cmd: Make --progress work in git bash on Windows - fixes #3531

This detects the presence of a VT100 terminal by using the TERM
environment variable and switches to using VT100 codes directly under
windows if it is found.

This makes --progress work correctly with git bash.

cmd/progress.go

@@ -23,10 +23,29 @@ const (
 	logTimeFormat = "2006-01-02 15:04:05"
 )
 
+var (
+	initTerminal    func() error
+	writeToTerminal func([]byte)
+)
+
+// Initialise the VT100 terminal
+func initTerminalVT100() error {
+	return nil
+}
+
+// Write to the VT100 terminal
+func writeToTerminalVT100(b []byte) {
+	_, _ = os.Stdout.Write(b)
+}
+
 // startProgress starts the progress bar printing
 //
 // It returns a func which should be called to stop the stats.
 func startProgress() func() {
+	if os.Getenv("TERM") != "" {
+		initTerminal = initTerminalVT100
+		writeToTerminal = writeToTerminalVT100
+	}
 	err := initTerminal()
 	if err != nil {
 		fs.Errorf(nil, "Failed to start progress: %v", err)

cmd/progress_other.go

@@ -2,12 +2,8 @@
 
 package cmd
 
-import "os"
-
-func initTerminal() error {
-	return nil
-}
-
-func writeToTerminal(b []byte) {
-	_, _ = os.Stdout.Write(b)
+func init() {
+	// Default terminal is VT100 for non Windows
+	initTerminal = initTerminalVT100
+	writeToTerminal = writeToTerminalVT100
 }

cmd/progress_windows.go

@@ -16,7 +16,13 @@ var (
 	ansiParser *ansiterm.AnsiParser
 )
 
-func initTerminal() error {
+func init() {
+	// Default terminal is Windows console for Windows
+	initTerminal = initTerminalWindows
+	writeToTerminal = writeToTerminalWindows
+}
+
+func initTerminalWindows() error {
 	winEventHandler := winterm.CreateWinEventHandler(os.Stdout.Fd(), os.Stdout)
 	if winEventHandler == nil {
 		err := syscall.GetLastError()
@@ -29,7 +35,7 @@ func initTerminal() error {
 	return nil
 }
 
-func writeToTerminal(b []byte) {
+func writeToTerminalWindows(b []byte) {
 	// Remove all non-ASCII characters until this is fixed
 	// https://github.com/Azure/go-ansiterm/issues/26
 	r := []rune(string(b))