Cache FID to avoid multiple disk reads. (#1571)

* Cache FID to avoid multiple disk reads.

* Addressing Rayo's comments.

Author: ankitaj224

firebase-installations/src/main/java/com/google/firebase/installations/FirebaseInstallations.java

@@ -67,6 +67,9 @@ public class FirebaseInstallations implements FirebaseInstallationsApi {
   private final Object lock = new Object();
   private final ExecutorService backgroundExecutor;
   private final ExecutorService networkExecutor;
+  /* FID of this Firebase Installations instance. Cached after successfully registering and
+  persisting the FID locally. NOTE: cachedFid resets if FID is deleted.*/
+  private String cachedFid = null;
 
   @GuardedBy("lock")
   private final List<StateListener> listeners = new ArrayList<>();
@@ -284,6 +287,9 @@ private void triggerOnException(PersistedInstallationEntry prefs, Exception exce
   }
 
   private String doGetId() {
+    if (cachedFid != null) {
+      return cachedFid;
+    }
     PersistedInstallationEntry prefs = getPrefsWithGeneratedIdMultiProcessSafe();
     // Execute network calls (CreateInstallations) to the FIS Servers on a separate executor
     // i.e networkExecutor
@@ -337,6 +343,11 @@ private void doNetworkCall(boolean forceRefresh) {
     // Store the prefs to persist the result of the previous step.
     insertOrUpdatePrefs(prefs);
 
+    // Update cachedFID, if FID is successfully REGISTERED and persisted.
+    if (prefs.isRegistered()) {
+      cachedFid = prefs.getFirebaseInstallationId();
+    }
+
     // Let the caller know about the result.
     if (prefs.isErrored()) {
       triggerOnException(prefs, new FirebaseInstallationsException(Status.BAD_CONFIG));
@@ -493,6 +504,7 @@ private PersistedInstallationEntry fetchAuthTokenFromServer(
       case AUTH_ERROR:
         // The the server refused to generate a new auth token due to bad credentials, clear the
         // FID to force the generation of a new one.
+        cachedFid = null;
         return prefs.withNoGeneratedFid();
       default:
         throw new IOException();
@@ -504,6 +516,7 @@ private PersistedInstallationEntry fetchAuthTokenFromServer(
    * storage.
    */
   private Void deleteFirebaseInstallationId() throws FirebaseInstallationsException, IOException {
+    cachedFid = null;
     PersistedInstallationEntry entry = getMultiProcessSafePrefs();
     if (entry.isRegistered()) {
       // Call the FIS servers to delete this Firebase Installation Id.
@@ -519,7 +532,6 @@ private Void deleteFirebaseInstallationId() throws FirebaseInstallationsExceptio
             "Failed to delete a Firebase Installation.", Status.BAD_CONFIG);
       }
     }
-
     insertOrUpdatePrefs(entry.withNoGeneratedFid());
     return null;
   }