Skip upload copy if we know the length

If the source length is known, we can skip copying the file.

communicator/ssh/communicator.go

@@ -292,8 +292,25 @@ func (c *Communicator) Upload(path string, input io.Reader) error {
 	// which works for unix and windows
 	targetDir = filepath.ToSlash(targetDir)
 
+	// Skip copying if we can get the file size directly from common io.Readers
+	size := int64(0)
+
+	switch src := input.(type) {
+	case *os.File:
+		fi, err := src.Stat()
+		if err != nil {
+			size = fi.Size()
+		}
+	case *bytes.Buffer:
+		size = int64(src.Len())
+	case *bytes.Reader:
+		size = int64(src.Len())
+	case *strings.Reader:
+		size = int64(src.Len())
+	}
+
 	scpFunc := func(w io.Writer, stdoutR *bufio.Reader) error {
-		return scpUploadFile(targetFile, input, w, stdoutR)
+		return scpUploadFile(targetFile, input, w, stdoutR, size)
 	}
 
 	return c.scpSession("scp -vt "+targetDir, scpFunc)
@@ -490,7 +507,8 @@ func checkSCPStatus(r *bufio.Reader) error {
 	return nil
 }
 
-func scpUploadFile(dst string, src io.Reader, w io.Writer, r *bufio.Reader) error {
+func scpUploadFile(dst string, src io.Reader, w io.Writer, r *bufio.Reader, size int64) error {
+	if size == 0 {
 		// Create a temporary file where we can copy the contents of the src
 		// so that we can determine the length, since SCP is length-prefixed.
 		tf, err := ioutil.TempFile("", "terraform-upload")
@@ -521,14 +539,18 @@ func scpUploadFile(dst string, src io.Reader, w io.Writer, r *bufio.Reader) erro
 			return fmt.Errorf("Error creating temporary file for upload: %s", err)
 		}
 
+		src = tf
+		size = fi.Size()
+	}
+
 	// Start the protocol
 	log.Println("Beginning file upload...")
-	fmt.Fprintln(w, "C0644", fi.Size(), dst)
+	fmt.Fprintln(w, "C0644", size, dst)
 	if err := checkSCPStatus(r); err != nil {
 		return err
 	}
 
-	if _, err := io.Copy(w, tf); err != nil {
+	if _, err := io.Copy(w, src); err != nil {
 		return err
 	}
 
@@ -592,7 +614,7 @@ func scpUploadDir(root string, fs []os.FileInfo, w io.Writer, r *bufio.Reader) e
 
 			err = func() error {
 				defer f.Close()
-				return scpUploadFile(fi.Name(), f, w, r)
+				return scpUploadFile(fi.Name(), f, w, r, fi.Size())
 			}()
 
 			if err != nil {