pnowojski commented on a change in pull request #18475:
URL: https://github.com/apache/flink/pull/18475#discussion_r794416248



##########
File path: 
flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/io/StreamMultipleInputProcessor.java
##########
@@ -157,4 +186,66 @@ private void fullCheckAndSetAvailable() {
             }
         }
     }
+
+    /** Visible for testing only. Do not use out side 
StreamMultipleInputProcessor. */
+    @VisibleForTesting
+    public static class MultipleInputAvailabilityHelper {
+        private final CompletableFuture<?>[] cachedAvailableFutures;
+        private final Consumer[] onCompletion;
+        private CompletableFuture<?> availableFuture;
+
+        public CompletableFuture<?> getAvailableFuture() {
+            return availableFuture;
+        }
+
+        public static MultipleInputAvailabilityHelper newInstance(int 
inputSize) {
+            MultipleInputAvailabilityHelper obj = new 
MultipleInputAvailabilityHelper(inputSize);
+            return obj;
+        }
+
+        private MultipleInputAvailabilityHelper(int inputSize) {
+            this.cachedAvailableFutures = new CompletableFuture[inputSize];
+            this.onCompletion = new Consumer[inputSize];
+        }
+
+        @VisibleForTesting
+        public void init() {
+            for (int i = 0; i < cachedAvailableFutures.length; i++) {
+                final int inputIdx = i;
+                onCompletion[i] = (Void) -> notifyCompletion(inputIdx);
+            }
+        }
+
+        public boolean isInvalid() {
+            return availableFuture == null || availableFuture.isDone();
+        }
+
+        public void resetToUnavailable() {
+            availableFuture = new CompletableFuture<>();
+        }
+
+        private void notifyCompletion(int idx) {
+            if (availableFuture != null && !availableFuture.isDone()) {
+                availableFuture.complete(null);
+            }
+            cachedAvailableFutures[idx] = AVAILABLE;
+        }

Review comment:
       > The AtomicReference without CAS operations is no difference with plain 
field reference.
   
   All atomic classes are wrappers around `volatile` field + indeed some extra 
atomic CAS operations and such [they provide happens before 
relationship](https://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html)
   




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