vetu clone: opportunistically zero-copy of files on Linux (#46)

Author: edigaryev

internal/storage/temporary/temporary.go

@@ -7,7 +7,9 @@ import (
 	"github.com/cirruslabs/vetu/internal/sparseio"
 	"github.com/cirruslabs/vetu/internal/vmconfig"
 	"github.com/cirruslabs/vetu/internal/vmdirectory"
+	"github.com/cirruslabs/vetu/internal/zerocopy"
 	"github.com/google/uuid"
+	"golang.org/x/sys/unix"
 	"os"
 	"path/filepath"
 )
@@ -61,8 +63,15 @@ func CreateFrom(srcDir string) (*vmdirectory.VMDirectory, error) {
 			return nil, err
 		}
 
-		if err := sparseio.Copy(dstFile, srcFile); err != nil {
-			return nil, err
+		if err := zerocopy.Clone(int(dstFile.Fd()), int(srcFile.Fd())); err != nil {
+			if !errors.Is(err, unix.ENOTSUP) {
+				return nil, err
+			}
+
+			// Fall back to slower sparse I/O copying if zero-copy is not supported
+			if err := sparseio.Copy(dstFile, srcFile); err != nil {
+				return nil, err
+			}
 		}
 
 		if err := srcFile.Close(); err != nil {

internal/zerocopy/zerocopy_linux.go

@@ -0,0 +1,9 @@
+package zerocopy
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func Clone(destFd int, srcFd int) error {
+	return unix.IoctlFileClone(destFd, srcFd)
+}

internal/zerocopy/zerocopy_unsupported.go

@@ -0,0 +1,9 @@
+//go:build !linux
+
+package zerocopy
+
+import "golang.org/x/sys/unix"
+
+func Clone(destFd int, srcFd int) error {
+	return unix.ENOTSUP
+}