ui: port syspolicy handler code to new app (#304)

* ui: port syspolicy handler code to new app

port over https://github.com/tailscale/tailscale-android/pull/199 from cmd/tailscale and legacy_android to libtailscale and android/

Updates tailscale/corp#18202

Signed-off-by: kari-ts <kari@tailscale.com>

* android: PR suggestions for syspolicyHandler (#308)

Updates tailscale/corp#18202

Signed-off-by: Percy Wegmann <percy@tailscale.com>

---------

Signed-off-by: kari-ts <kari@tailscale.com>
Signed-off-by: Percy Wegmann <percy@tailscale.com>
Co-authored-by: Percy Wegmann <percy@tailscale.com>

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

@@ -37,6 +37,7 @@ import androidx.core.app.NotificationCompat
 import androidx.core.app.NotificationManagerCompat
 import androidx.security.crypto.EncryptedSharedPreferences
 import androidx.security.crypto.MasterKey
+import com.tailscale.ipn.mdm.MDMSettings
 import com.tailscale.ipn.ui.localapi.Client
 import com.tailscale.ipn.ui.localapi.Request
 import com.tailscale.ipn.ui.model.Ipn
@@ -451,4 +452,18 @@ class App : Application(), libtailscale.AppContext {
 
     return downloads
   }
+
+  @Throws(IOException::class, GeneralSecurityException::class)
+  override fun getSyspolicyBooleanValue(key: String): Boolean {
+    return getSyspolicyStringValue(key) == "true"
+  }
+
+  @Throws(IOException::class, GeneralSecurityException::class)
+  override fun getSyspolicyStringValue(key: String): String {
+    return MDMSettings.allSettingsByKey[key]?.flow?.value?.toString()
+        ?: run {
+          Log.d("MDM", "$key is not defined on Android. Returning empty.")
+          ""
+        }
+  }
 }

android/src/main/java/com/tailscale/ipn/mdm/MDMSettings.kt

@@ -57,6 +57,8 @@ object MDMSettings {
         .map { it.call(MDMSettings) as MDMSetting<*> }
   }
 
+  val allSettingsByKey by lazy { allSettings.associateBy { it.key } }
+
   fun update(app: App, restrictionsManager: RestrictionsManager?) {
     val bundle = restrictionsManager?.applicationRestrictions
     allSettings.forEach { it.setFrom(bundle, app) }

libtailscale/interfaces.go

@@ -45,6 +45,12 @@ type AppContext interface {
 	// GetPlatformDNSConfig gets a string representation of the current DNS
 	// configuration.
 	GetPlatformDNSConfig() string
+
+	// GetSyspolicyStringValue returns the current string value for the given system policy.
+	GetSyspolicyStringValue(key string) (string, error)
+
+	// GetSyspolicyBooleanValue returns whether the given system policy is enabled.
+	GetSyspolicyBooleanValue(key string) (bool, error)
 }
 
 // IPNService corresponds to our IPNService in Java.

libtailscale/syspolicy_handler.go

@@ -0,0 +1,47 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package libtailscale
+
+import (
+	"log"
+
+	"tailscale.com/util/syspolicy"
+)
+
+// syspolicyHandler is a syspolicy handler for the Android version of the Tailscale client,
+// which lets the main networking code read values set via the Android RestrictionsManager.
+type syspolicyHandler struct {
+	a *App
+}
+
+func (h syspolicyHandler) ReadString(key string) (string, error) {
+	if key == "" {
+		return "", syspolicy.ErrNoSuchKey
+	}
+	retVal, err := h.a.appCtx.GetSyspolicyStringValue(key)
+	if err != nil {
+		log.Printf("syspolicy: failed to get string value via gomobile: %v", err)
+	}
+	return retVal, err
+}
+
+func (h syspolicyHandler) ReadBoolean(key string) (bool, error) {
+	if key == "" {
+		return false, syspolicy.ErrNoSuchKey
+	}
+	retVal, err := h.a.appCtx.GetSyspolicyBooleanValue(key)
+	if err != nil {
+		log.Printf("syspolicy: failed to get bool value via gomobile: %v", err)
+	}
+	return retVal, err
+}
+
+func (h syspolicyHandler) ReadUInt64(key string) (uint64, error) {
+	if key == "" {
+		return 0, syspolicy.ErrNoSuchKey
+	}
+	// TODO(angott): drop ReadUInt64 everywhere. We are not using it.
+	log.Fatalf("ReadUInt64 is not implemented on Android")
+	return 0, nil
+}

libtailscale/tailscale.go

@@ -18,6 +18,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/syspolicy"
 )
 
 const defaultMTU = 1280 // minimalMTU from wgengine/userspace.go
@@ -38,6 +39,7 @@ func newApp(dataDir, directFileRoot string, appCtx AppContext) Application {
 
 	a.store = newStateStore(a.appCtx)
 	interfaces.RegisterInterfaceGetter(a.getInterfaces)
+	syspolicy.RegisterHandler(syspolicyHandler{a: a})
 	go func() {
 		defer func() {
 			if p := recover(); p != nil {