This change ensures that we send back only one 'isComplete=true' message for a given result set (#763)

Fixes:
The streaming protocol is now tightened to ensure that only the last message for a result set contain isCompleted=true. Now isCompleted=true is never sent in isAvailable message.
Tightened logic in sending messages to make sure that no duplicate messages get sent out due to concurrent processing.
Made a fix to a null reference exception when processing special action which was a pre-existing benign bug.
Testing: Added 1 more new test that runs existing tests concurrently 1000 times to make sure no random timing issues are observed and tightened verifications to existing tests to ensure no duplicate messages and only one isComplete=true message is sent across.

* tightening the protocal to ensure only one message (the last one) with completed=true is sent back for a single result set within a query batch

test/Microsoft.SqlTools.ServiceLayer.UnitTests/QueryExecution/Execution/ResultSetTests.cs

@@ -125,6 +125,30 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.QueryExecution.Execution
             //
             VerifyReadResultToEnd(resultSet, resultSummaryFromAvailableCallback, resultSummaryFromCompleteCallback, resultSummariesFromUpdatedCallback);
         }
+
+        /// <summary>
+        /// Read to End test
+        /// </summary>
+        /// <param name="testDataSet"></param>
+        [Theory]
+        [MemberData(nameof(ReadToEndSuccessData), parameters: 3)]
+        public async Task ReadToEndSuccessSeveralTimes(TestResultSet[] testDataSet)
+        {
+            const int NumberOfInvocations = 5000;
+            List<Task> allTasks = new List<Task>();
+            Parallel.ForEach(Partitioner.Create(0, NumberOfInvocations), (range) =>
+            {
+                int start = range.Item1 == 0 ? 1 : range.Item1;
+                Task[] tasks = new Task[range.Item2 - start];
+                for (int i = start; i < range.Item2; i++)
+                {
+                    allTasks.Add(ReadToEndSuccess(testDataSet));
+                }
+
+            });
+            await Task.WhenAll(allTasks);
+        }
+
         public static IEnumerable<object[]> ReadToEndSuccessData(int numTests) => Common.TestResultSetsEnumeration.Select(r => new object[] { new TestResultSet[] { r } }).Take(numTests);
 
         [Theory]
@@ -160,7 +184,11 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.QueryExecution.Execution
             // ... The callback for result set available, update and completion callbacks should have been fired.
             //
             Assert.True(null != resultSummaryFromCompleteCallback, "completeResultSummary is null" + $"\r\n\t\tupdateResultSets: {string.Join("\r\n\t\t\t", resultSummariesFromUpdatedCallback)}");
-            Assert.True(null != resultSummaryFromAvailableCallback, "availableResultSummary is null" + $"\r\n\t\tupdateResultSets: {string.Join("\r\n\t\t\t", resultSummariesFromUpdatedCallback)}");
+            Assert.True(null != resultSummaryFromAvailableCallback, "availableResultSummary is null" + $"\r\n\t\tavailableResultSet: {resultSummaryFromAvailableCallback}");
+
+            // ... resultSetAvailable is not marked Complete
+            //
+            Assert.True(false == resultSummaryFromAvailableCallback.Complete, "availableResultSummary.Complete is true" + $"\r\n\t\tavailableResultSet: {resultSummaryFromAvailableCallback}");
 
             // insert availableResult at the top of the resultSummariesFromUpdatedCallback list as the available result set is the first update in that series.
             //
@@ -175,6 +203,7 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.QueryExecution.Execution
             Assert.True(resultSummariesFromUpdatedCallback.Last().Complete,
                 $"Complete Check failed.\r\n\t\t resultSummariesFromUpdatedCallback:{string.Join("\r\n\t\t\t", resultSummariesFromUpdatedCallback)}");
 
+
             // ... The no of rows in the final updateResultSet/AvailableResultSet should be equal to that in the Complete Result Set. 
             //
             Assert.True(resultSummaryFromCompleteCallback.RowCount == resultSummariesFromUpdatedCallback.Last().RowCount,
@@ -184,7 +213,9 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.QueryExecution.Execution
             );
 
             // ... RowCount should be in increasing order in updateResultSet callbacks
+            // ..... and there should be only one resultSummary with Complete flag set to true.
             //
+            int completeFlagCount = 0;
             Parallel.ForEach(Partitioner.Create(0, resultSummariesFromUpdatedCallback.Count), (range) =>
             {
                 int start = range.Item1 == 0 ? 1 : range.Item1;
@@ -194,8 +225,13 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.QueryExecution.Execution
                         $"Row Count of {i}th updateResultSet was smaller than that of the previous one"
                       + $"\r\n\t\tupdateResultSets: {string.Join("\r\n\t\t\t", resultSummariesFromUpdatedCallback)}"
                     );
+                    if (resultSummariesFromUpdatedCallback[i].Complete)
+                    {
+                        Interlocked.Increment(ref completeFlagCount);
+                    }
                 }
             });
+            Assert.True(completeFlagCount == 1, "Number of update events with complete flag event set should be 1" + $"\r\n\t\tupdateResultSets: {string.Join("\r\n\t\t\t", resultSummariesFromUpdatedCallback)}");
         }
 
         /// <summary>