From 3001fc2b49ce0e11e9e8d012345c80845957f60b Mon Sep 17 00:00:00 2001
From: Kristoffer Dalby <kristoffer@tailscale.com>
Date: Fri, 12 Sep 2025 16:10:49 +0200
Subject: [PATCH] util: add YesNo prompt

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
---
 hscontrol/util/prompt.go      |  24 ++++
 hscontrol/util/prompt_test.go | 209 ++++++++++++++++++++++++++++++++++
 2 files changed, 233 insertions(+)
 create mode 100644 hscontrol/util/prompt.go
 create mode 100644 hscontrol/util/prompt_test.go

diff --git a/hscontrol/util/prompt.go b/hscontrol/util/prompt.go
new file mode 100644
index 00000000..098f1979
--- /dev/null
+++ b/hscontrol/util/prompt.go
@@ -0,0 +1,24 @@
+package util
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// YesNo takes a question and prompts the user to answer the
+// question with a yes or no. It appends a [y/n] to the message.
+// The question is written to stderr so that content can be redirected
+// without interfering with the prompt.
+func YesNo(msg string) bool {
+	fmt.Fprint(os.Stderr, msg+" [y/n] ")
+
+	var resp string
+	fmt.Scanln(&resp)
+	resp = strings.ToLower(resp)
+	switch resp {
+	case "y", "yes", "sure":
+		return true
+	}
+	return false
+}
diff --git a/hscontrol/util/prompt_test.go b/hscontrol/util/prompt_test.go
new file mode 100644
index 00000000..d726ec60
--- /dev/null
+++ b/hscontrol/util/prompt_test.go
@@ -0,0 +1,209 @@
+package util
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestYesNo(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected bool
+	}{
+		{
+			name:     "y answer",
+			input:    "y\n",
+			expected: true,
+		},
+		{
+			name:     "Y answer",
+			input:    "Y\n",
+			expected: true,
+		},
+		{
+			name:     "yes answer",
+			input:    "yes\n",
+			expected: true,
+		},
+		{
+			name:     "YES answer",
+			input:    "YES\n",
+			expected: true,
+		},
+		{
+			name:     "sure answer",
+			input:    "sure\n",
+			expected: true,
+		},
+		{
+			name:     "SURE answer",
+			input:    "SURE\n",
+			expected: true,
+		},
+		{
+			name:     "n answer",
+			input:    "n\n",
+			expected: false,
+		},
+		{
+			name:     "no answer",
+			input:    "no\n",
+			expected: false,
+		},
+		{
+			name:     "empty answer",
+			input:    "\n",
+			expected: false,
+		},
+		{
+			name:     "invalid answer",
+			input:    "maybe\n",
+			expected: false,
+		},
+		{
+			name:     "random text",
+			input:    "foobar\n",
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Capture stdin
+			oldStdin := os.Stdin
+			r, w, _ := os.Pipe()
+			os.Stdin = r
+
+			// Capture stderr
+			oldStderr := os.Stderr
+			stderrR, stderrW, _ := os.Pipe()
+			os.Stderr = stderrW
+
+			// Write test input
+			go func() {
+				defer w.Close()
+				w.WriteString(tt.input)
+			}()
+
+			// Call the function
+			result := YesNo("Test question")
+
+			// Restore stdin and stderr
+			os.Stdin = oldStdin
+			os.Stderr = oldStderr
+			stderrW.Close()
+
+			// Check the result
+			if result != tt.expected {
+				t.Errorf("YesNo() = %v, want %v", result, tt.expected)
+			}
+
+			// Check that the prompt was written to stderr
+			var stderrBuf bytes.Buffer
+			io.Copy(&stderrBuf, stderrR)
+			stderrR.Close()
+
+			expectedPrompt := "Test question [y/n] "
+			actualPrompt := stderrBuf.String()
+			if actualPrompt != expectedPrompt {
+				t.Errorf("Expected prompt %q, got %q", expectedPrompt, actualPrompt)
+			}
+		})
+	}
+}
+
+func TestYesNoPromptMessage(t *testing.T) {
+	// Capture stdin
+	oldStdin := os.Stdin
+	r, w, _ := os.Pipe()
+	os.Stdin = r
+
+	// Capture stderr
+	oldStderr := os.Stderr
+	stderrR, stderrW, _ := os.Pipe()
+	os.Stderr = stderrW
+
+	// Write test input
+	go func() {
+		defer w.Close()
+		w.WriteString("n\n")
+	}()
+
+	// Call the function with a custom message
+	customMessage := "Do you want to continue with this dangerous operation?"
+	YesNo(customMessage)
+
+	// Restore stdin and stderr
+	os.Stdin = oldStdin
+	os.Stderr = oldStderr
+	stderrW.Close()
+
+	// Check that the custom message was included in the prompt
+	var stderrBuf bytes.Buffer
+	io.Copy(&stderrBuf, stderrR)
+	stderrR.Close()
+
+	expectedPrompt := customMessage + " [y/n] "
+	actualPrompt := stderrBuf.String()
+	if actualPrompt != expectedPrompt {
+		t.Errorf("Expected prompt %q, got %q", expectedPrompt, actualPrompt)
+	}
+}
+
+func TestYesNoCaseInsensitive(t *testing.T) {
+	testCases := []struct {
+		input    string
+		expected bool
+	}{
+		{"y\n", true},
+		{"Y\n", true},
+		{"yes\n", true},
+		{"Yes\n", true},
+		{"YES\n", true},
+		{"yEs\n", true},
+		{"sure\n", true},
+		{"Sure\n", true},
+		{"SURE\n", true},
+		{"SuRe\n", true},
+	}
+
+	for _, tc := range testCases {
+		t.Run("input_"+strings.TrimSpace(tc.input), func(t *testing.T) {
+			// Capture stdin
+			oldStdin := os.Stdin
+			r, w, _ := os.Pipe()
+			os.Stdin = r
+
+			// Capture stderr to avoid output during tests
+			oldStderr := os.Stderr
+			stderrR, stderrW, _ := os.Pipe()
+			os.Stderr = stderrW
+
+			// Write test input
+			go func() {
+				defer w.Close()
+				w.WriteString(tc.input)
+			}()
+
+			// Call the function
+			result := YesNo("Test")
+
+			// Restore stdin and stderr
+			os.Stdin = oldStdin
+			os.Stderr = oldStderr
+			stderrW.Close()
+
+			// Drain stderr
+			io.Copy(io.Discard, stderrR)
+			stderrR.Close()
+
+			if result != tc.expected {
+				t.Errorf("Input %q: expected %v, got %v", strings.TrimSpace(tc.input), tc.expected, result)
+			}
+		})
+	}
+}