chore: Pull instead of throw exception in groupBy operator. (#1210)

When a sub stream is already closed, but the main stream is still continue, instead of throw a TooManySubStreamOpenException, just drop the current element and continue pulling.

stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowGroupBySpec.scala

@@ -694,6 +694,19 @@ class FlowGroupBySpec extends StreamSpec("""
       queue.complete()
     }
 
+    "should not fail when one sub stream completed" in {
+      Source(1 to 10)
+        .groupBy(2, _ % 2, allowClosedSubstreamRecreation = false)
+        .take(2)
+        .mergeSubstreams
+        .fold(Set.empty[Int])((set, elem) => set + elem)
+        .runWith(TestSink())
+        .ensureSubscription()
+        .request(10)
+        .expectNext(Set(1, 2, 3, 4))
+        .expectComplete()
+    }
+
   }
 
 }

stream/src/main/scala/org/apache/pekko/stream/impl/fusing/StreamOfStreams.scala

@@ -364,11 +364,12 @@ import pekko.util.ccompat.JavaConverters._
               nextElementValue = elem
             }
           } else {
-            if (activeSubstreamsMap.size + closedSubstreams.size == maxSubstreams)
-              throw tooManySubstreamsOpenException
-            else if (closedSubstreams.contains(key) && !hasBeenPulled(in))
+            if (closedSubstreams.contains(key)) {
+              // If the sub stream is already closed, we just skip the current element and pull the next element.
               pull(in)
-            else runSubstream(key, elem)
+            } else if (activeSubstreamsMap.size + closedSubstreams.size == maxSubstreams) {
+              throw tooManySubstreamsOpenException
+            } else runSubstream(key, elem)
           }
         } catch {
           case NonFatal(ex) =>