally. Thank you for reporting the issue and helping me confirm this fix works.