From b9cdef18c04b48a52235af4eadcd9a3193cafb3c Mon Sep 17 00:00:00 2001
From: James Tucker <james@tailscale.com>
Date: Fri, 12 Sep 2025 12:33:46 -0700
Subject: [PATCH] util/prompt: add a default and take default in
 non-interactive cases

The Tailscale CLI is the primary configuration interface and as such it
is used in scripts, container setups, and many other places that do not
have a terminal available and should not be made to respond to prompts.

The default is set to false where the "risky" API is being used by the
CLI and true otherwise, this means that the `--yes` flags are only
required under interactive runs and scripts do not need to be concerned
with prompts or extra flags.

Updates #19445

Signed-off-by: James Tucker <james@tailscale.com>
---
 cmd/tailscale/cli/network-lock.go |  2 +-
 cmd/tailscale/cli/risks.go        |  2 +-
 cmd/tailscale/cli/serve_v2.go     |  2 +-
 cmd/tailscale/cli/update.go       |  2 +-
 util/prompt/prompt.go             | 19 +++++++++++++++++--
 5 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/cmd/tailscale/cli/network-lock.go b/cmd/tailscale/cli/network-lock.go
index ec3b01ad6..9b2f6fbdb 100644
--- a/cmd/tailscale/cli/network-lock.go
+++ b/cmd/tailscale/cli/network-lock.go
@@ -384,7 +384,7 @@ Removal of a signing key(s) without resigning nodes (--re-sign=false)
 will cause any nodes signed by the the given key(s) to be locked out
 of the Tailscale network. Proceed with caution.
 `)
-			if !prompt.YesNo("Are you sure you want to remove the signing key(s)?") {
+			if !prompt.YesNo("Are you sure you want to remove the signing key(s)?", true) {
 				fmt.Printf("aborting removal of signing key(s)\n")
 				os.Exit(0)
 			}
diff --git a/cmd/tailscale/cli/risks.go b/cmd/tailscale/cli/risks.go
index dfde87f64..d4572842b 100644
--- a/cmd/tailscale/cli/risks.go
+++ b/cmd/tailscale/cli/risks.go
@@ -66,7 +66,7 @@ func presentRiskToUser(riskType, riskMessage, acceptedRisks string) error {
 	outln(riskMessage)
 	printf("To skip this warning, use --accept-risk=%s\n", riskType)
 
-	if prompt.YesNo("Continue?") {
+	if prompt.YesNo("Continue?", false) {
 		return nil
 	}
 
diff --git a/cmd/tailscale/cli/serve_v2.go b/cmd/tailscale/cli/serve_v2.go
index 058d80649..8831db2a9 100644
--- a/cmd/tailscale/cli/serve_v2.go
+++ b/cmd/tailscale/cli/serve_v2.go
@@ -1086,7 +1086,7 @@ func (e *serveEnv) removeWebServe(sc *ipn.ServeConfig, dnsName string, srvPort u
 
 	if len(mounts) > 1 {
 		msg := fmt.Sprintf("Are you sure you want to delete %d handlers under port %s?", len(mounts), portStr)
-		if !e.yes && !prompt.YesNo(msg) {
+		if !e.yes && !prompt.YesNo(msg, true) {
 			return nil
 		}
 	}
diff --git a/cmd/tailscale/cli/update.go b/cmd/tailscale/cli/update.go
index 7c0269f6a..7eb0dccac 100644
--- a/cmd/tailscale/cli/update.go
+++ b/cmd/tailscale/cli/update.go
@@ -87,5 +87,5 @@ func confirmUpdate(ver string) bool {
 	}
 
 	msg := fmt.Sprintf("This will update Tailscale from %v to %v. Continue?", version.Short(), ver)
-	return prompt.YesNo(msg)
+	return prompt.YesNo(msg, true)
 }
diff --git a/util/prompt/prompt.go b/util/prompt/prompt.go
index 4e589ceb3..a6d86fb48 100644
--- a/util/prompt/prompt.go
+++ b/util/prompt/prompt.go
@@ -6,19 +6,34 @@ package prompt
 
 import (
 	"fmt"
+	"os"
 	"strings"
+
+	"github.com/mattn/go-isatty"
 )
 
 // 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.
-func YesNo(msg string) bool {
-	fmt.Print(msg + " [y/n] ")
+//
+// If there is no TTY on both Stdin and Stdout, assume that we're in a script
+// and return the dflt result.
+func YesNo(msg string, dflt bool) bool {
+	if !(isatty.IsTerminal(os.Stdin.Fd()) && isatty.IsTerminal(os.Stdout.Fd())) {
+		return dflt
+	}
+	if dflt {
+		fmt.Print(msg + " [Y/n] ")
+	} else {
+		fmt.Print(msg + " [y/N] ")
+	}
 	var resp string
 	fmt.Scanln(&resp)
 	resp = strings.ToLower(resp)
 	switch resp {
 	case "y", "yes", "sure":
 		return true
+	case "":
+		return dflt
 	}
 	return false
 }