terminal: add display command (#1917)

Implements #1256

Documentation/cli/README.md

@@ -39,6 +39,7 @@ Command | Description
 Command | Description
 --------|------------
 [args](#args) | Print function arguments.
+[display](#display) | Print value of an expression every time the program stops.
 [examinemem](#examinemem) | Examine memory:
 [locals](#locals) | Print local variables.
 [print](#print) | Evaluate an expression.
@@ -225,6 +226,17 @@ If no argument is specified the function being executed in the selected stack fr
 
 Aliases: disass
 
+## display
+Print value of an expression every time the program stops.
+
+	display -a <expression>
+	display -d <number>
+
+The '-a' option adds an expression to the list of expression printed every time the program stops. The '-d' option removes the specified expression from the list.
+
+If display is called without arguments it will print the value of all expression in the list.
+
+
 ## down
 Move the current frame down.
 

pkg/terminal/command.go

@@ -384,6 +384,15 @@ Address is the memory location of the target to examine.
 For example:
 
     x -fmt hex -len 20 0xc00008af38`},
+
+		{aliases: []string{"display"}, group: dataCmds, cmdFn: display, helpMsg: `Print value of an expression every time the program stops.
+
+	display -a <expression>
+	display -d <number>
+
+The '-a' option adds an expression to the list of expression printed every time the program stops. The '-d' option removes the specified expression from the list.
+
+If display is called without arguments it will print the value of all expression in the list.`},
 	}
 
 	if client == nil || client.Recorded() {
@@ -981,6 +990,7 @@ func restartRecorded(t *Term, ctx callContext, args string) error {
 	}
 	printcontext(t, state)
 	printfile(t, state.CurrentThread.File, state.CurrentThread.Line, true)
+	t.onStop()
 	return nil
 }
 
@@ -1057,6 +1067,7 @@ func (c *Commands) cont(t *Term, ctx callContext, args string) error {
 	if ctx.Prefix == revPrefix {
 		return c.rewind(t, ctx, args)
 	}
+	defer t.onStop()
 	c.frame = 0
 	stateChan := t.client.Continue()
 	var state *api.DebuggerState
@@ -1072,6 +1083,7 @@ func (c *Commands) cont(t *Term, ctx callContext, args string) error {
 }
 
 func continueUntilCompleteNext(t *Term, state *api.DebuggerState, op string, shouldPrintFile bool) error {
+	defer t.onStop()
 	if !state.NextInProgress {
 		if shouldPrintFile {
 			printfile(t, state.CurrentThread.File, state.CurrentThread.Line, true)
@@ -1141,6 +1153,8 @@ func (c *Commands) stepInstruction(t *Term, ctx callContext, args string) error
 		return notOnFrameZeroErr
 	}
 
+	defer t.onStop()
+
 	var fn func() (*api.DebuggerState, error)
 	if ctx.Prefix == revPrefix {
 		fn = t.client.ReverseStepInstruction
@@ -2386,6 +2400,37 @@ func clearCheckpoint(t *Term, ctx callContext, args string) error {
 	return t.client.ClearCheckpoint(id)
 }
 
+func display(t *Term, ctx callContext, args string) error {
+	const (
+		addOption = "-a "
+		delOption = "-d "
+	)
+	switch {
+	case args == "":
+		t.printDisplays()
+
+	case strings.HasPrefix(args, addOption):
+		args = strings.TrimSpace(args[len(addOption):])
+		if args == "" {
+			return fmt.Errorf("not enough arguments")
+		}
+		t.addDisplay(args)
+		t.printDisplay(len(t.displays) - 1)
+
+	case strings.HasPrefix(args, delOption):
+		args = strings.TrimSpace(args[len(delOption):])
+		n, err := strconv.Atoi(args)
+		if err != nil {
+			return fmt.Errorf("%q is not a number", args)
+		}
+		return t.removeDisplay(n)
+
+	default:
+		return fmt.Errorf("wrong arguments")
+	}
+	return nil
+}
+
 func formatBreakpointName(bp *api.Breakpoint, upcase bool) string {
 	thing := "breakpoint"
 	if bp.Tracepoint {

pkg/terminal/terminal.go

@@ -54,6 +54,7 @@ type Term struct {
 	dumb     bool
 	stdout   io.Writer
 	InitFile string
+	displays []string
 
 	starlarkEnv *starbind.Env
 
@@ -432,6 +433,50 @@ func (t *Term) loadConfig() api.LoadConfig {
 	return r
 }
 
+func (t *Term) removeDisplay(n int) error {
+	if n < 0 || n >= len(t.displays) {
+		return fmt.Errorf("%d is out of range", n)
+	}
+	t.displays[n] = ""
+	for i := len(t.displays) - 1; i >= 0; i-- {
+		if t.displays[i] != "" {
+			t.displays = t.displays[:i+1]
+			return nil
+		}
+	}
+	t.displays = t.displays[:0]
+	return nil
+}
+
+func (t *Term) addDisplay(expr string) {
+	t.displays = append(t.displays, expr)
+}
+
+func (t *Term) printDisplay(i int) {
+	expr := t.displays[i]
+	val, err := t.client.EvalVariable(api.EvalScope{GoroutineID: -1}, expr, ShortLoadConfig)
+	if err != nil {
+		if isErrProcessExited(err) {
+			return
+		}
+		fmt.Printf("%d: %s = error %v\n", i, expr, err)
+		return
+	}
+	fmt.Printf("%d: %s = %s\n", i, val.Name, val.SinglelineString())
+}
+
+func (t *Term) printDisplays() {
+	for i := range t.displays {
+		if t.displays[i] != "" {
+			t.printDisplay(i)
+		}
+	}
+}
+
+func (t *Term) onStop() {
+	t.printDisplays()
+}
+
 // isErrProcessExited returns true if `err` is an RPC error equivalent of proc.ErrProcessExited
 func isErrProcessExited(err error) bool {
 	rpcError, ok := err.(rpc.ServerError)