Don't assert that files exists if recovery has been cancled

Today we assert that the tmp files are present but if the recovery
was canceled this might not be the case while still a valid state.
This chance only throws the AssertionError if the recovery is still active.

core/src/main/java/org/elasticsearch/indices/recovery/RecoveryTarget.java

@@ -394,10 +394,23 @@ public class RecoveryTarget extends AbstractRefCounted implements RecoveryTarget
                 indexOutput.close();
             }
             final String temporaryFileName = getTempNameForFile(name);
-            assert Arrays.asList(store.directory().listAll()).contains(temporaryFileName);
+            assert assertTempFileExists(temporaryFileName);
             store.directory().sync(Collections.singleton(temporaryFileName));
             IndexOutput remove = removeOpenIndexOutputs(name);
             assert remove == null || remove == indexOutput; // remove maybe null if we got finished
         }
     }
+
+    private boolean assertTempFileExists(String temporaryFileName) throws IOException {
+        try {
+            assert Arrays.asList(store.directory().listAll()).contains(temporaryFileName) :
+                "expected: [" + temporaryFileName + "] in " + Arrays.toString(store.directory().listAll());
+        } catch (AssertionError error) {
+            if (finished.get() == false) {
+                // if we got canceled stuff might not be here anymore..
+                throw error;
+            }
+        }
+        return true;
+    }
 }