⚙️ Add ability to build from Dockerfiles directly

vendor/github.com/moby/sys/mountinfo/mounted_linux.go

@@ -7,6 +7,34 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// MountedFast is a method of detecting a mount point without reading
+// mountinfo from procfs. A caller can only trust the result if no error
+// and sure == true are returned. Otherwise, other methods (e.g. parsing
+// /proc/mounts) have to be used. If unsure, use Mounted instead (which
+// uses MountedFast, but falls back to parsing mountinfo if needed).
+//
+// If a non-existent path is specified, an appropriate error is returned.
+// In case the caller is not interested in this particular error, it should
+// be handled separately using e.g. errors.Is(err, os.ErrNotExist).
+//
+// This function is only available on Linux. When available (since kernel
+// v5.6), openat2(2) syscall is used to reliably detect all mounts. Otherwise,
+// the implementation falls back to using stat(2), which can reliably detect
+// normal (but not bind) mounts.
+func MountedFast(path string) (mounted, sure bool, err error) {
+	// Root is always mounted.
+	if path == string(os.PathSeparator) {
+		return true, true, nil
+	}
+
+	path, err = normalizePath(path)
+	if err != nil {
+		return false, false, err
+	}
+	mounted, sure, err = mountedFast(path)
+	return
+}
+
 // mountedByOpenat2 is a method of detecting a mount that works for all kinds
 // of mounts (incl. bind mounts), but requires a recent (v5.6+) linux kernel.
 func mountedByOpenat2(path string) (bool, error) {
@@ -16,9 +44,6 @@ func mountedByOpenat2(path string) (bool, error) {
 		Flags: unix.O_PATH | unix.O_CLOEXEC,
 	})
 	if err != nil {
-		if err == unix.ENOENT { // not a mount
-			return false, nil
-		}
 		return false, &os.PathError{Op: "openat2", Path: dir, Err: err}
 	}
 	fd, err := unix.Openat2(dirfd, last, &unix.OpenHow{
@@ -26,33 +51,51 @@ func mountedByOpenat2(path string) (bool, error) {
 		Resolve: unix.RESOLVE_NO_XDEV,
 	})
 	_ = unix.Close(dirfd)
-	switch err {
+	switch err { //nolint:errorlint // unix errors are bare
 	case nil: // definitely not a mount
 		_ = unix.Close(fd)
 		return false, nil
 	case unix.EXDEV: // definitely a mount
 		return true, nil
-	case unix.ENOENT: // not a mount
-		return false, nil
 	}
 	// not sure
 	return false, &os.PathError{Op: "openat2", Path: path, Err: err}
 }
 
-func mounted(path string) (bool, error) {
-	// Try a fast path, using openat2() with RESOLVE_NO_XDEV.
-	mounted, err := mountedByOpenat2(path)
-	if err == nil {
-		return mounted, nil
+// mountedFast is similar to MountedFast, except it expects a normalized path.
+func mountedFast(path string) (mounted, sure bool, err error) {
+	// Root is always mounted.
+	if path == string(os.PathSeparator) {
+		return true, true, nil
 	}
+
+	// Try a fast path, using openat2() with RESOLVE_NO_XDEV.
+	mounted, err = mountedByOpenat2(path)
+	if err == nil {
+		return mounted, true, nil
+	}
+
 	// Another fast path: compare st.st_dev fields.
 	mounted, err = mountedByStat(path)
 	// This does not work for bind mounts, so false negative
 	// is possible, therefore only trust if return is true.
 	if mounted && err == nil {
+		return true, true, nil
+	}
+
+	return
+}
+
+func mounted(path string) (bool, error) {
+	path, err := normalizePath(path)
+	if err != nil {
+		return false, err
+	}
+	mounted, sure, err := mountedFast(path)
+	if sure && err == nil {
 		return mounted, nil
 	}
 
-	// Fallback to parsing mountinfo
+	// Fallback to parsing mountinfo.
 	return mountedByMountinfo(path)
 }