ioutilx, hostagent: support reverse-sshfs on plain Windows

ioutilx.WindowsSubsystemPath: keep cygpath as the preferred backend
(it respects any custom fstab the user has configured for MSYS2 / Git
for Windows), but add a native fallback for the common drive-letter
case (C:\Users\jan -> /c/Users/jan). Without the fallback, plain
Windows installs that have neither Git for Windows nor MSYS2 hit a
fatal error during fillDefault when computing the default mountPoint
for a host mount. After this change, the default mountPoint is
computed correctly without external tooling.

hostagent.setupMount: switch the host-path translation from
ioutilx.WindowsSubsystemPath to sshutil.PathForSSH. The path is
consumed by the sftp-server that sshocker spawns, and the format that
binary expects depends on toolchain: Cygwin sftp-server (Git for
Windows / MSYS2) wants Cygwin paths, native Windows sftp-server wants
native forward-slash paths. PathForSSH already encodes that decision.

Verified end-to-end on Windows 11 with QEMU 10.2.0 and only native
Windows OpenSSH on PATH (no Git for Windows, no MSYS2): reverse-sshfs
mounts a host directory into the guest, both sides see the same files,
read and write both work, and the host-side sftp-server is the
ssh-built-in C:\Windows\System32\OpenSSH\sftp-server.exe (auto-detected
by sshocker via exec.LookPath).

Signed-off-by: Jan Dubois <jan.dubois@suse.com>

Author: jandubois

pkg/hostagent/mount.go

@@ -13,7 +13,6 @@ import (
 	"github.com/lima-vm/sshocker/pkg/reversesshfs"
 	"github.com/sirupsen/logrus"
 
-	"github.com/lima-vm/lima/v2/pkg/ioutilx"
 	"github.com/lima-vm/lima/v2/pkg/limatype"
 	"github.com/lima-vm/lima/v2/pkg/sshutil"
 )
@@ -54,8 +53,16 @@ func (a *HostAgent) setupMount(ctx context.Context, m limatype.Mount) (*mount, e
 
 	resolvedLocation := m.Location
 	if runtime.GOOS == "windows" {
-		var err error
-		resolvedLocation, err = ioutilx.WindowsSubsystemPath(ctx, m.Location)
+		// Convert the host path to the form the sftp-server invoked by sshocker
+		// expects. PathForSSH tracks the toolchain Lima is using: Cygwin-style
+		// (e.g. /c/Users/jan) when ssh comes from Git for Windows / MSYS2 (where
+		// sftp-server is also Cygwin-based), and native forward-slash form
+		// (e.g. C:/Users/jan) when ssh is native Windows OpenSSH.
+		sshExe, err := sshutil.NewSSHExe()
+		if err != nil {
+			return nil, err
+		}
+		resolvedLocation, err = sshutil.PathForSSH(ctx, sshExe, m.Location)
 		if err != nil {
 			return nil, err
 		}

pkg/ioutilx/ioutilx.go

@@ -50,13 +50,23 @@ func FromUTF16leToString(r io.Reader) (string, error) {
 	return string(out), nil
 }
 
+// WindowsSubsystemPath converts a Windows path to a Cygwin/MSYS-style path
+// (e.g. C:\Users\jan -> /c/Users/jan). It prefers cygpath, since that respects
+// any custom fstab the user has configured for MSYS2 / Git for Windows. When
+// cygpath is unavailable (plain Windows install with neither Git for Windows
+// nor MSYS2), it falls back to a native conversion that handles the common
+// drive-letter case. UNC paths and other inputs without a drive letter return
+// an error.
 func WindowsSubsystemPath(ctx context.Context, orig string) (string, error) {
-	out, err := exec.CommandContext(ctx, "cygpath", filepath.ToSlash(orig)).CombinedOutput()
-	if err != nil {
-		logrus.WithError(err).Errorf("failed to convert path to mingw, maybe not using Git ssh?")
-		return "", err
+	if out, err := exec.CommandContext(ctx, "cygpath", filepath.ToSlash(orig)).CombinedOutput(); err == nil {
+		return strings.TrimSpace(string(out)), nil
+	} else {
+		logrus.WithError(err).Debugf("cygpath unavailable for %q, attempting native conversion", orig)
 	}
-	return strings.TrimSpace(string(out)), nil
+	if vol := filepath.VolumeName(orig); len(vol) == 2 && vol[1] == ':' {
+		return "/" + strings.ToLower(vol[:1]) + filepath.ToSlash(orig[2:]), nil
+	}
+	return "", fmt.Errorf("cannot convert %q to a Cygwin-style path: cygpath unavailable and input is not a drive-letter path", orig)
 }
 
 func WindowsSubsystemPathForLinux(ctx context.Context, orig, distro string) (string, error) {