repository set-url fails when Git fsmonitor leaves a socket under .git

[smorimoto]

# opam config report
# opam-version         2.5.2
# self-upgrade         no
# system               arch=arm64 os=macos os-distribution=homebrew os-version=26.5.2
# solver               builtin-mccs+glpk
# install-criteria     -removed,-count[avoid-version,changed],-count[version-lag,request],-count[version-lag,changed],-count[missing-depexts,changed],-changed
# upgrade-criteria     -removed,-count[avoid-version,changed],-count[version-lag,solution],-count[missing-depexts,changed],-new
# jobs                 9
# repositories         1 (http), 2 (version-controlled)
# current-switch       none set

Description

With git config --global core.fsmonitor true, Git creates a Unix socket at .git/fsmonitor--daemon.ipc. Commands that copy a repository root (notably opam repository set-url) then fail:

Fatal error: Copying sockets (.../.opam/repo/default/.git/fsmonitor--daemon.ipc) is unsupported

or, during the subsequent update:

[ERROR] Could not update repository "default": Copying sockets (.../.opam/repo/default/.git/fsmonitor--daemon.ipc) is unsupported

Reproducer

1. git config --global core.fsmonitor true
2. Ensure the default repository is a git clone under ~/.opam/repo/default (or run any git command there so the fsmonitor daemon starts).
3. opam repository set-url default git+https://github.com/ocaml/opam-repository.git

Notes

• OpamSystem.copy_dir raises on Unix.S_SOCK (src/core/opamSystem.ml).
• On 2.5.2 this hits the backup copy_dir in repository set-url, and again the quarantine copy_dir in the VCS first-init path (OpamFilename.copy_dir of repo → repo.new after git fetch, which itself creates the socket when fsmonitor is enabled).
• Master isolates git via GIT_CONFIG_GLOBAL/GIT_CONFIG_SYSTEM (2b1536a9a), which avoids creating the socket during opam's own git calls, but an already-present socket (or one created by an external git in that tree) still breaks any full directory copy.

Workaround

git -C ~/.opam/repo/default fsmonitor--daemon stop
rm -f ~/.opam/repo/default/.git/fsmonitor--daemon.ipc
# temporarily:
git config --global core.fsmonitor false

A robust fix would likely skip uncopyable runtime artefacts such as sockets in copy_dir (or stop/remove the fsmonitor daemon before copying .git).

[kit-ty-kate]

#6992 should have fixed that. Could you try the latest alpha to confirm this is fixed?