android: add All() to state store implementation

Android has its own SharedPreferences-backed implementation of ipn.StateStore. Due to https://github.com/golang/go/issues/13445, we bundle the key list into a single primitive and unpack it in Go in our All() implementation.
This also adds a compile-time check to prevent drift the interface.

Updates tailscale/tailscale#15830

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

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

@@ -248,6 +248,16 @@ class App : UninitializedApp(), libtailscale.AppContext, ViewModelStoreOwner {
     return getEncryptedPrefs().getString(prefKey, null)
   }
 
+  override fun getStateStoreKeysJSON(): String {
+    val prefix = "statestore-"
+    val keys = getEncryptedPrefs()
+                  .getAll()
+                  .keys
+                  .filter { it.startsWith(prefix) }
+                  .map   { it.removePrefix(prefix) }
+    return org.json.JSONArray(keys).toString()
+  }
+
   @Throws(IOException::class, GeneralSecurityException::class)
   fun getEncryptedPrefs(): SharedPreferences {
     val key = MasterKey.Builder(this).setKeyScheme(MasterKey.KeyScheme.AES256_GCM).build()

libtailscale/interfaces.go

@@ -29,6 +29,10 @@ type AppContext interface {
 	// at the given key, or returns empty string if unset.
 	DecryptFromPref(key string) (string, error)
 
+	// GetStateStoreKeysJson retrieves all keys stored in the encrypted SharedPreferences,
+	// strips off the "statestore-" prefix, and returns them as a JSON array.
+	GetStateStoreKeysJSON() string
+
 	// GetOSVersion gets the Android version.
 	GetOSVersion() (string, error)
 

libtailscale/store.go

@@ -5,6 +5,8 @@ package libtailscale
 
 import (
 	"encoding/base64"
+	"encoding/json"
+	"iter"
 
 	"tailscale.com/ipn"
 )
@@ -23,6 +25,28 @@ func newStateStore(appCtx AppContext) *stateStore {
 	}
 }
 
+func (s *stateStore) All() iter.Seq2[ipn.StateKey, []byte] {
+	rawJSON := s.appCtx.GetStateStoreKeysJSON()
+	var keys []string
+	if err := json.Unmarshal([]byte(rawJSON), &keys); err != nil {
+		return func(yield func(ipn.StateKey, []byte) bool) {}
+	}
+	return func(yield func(ipn.StateKey, []byte) bool) {
+		for _, k := range keys {
+			blob, err := s.ReadState(ipn.StateKey(k))
+			if err != nil {
+				continue
+			}
+			if !yield(ipn.StateKey(k), blob) {
+				return
+			}
+		}
+	}
+}
+
+// compile-time assertion that store must implement ipn.StateStore to give immediate feedback on interface drift.
+var _ ipn.StateStore = (*stateStore)(nil)
+
 func prefKeyFor(id ipn.StateKey) string {
 	return "statestore-" + string(id)
 }