Link VHD instead of copy where allowed by OS

No need to incur that copy I/O if we don't even change the file!

post-processor/vagrant/hyperv.go

@@ -2,10 +2,11 @@ package vagrant
 
 import (
 	"fmt"
-	"github.com/hashicorp/packer/packer"
 	"os"
 	"path/filepath"
 	"strings"
+
+	"github.com/hashicorp/packer/packer"
 )
 
 type HypervProvider struct{}
@@ -55,10 +56,18 @@ func (p *HypervProvider) Process(ui packer.Ui, artifact packer.Artifact, dir str
 
 		dstPath := filepath.Join(dstDir, filepath.Base(path))
 
-		if err = CopyContents(dstPath, path); err != nil {
-			ui.Message(fmt.Sprintf("err in copying: %s to %s", path, dstPath))
-			return
+		// We prefer to link the files where possible because they are often very huge.
+		// Some filesystem configurations do not allow hardlinks. As the possibilities
+		// of mounting different devices in different paths are flexible, we just try to
+		// link the file and copy if the link fails, thereby automatically optimizing with a safe fallback.
+		if err = LinkFile(dstPath, path); err != nil {
+			// ui.Message(fmt.Sprintf("err in linking: %s to %s", path, dstPath))
+			if err = CopyContents(dstPath, path); err != nil {
+				ui.Message(fmt.Sprintf("err in copying: %s to %s", path, dstPath))
+				return
+			}
 		}
+
 		ui.Message(fmt.Sprintf("Copyed %s to %s", path, dstPath))
 	}
 

post-processor/vagrant/util.go

@@ -51,6 +51,23 @@ func CopyContents(dst, src string) error {
 	return nil
 }
 
+// Creates a (hard) link to a file, ensuring that all parent directories also exist.
+func LinkFile(dst, src string) error {
+	dstDir, _ := filepath.Split(dst)
+	if dstDir != "" {
+		err := os.MkdirAll(dstDir, os.ModePerm)
+		if err != nil {
+			return err
+		}
+	}
+
+	if err := os.Link(src, dst); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 // DirToBox takes the directory and compresses it into a Vagrant-compatible
 // box. This function does not perform checks to verify that dir is
 // actually a proper box. This is an expected precondition.