Fix image not loading issue in peer review (commons-app#2902)

* Fix image loading issue in peer review

* With more test cases

Author: maskaravivek

app/src/main/java/fr/free/nrw/commons/review/ReviewHelper.java

@@ -46,10 +46,10 @@ Single<Media> getRandomMedia() {
                 .flatMap(title -> mediaWikiApi.pageExists("Commons:Deletion_requests/" + title)
                         .map(pageExists -> new Pair<>(title, pageExists)))
                 .map((Pair<String, Boolean> pair) -> {
-                    if (pair.second) {
+                    if (!pair.second) {
                         return new Media(pair.first.replace("File:", ""));
                     }
-                    throw new Exception("Page does not exist");
+                    throw new Exception("Already nominated for deletion");
                 }).retry(MAX_RANDOM_TRIES);
     }
 
@@ -58,7 +58,7 @@ Single<MwQueryPage.Revision> getFirstRevisionOfFile(String fileName) {
     }
 
     @Nullable
-    public String findImageInRecentChanges(List<RecentChange> recentChanges) {
+    private String findImageInRecentChanges(List<RecentChange> recentChanges) {
         String imageTitle;
         Random r = new Random();
         int count = recentChanges.size();

app/src/test/kotlin/fr/free/nrw/commons/review/ReviewHelperTest.kt

@@ -50,12 +50,28 @@ class ReviewHelperTest {
                 .thenReturn(Single.just(listOf(recentChange, recentChange1, recentChange2)))
 
         `when`(mediaWikiApi?.pageExists(ArgumentMatchers.anyString()))
-                .thenReturn(Single.just(true))
+                .thenReturn(Single.just(false))
         val randomMedia = reviewHelper?.randomMedia?.blockingGet()
 
         assertTrue(randomMedia is Media)
     }
 
+    /**
+     * Test scenario when all media is already nominated for deletion
+     */
+    @Test(expected = Exception::class)
+    fun getRandomMediaWithWithAllMediaNominatedForDeletion() {
+        val recentChange = getMockRecentChange("test", "File:Test1.jpeg", 0)
+        val recentChange1 = getMockRecentChange("test", "File:Test2.png", 0)
+        val recentChange2 = getMockRecentChange("test", "File:Test3.jpg", 0)
+        `when`(okHttpJsonApiClient?.recentFileChanges)
+                .thenReturn(Single.just(listOf(recentChange, recentChange1, recentChange2)))
+
+        `when`(mediaWikiApi?.pageExists(ArgumentMatchers.anyString()))
+                .thenReturn(Single.just(true))
+        reviewHelper?.randomMedia?.blockingGet()
+    }
+
     fun getMockRecentChange(type: String, title: String, oldRevisionId: Long): RecentChange {
         val recentChange = mock(RecentChange::class.java)
         `when`(recentChange!!.type).thenReturn(type)