android, libtailscale: pass BuildConfig to Go code; use for DNS config

This commit wires up a method to allow the Tailscale Go backend to
obtain the build configuration, and then adds a new build configuration
to the build to control whether we fall back to the Google public DNS
servers if we can't determine the platform's DNS configuration.

This replaces the previous "IsPlayVersion" / "MaybeGoogle" check for
whether to use the DNS servers as fallbacks, to allow users to decide
this independently of what version of the Android app this is.

Updates tailscale/tailscale#13431

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

android/build.gradle

@@ -39,6 +39,14 @@ android {
         targetSdkVersion 34
         versionCode 241
         versionName "1.73.114-t0970615b1-gab7ab737364"
+
+        // This setting, which defaults to 'true', will cause Tailscale to fall
+        // back to the Google DNS servers if it cannot determine what the
+        // operating system's DNS configuration is.
+        //
+        // Set it to false either here or in your local.properties file to
+        // disable this behaviour.
+        buildConfigField "boolean", "USE_GOOGLE_DNS_FALLBACK", getLocalProperty("tailscale.useGoogleDnsFallback", "true")
     }
 
     compileOptions {
@@ -54,6 +62,7 @@ android {
         jvmTarget = "17"
     }
     buildFeatures {
+        buildConfig true
         compose true
     }
     composeOptions {
@@ -66,9 +75,9 @@ android {
         applicationTest {
             initWith debug
             manifestPlaceholders.leanbackRequired = false
-            buildConfigField "String", "GITHUB_USERNAME", "\"" + getLocalProperty("githubUsername")+"\""
-            buildConfigField "String", "GITHUB_PASSWORD", "\"" + getLocalProperty("githubPassword")+"\""
-            buildConfigField "String", "GITHUB_2FA_SECRET", "\"" + getLocalProperty("github2FASecret")+"\""
+            buildConfigField "String", "GITHUB_USERNAME", "\"" + getLocalProperty("githubUsername", "")+"\""
+            buildConfigField "String", "GITHUB_PASSWORD", "\"" + getLocalProperty("githubPassword", "")+"\""
+            buildConfigField "String", "GITHUB_2FA_SECRET", "\"" + getLocalProperty("github2FASecret", "")+"\""
         }
         debug {
             manifestPlaceholders.leanbackRequired = false
@@ -156,12 +165,12 @@ dependencies {
     implementation("androidx.compose.ui:ui-tooling-preview")
 }
 
-def getLocalProperty(key) {
+def getLocalProperty(key, defaultValue) {
     try {
         Properties properties = new Properties()
         properties.load(project.file('local.properties').newDataInputStream())
-        return properties.getProperty(key)
+        return properties.getProperty(key) ?: defaultValue
     } catch(Throwable ignored) {
-        return ""
+        return defaultValue
     }
 }

android/src/main/java/com/tailscale/ipn/App.kt

@@ -38,6 +38,7 @@ import kotlinx.coroutines.cancel
 import kotlinx.coroutines.launch
 import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
+import libtailscale.BuildConfig as GoBuildConfig
 import libtailscale.Libtailscale
 import java.io.File
 import java.io.IOException
@@ -311,6 +312,13 @@ class App : UninitializedApp(), libtailscale.AppContext, ViewModelStoreOwner {
     }
   }
 
+  // getBuildConfig implements the libtailscale.AppContext interface.
+  override fun getBuildConfig(): GoBuildConfig {
+    var buildConfig = GoBuildConfig()
+    buildConfig.useGoogleDNSFallback = BuildConfig.USE_GOOGLE_DNS_FALLBACK
+    return buildConfig
+  }
+
   fun notifyPolicyChanged() {
     app.notifyPolicyChanged()
   }

libtailscale/backend.go

@@ -44,6 +44,9 @@ type App struct {
 	// appCtx is a global reference to the com.tailscale.ipn.App instance.
 	appCtx AppContext
 
+	// buildConfig is the build configuration for the app.
+	buildConfig *BuildConfig
+
 	store             *stateStore
 	policyStore       *syspolicyHandler
 	logIDPublicAtomic atomic.Pointer[logid.PublicID]
@@ -97,7 +100,8 @@ type backend struct {
 	// when no nameservers are provided by Tailscale.
 	avoidEmptyDNS bool
 
-	appCtx AppContext
+	appCtx      AppContext
+	buildConfig *BuildConfig
 }
 
 type settingsFunc func(*router.Config, *dns.OSConfig) error
@@ -262,9 +266,10 @@ func (a *App) newBackend(dataDir, directFileRoot string, appCtx AppContext, stor
 
 	logf := logger.RusagePrefixLog(log.Printf)
 	b := &backend{
-		devices:  newTUNDevices(),
-		settings: settings,
-		appCtx:   appCtx,
+		devices:     newTUNDevices(),
+		settings:    settings,
+		appCtx:      appCtx,
+		buildConfig: a.buildConfig,
 	}
 	var logID logid.PrivateID
 	logID.UnmarshalText([]byte("dead0000dead0000dead0000dead0000dead0000dead0000dead0000dead0000"))

libtailscale/interfaces.go

@@ -57,6 +57,12 @@ type AppContext interface {
 	// GetSyspolicyStringArrayValue returns the current string array value for the given system policy,
 	// expressed as a JSON string.
 	GetSyspolicyStringArrayJSONValue(key string) (string, error)
+
+	// GetBuildConfig gets the build configuration of the Android app.
+	//
+	// The returned BuildConfig should not change during the lifetime of
+	// the app.
+	GetBuildConfig() *BuildConfig
 }
 
 // IPNService corresponds to our IPNService in Java.
@@ -166,3 +172,11 @@ func RequestVPN(service IPNService) {
 func ServiceDisconnect(service IPNService) {
 	onDisconnect <- service
 }
+
+// BuildConfig is a struct that represents the build configuration of the
+// Android application, as set in BuildConfig.java.
+type BuildConfig struct {
+	// UseGoogleDNSFallback is whether to fall back to the Google public
+	// DNS servers if the platform's DNS servers cannot be determined.
+	UseGoogleDNSFallback bool
+}

libtailscale/net.go

@@ -287,7 +287,7 @@ func (b *backend) getDNSBaseConfig() (ret dns.OSConfig, _ error) {
 		// DNS config are lacking, and almost all Android phones use Google
 		// services anyway, so it's a reasonable default: it's an ecosystem the
 		// user has selected by having an Android device.
-		if len(ret.Nameservers) == 0 && b.appCtx.IsPlayVersion() {
+		if len(ret.Nameservers) == 0 && b.buildConfig.UseGoogleDNSFallback {
 			log.Printf("getDNSBaseConfig: none found; falling back to Google public DNS")
 			ret.Nameservers = append(ret.Nameservers, googleDNSServers...)
 		}

libtailscale/tailscale.go

@@ -38,6 +38,11 @@ func newApp(dataDir, directFileRoot string, appCtx AppContext) Application {
 	}
 	a.ready.Add(2)
 
+	// Get the build configuration, if any.
+	if bc := appCtx.GetBuildConfig(); bc != nil {
+		a.buildConfig = bc
+	}
+
 	a.store = newStateStore(a.appCtx)
 	a.policyStore = &syspolicyHandler{a: a}
 	netmon.RegisterInterfaceGetter(a.getInterfaces)