mdm: read user defaults + registry proto

Signed-off-by: Andrea Gottardo <andrea@tailscale.com>

mdm/mdm.go

@@ -0,0 +1,89 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package mdm contains functions to read platform-specific MDM-enforced flags
+// in a platform-independent manner.
+package mdm
+
+import (
+	"fmt"
+	"os/exec"
+	"runtime"
+	"tailscale.com/version"
+)
+
+func ReadBool(key string) (bool, error) {
+	if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
+		return readUserDefaultsBool(key)
+	} else if runtime.GOOS == "windows" {
+		return readRegistryBool(key)
+	} else {
+		return false, fmt.Errorf("unsupported platform")
+	}
+}
+
+func ReadString(key string) (string, error) {
+	if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
+		return readUserDefaultsString(key)
+	} else if runtime.GOOS == "windows" {
+		// TODO(angott): Windows
+		return readRegistryString(key)
+	} else {
+		return "", fmt.Errorf("unsupported platform")
+	}
+}
+
+/// Darwin
+
+// readUserDefaultsBool reads a boolean value with the given key from the macOS/iOS UserDefaults.
+func readUserDefaultsBool(key string) (bool, error) {
+	cmd := exec.Command("defaults", "read", userDefaultsDomain(), key)
+	output, err := cmd.Output()
+	if err != nil {
+		return false, err
+	}
+	asString := string(output)
+	if asString == "0" {
+		return false, nil
+	} else if asString == "1" {
+		return true, nil
+	} else {
+		return false, fmt.Errorf("unexpected user defaults value for", key, ":", err)
+	}
+}
+
+// readRegistryString reads a string value with the given key from the macOS/iOS UserDefaults.
+func readUserDefaultsString(key string) (string, error) {
+	cmd := exec.Command("defaults", "read", userDefaultsDomain(), key)
+	output, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	asString := string(output)
+	return asString, nil
+}
+
+// userDefaultsDomain returns the domain iOS or macOS store the Tailscale settings in.
+func userDefaultsDomain() string {
+	var bundleIdentifierSuffix string
+	if version.IsMacSysExt() {
+		bundleIdentifierSuffix = "macsys"
+	} else {
+		bundleIdentifierSuffix = "macos"
+	}
+	return "io.tailscale.ipn." + bundleIdentifierSuffix
+}
+
+/// Windows
+
+// readRegistryBool reads a boolean value with the given key from the Windows registry.
+func readRegistryBool(key string) (bool, error) {
+	// TODO(angott): Windows support
+	return false, nil
+}
+
+// readRegistryBool reads a string value with the given key from the Windows registry.
+func readRegistryString(key string) (string, error) {
+	// TODO(angott): Windows support
+	return "", nil
+}