Fix #2345 , split test for non-throwing base enumerable.

Author: Clockwork-Muse

src/System.Linq.Parallel/tests/ExchangeTests.cs

@@ -5,6 +5,7 @@
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using Xunit;
 
 namespace Test
@@ -131,22 +132,26 @@ public static void Merge_Ordered_Longrunning(Labeled<ParallelQuery<int>> labeled
         }
 
         [Theory]
-        [MemberData("MergeData", (object)(new int[] { 16, 1024 }))]
-        [MemberData("ThrowOnCount_AllMergeOptions_MemberData", (object)(new int[] { 16, 1024 }))]
+        [MemberData("ThrowOnCount_AllMergeOptions_MemberData", (object)(new int[] { 4, 8 }))]
         // FailingMergeData has enumerables that throw errors when attempting to perform the nth enumeration.
         // This test checks whether the query runs in a pipelined or buffered fashion.
         public static void Merge_Ordered_Pipelining(Labeled<ParallelQuery<int>> labeled, int count, ParallelMergeOptions options)
         {
-            Assert.Equal(0, labeled.Item.WithMergeOptions(options).Select(x => x).First());
+            Assert.Equal(0, labeled.Item.WithDegreeOfParallelism(count - 1).WithMergeOptions(options).First());
         }
 
         [Theory]
-        [OuterLoop]
-        [MemberData("MergeData", (object)(new int[] { 1024 * 4, 1024 * 1024 }))]
-        [MemberData("ThrowOnCount_AllMergeOptions_MemberData", (object)(new int[] { 1024 * 4, 1024 * 1024 }))]
-        public static void Merge_Ordered_Pipelining_Longrunning(Labeled<ParallelQuery<int>> labeled, int count, ParallelMergeOptions options)
+        [MemberData("MergeData", (object)(new int[] { 4, 8 }))]
+        // This test checks whether the query runs in a pipelined or buffered fashion.
+        public static void Merge_Ordered_Pipelining_Select(Labeled<ParallelQuery<int>> labeled, int count, ParallelMergeOptions options)
         {
-            Merge_Ordered_Pipelining(labeled, count, options);
+            int countdown = count;
+            Func<int, int> down = i =>
+            {
+                if (Interlocked.Decrement(ref countdown) == 0) throw new DeliberateTestException();
+                return i;
+            };
+            Assert.Equal(0, labeled.Item.WithDegreeOfParallelism(count - 1).WithMergeOptions(options).Select(down).First());
         }
 
         [Theory]