cmd/tailscale/cli: relax & improve the running-as-CLI check for macOS

On macOS, we link the CLI into the GUI executable so it can be included in
the Mac App Store build.

You then need to run it like:

/Applications/Tailscale.app/Contents/MacOS/Tailscale <command>

But our old detection of whether you're running that Tailscale binary
in CLI mode wasn't accurate and often bit people. For instance, when
they made a typo, it then launched in GUI mode and broke their
existing GUI connection (starting a new IPNExtension) and took down
their network.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -15,6 +15,7 @@ import (
 	"os"
 	"os/signal"
 	"runtime"
+	"strconv"
 	"strings"
 	"syscall"
 	"text/tabwriter"
@@ -31,15 +32,37 @@ import (
 // CLI based on os.Args, GOOS, the context the process is running in
 // (pty, parent PID), etc.
 func ActLikeCLI() bool {
-	if len(os.Args) < 2 {
+	// This function is only used on macOS.
+	if runtime.GOOS != "darwin" {
 		return false
 	}
-	switch os.Args[1] {
-	case "up", "down", "status", "netcheck", "ping", "version",
-		"debug",
-		"-V", "--version", "-h", "--help":
+
+	// Escape hatch to let people force running the macOS
+	// GUI Tailscale binary as the CLI.
+	if v, _ := strconv.ParseBool(os.Getenv("TAILSCALE_BE_CLI")); v {
 		return true
 	}
+
+	// If our parent is launchd, we're definitely not
+	// being run as a CLI.
+	if os.Getppid() == 1 {
+		return false
+	}
+
+	// Looking at the environment of the GUI Tailscale app (ps eww
+	// $PID), empirically none of these environment variables are
+	// present. But all or some of these should be present with
+	// Terminal.all and bash or zsh.
+	for _, e := range []string{
+		"SHLVL",
+		"TERM",
+		"TERM_PROGRAM",
+		"PS1",
+	} {
+		if os.Getenv(e) != "" {
+			return true
+		}
+	}
 	return false
 }