[Appcheck] Use default message if getToken exception doesn't include one (#7640)

This addresses issue
#7624.
Previously, missing error messages in custom providers would the Task to
fail with an null argument exception. The app check contract states that
the task **will** succeed. In the case of error, the
`AppCheckTokenResult` object returned will include the error
information.

Author: rlazo

appcheck/firebase-appcheck/CHANGELOG.md

@@ -1,5 +1,8 @@
 # Unreleased
 
+- [fixed] Fixed a bug causing custom `AppCheckProvider` returning errors without a message to throw an exception.
+  [#7624](//github.com/firebase/firebase-android-sdk/pull/7624)
+
 # 19.0.1
 
 - [changed] Bumped internal dependencies.

appcheck/firebase-appcheck/src/main/java/com/google/firebase/appcheck/internal/DefaultFirebaseAppCheck.java

@@ -211,7 +211,9 @@ public Task<AppCheckTokenResult> getToken(boolean forceRefresh) {
                 return Tasks.forResult(
                     DefaultAppCheckTokenResult.constructFromError(
                         new FirebaseException(
-                            appCheckTokenTask.getException().getMessage(),
+                            getErrorMessageOrDefault(
+                                appCheckTokenTask.getException(),
+                                "Unknown error generating App Check token"),
                             appCheckTokenTask.getException())));
               });
         });
@@ -234,7 +236,9 @@ public Task<AppCheckTokenResult> getLimitedUseToken() {
               return Tasks.forResult(
                   DefaultAppCheckTokenResult.constructFromError(
                       new FirebaseException(
-                          appCheckTokenTask.getException().getMessage(),
+                          getErrorMessageOrDefault(
+                              appCheckTokenTask.getException(),
+                              "Unknown error generating limited use App Check token"),
                           appCheckTokenTask.getException())));
             });
   }
@@ -319,4 +323,8 @@ private boolean hasValidToken() {
     return cachedToken != null
         && cachedToken.getExpireTimeMillis() - clock.currentTimeMillis() > BUFFER_TIME_MILLIS;
   }
+
+  private String getErrorMessageOrDefault(@NonNull Exception e, String defaultMessage) {
+    return e.getMessage() != null ? e.getMessage() : defaultMessage;
+  }
 }

appcheck/firebase-appcheck/src/test/java/com/google/firebase/appcheck/internal/DefaultFirebaseAppCheckTest.java

@@ -450,4 +450,36 @@ public void testGetLimitedUseToken_existingToken_requestsNewToken() {
 
     verify(mockAppCheckProvider).getToken();
   }
+
+  @Test
+  public void getToken_providerThrowsExceptionWithNullMessage_returnsResultWithError()
+      throws Exception {
+    defaultFirebaseAppCheck.installAppCheckProviderFactory(mockAppCheckProviderFactory);
+    when(mockAppCheckProvider.getToken())
+        .thenReturn(Tasks.forException(new Exception((String) null)));
+    when(mockAppCheckProvider.getToken())
+        .thenReturn(Tasks.forException(new Exception((String) null)));
+
+    Task<AppCheckTokenResult> tokenTask =
+        defaultFirebaseAppCheck.getToken(/* forceRefresh= */ false);
+
+    assertThat(tokenTask.isComplete()).isTrue();
+    assertThat(tokenTask.isSuccessful()).isTrue();
+    assertThat(tokenTask.getResult().getError()).isNotNull();
+  }
+
+  @Test
+  public void getLimitedUseToken_providerThrowsExceptionWithNullMessage_returnsResultWithError()
+      throws Exception {
+    defaultFirebaseAppCheck.installAppCheckProviderFactory(mockAppCheckProviderFactory);
+    when(mockAppCheckProvider.getToken())
+        .thenReturn(Tasks.forException(new Exception((String) null)));
+    when(mockAppCheckProvider.getToken())
+        .thenReturn(Tasks.forException(new Exception((String) null)));
+
+    Task<AppCheckTokenResult> tokenTask = defaultFirebaseAppCheck.getLimitedUseToken();
+    assertThat(tokenTask.isComplete()).isTrue();
+    assertThat(tokenTask.isSuccessful()).isTrue();
+    assertThat(tokenTask.getResult().getError()).isNotNull();
+  }
 }