pkg/fileutils: add ReflinkOrCopy()

Add a function to copy via reflink if possible or fall back to the
normal io.Copy().
c/image uses this function internally[1] however I like to export this
and use it in podman and I think this package here in c/storage is the
most logical location for it. Note compared to the c/image version I am
calling unix.IoctlFileClone() directly instead of the raw syscall
function.

[1] https://github.com/containers/image/tree/main/internal/reflink

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

pkg/fileutils/reflink_linux.go

@@ -0,0 +1,20 @@
+package fileutils
+
+import (
+	"io"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// ReflinkOrCopy attempts to reflink the source to the destination fd.
+// If reflinking fails or is unsupported, it falls back to io.Copy().
+func ReflinkOrCopy(src, dst *os.File) error {
+	err := unix.IoctlFileClone(int(dst.Fd()), int(src.Fd()))
+	if err == nil {
+		return nil
+	}
+
+	_, err = io.Copy(dst, src)
+	return err
+}

pkg/fileutils/reflink_unsupported.go

@@ -0,0 +1,15 @@
+//go:build !linux
+
+package fileutils
+
+import (
+	"io"
+	"os"
+)
+
+// ReflinkOrCopy attempts to reflink the source to the destination fd.
+// If reflinking fails or is unsupported, it falls back to io.Copy().
+func ReflinkOrCopy(src, dst *os.File) error {
+	_, err := io.Copy(dst, src)
+	return err
+}