lib/fs: Unwrap mtimeFile, get fd the "correct" way (ref #6875) (#6877)

lib/fs/basicfs_copy_range.go

@@ -14,6 +14,9 @@ type copyRangeImplementationBasicFile func(src, dst basicFile, srcOffset, dstOff
 
 func copyRangeImplementationForBasicFile(impl copyRangeImplementationBasicFile) copyRangeImplementation {
 	return func(src, dst File, srcOffset, dstOffset, size int64) error {
+		src = unwrap(src)
+		dst = unwrap(dst)
+		// Then see if it's basic files
 		srcFile, srcOk := src.(basicFile)
 		dstFile, dstOk := dst.(basicFile)
 		if !srcOk || !dstOk {
@@ -22,3 +25,38 @@ func copyRangeImplementationForBasicFile(impl copyRangeImplementationBasicFile)
 		return impl(srcFile, dstFile, srcOffset, dstOffset, size)
 	}
 }
+
+func withFileDescriptors(first, second basicFile, fn func(first, second uintptr) (int, error)) (int, error) {
+	fc, err := first.SyscallConn()
+	if err != nil {
+		return 0, err
+	}
+	sc, err := second.SyscallConn()
+	if err != nil {
+		return 0, err
+	}
+	var n int
+	var ferr, serr, fnerr error
+	ferr = fc.Control(func(first uintptr) {
+		serr = sc.Control(func(second uintptr) {
+			n, fnerr = fn(first, second)
+		})
+	})
+	if ferr != nil {
+		return n, ferr
+	}
+	if serr != nil {
+		return n, serr
+	}
+	return n, fnerr
+}
+
+func unwrap(f File) File {
+	for {
+		if wrapped, ok := f.(interface{ unwrap() File }); ok {
+			f = wrapped.unwrap()
+		} else {
+			return f
+		}
+	}
+}