termstatus: Fix panic for non-terminal runs

Closes #1803

internal/ui/termstatus/status.go

@@ -290,6 +290,20 @@ func (t *Terminal) Errorf(msg string, args ...interface{}) {
 	t.Error(s)
 }
 
+// truncate returns a string that has at most maxlen characters. If maxlen is
+// negative, the empty string is returned.
+func truncate(s string, maxlen int) string {
+	if maxlen < 0 {
+		return ""
+	}
+
+	if len(s) < maxlen {
+		return s
+	}
+
+	return s[:maxlen]
+}
+
 // SetStatus updates the status lines.
 func (t *Terminal) SetStatus(lines []string) {
 	if len(lines) == 0 {
@@ -297,7 +311,7 @@ func (t *Terminal) SetStatus(lines []string) {
 	}
 
 	width, _, err := getTermSize(t.fd)
-	if err != nil || width < 0 {
+	if err != nil || width <= 0 {
 		// use 80 columns by default
 		width = 80
 	}
@@ -305,11 +319,7 @@ func (t *Terminal) SetStatus(lines []string) {
 	// make sure that all lines have a line break and are not too long
 	for i, line := range lines {
 		line = strings.TrimRight(line, "\n")
-
-		if len(line) >= width-2 {
-			line = line[:width-2]
-		}
-		line += "\n"
+		line = truncate(line, width-2) + "\n"
 		lines[i] = line
 	}
 

internal/ui/termstatus/status_test.go

@@ -0,0 +1,32 @@
+package termstatus
+
+import "testing"
+
+func TestTruncate(t *testing.T) {
+	var tests = []struct {
+		input  string
+		maxlen int
+		output string
+	}{
+		{"", 80, ""},
+		{"", 0, ""},
+		{"", -1, ""},
+		{"foo", 80, "foo"},
+		{"foo", 4, "foo"},
+		{"foo", 3, "foo"},
+		{"foo", 2, "fo"},
+		{"foo", 1, "f"},
+		{"foo", 0, ""},
+		{"foo", -1, ""},
+	}
+
+	for _, test := range tests {
+		t.Run("", func(t *testing.T) {
+			out := truncate(test.input, test.maxlen)
+			if out != test.output {
+				t.Fatalf("wrong output for input %v, maxlen %d: want %q, got %q",
+					test.input, test.maxlen, test.output, out)
+			}
+		})
+	}
+}