3.x: Fix parallel() on grouped flowable not replenishing properly

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableGroupBy.java

@@ -743,19 +743,27 @@ public T poll() {
                 produced++;
                 return v;
             }
+            tryReplenish();
+            return null;
+        }
+
+        @Override
+        public boolean isEmpty() {
+            if (queue.isEmpty()) {
+                tryReplenish();
+                return true;
+            }
+            return false;
+        }
+
+        void tryReplenish() {
             int p = produced;
             if (p != 0) {
                 produced = 0;
                 if ((once.get() & ABANDONED) == 0) {
                     parent.upstream.request(p);
                 }
             }
-            return null;
-        }
-
-        @Override
-        public boolean isEmpty() {
-            return queue.isEmpty();
         }
 
         @Override

src/test/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableGroupByTest.java

@@ -2413,4 +2413,34 @@ public void run() {
             }
         }
     }
+
+    @Test
+    public void fusedParallelGroupProcessing() {
+        Flowable.range(0, 500000)
+        .subscribeOn(Schedulers.single())
+        .groupBy(new Function<Integer, Integer>() {
+            @Override
+            public Integer apply(Integer i) throws Throwable {
+                return i % 2;
+            }
+        })
+        .flatMap(new Function<GroupedFlowable<Integer, Integer>, Publisher<Integer>>() {
+            @Override
+            public Publisher<Integer> apply(GroupedFlowable<Integer, Integer> g) {
+                return g.getKey() == 0
+                    ? g
+                        .parallel()
+                        .runOn(Schedulers.computation())
+                        .map(Functions.<Integer>identity())
+                        .sequential()
+                    : g.map(Functions.<Integer>identity()) // no need to use hide
+                ;
+            }
+        })
+        .test()
+        .awaitDone(20, TimeUnit.SECONDS)
+        .assertValueCount(500000)
+        .assertComplete()
+        .assertNoErrors();
+    }
 }

src/test/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableObserveOnTest.java

@@ -1972,4 +1972,20 @@ public void fusedNoConcurrentCleanDueToCancel() {
             }
         }
     }
+
+    @Test
+    public void fusedParallelProcessing() {
+        Flowable.range(0, 500000)
+        .subscribeOn(Schedulers.single())
+        .observeOn(Schedulers.computation())
+        .parallel()
+        .runOn(Schedulers.computation())
+        .map(Functions.<Integer>identity())
+        .sequential()
+        .test()
+        .awaitDone(20, TimeUnit.SECONDS)
+        .assertValueCount(500000)
+        .assertComplete()
+        .assertNoErrors();
+    }
 }