Implement initial 'wdcli backup_instance' command

This commit performs an initial implementation of the 'backup_instance'
command.

internal/fsx/copy.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"path/filepath"
 )
 
 var ErrCopySameFile = errors.New("src and dst must be different files")
@@ -39,3 +40,70 @@ func CopyFile(dst, src string) error {
 	_, err = io.Copy(dstFile, srcFile)
 	return err
 }
+
+var ErrCopyNoDirectory = errors.New("dst is not a directory")
+
+// CopyDirectory copies the directory src to dst recursively.
+// The destination directory must exist, or an error is returned.
+//
+// onCopy, when not nil, is called for each file or directory being copied.
+func CopyDirectory(dst, src string, onCopy func(dst, src string)) error {
+	// TODO: Allow copying in parallel? Maybe with a mutex?
+
+	// sanity checks
+	if src == dst {
+		return ErrCopySameFile
+	}
+	if !IsDirectory(dst) {
+		return ErrCopyNoDirectory
+	}
+
+	// call onCopy for this directory!
+	if onCopy != nil {
+		onCopy(dst, src)
+	}
+
+	// iterate over the entries or bail out
+	entries, err := os.ReadDir(src)
+	if err != nil {
+		return err
+	}
+
+	for _, entry := range entries {
+		name := entry.Name()
+		eDest := filepath.Join(dst, name)
+		eSrc := filepath.Join(src, name)
+
+		// it is not a directory => Use CopyFile
+		if !entry.IsDir() {
+			if onCopy != nil {
+				onCopy(eDest, eSrc)
+			}
+
+			// do the copy!
+			if err := CopyFile(eDest, eSrc); err != nil {
+				return err
+			}
+
+			continue
+		}
+
+		// find out the mode of the entry
+		eInfo, err := entry.Info()
+		if err != nil {
+			return err
+		}
+
+		// make the target directory
+		if err := os.Mkdir(eDest, eInfo.Mode()); err != nil {
+			return err
+		}
+
+		// do the copy!
+		if err := CopyDirectory(eDest, eSrc, onCopy); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}