Allow for inline write completion

Author: benaadams

src/Microsoft.AspNet.Server.Kestrel/Http/SocketOutput.cs

@@ -351,7 +351,7 @@ private void OnWriteCompleted(WriteContext writeContext)
 
             if (scheduleWrite)
             {
-                WriteAllPending();
+                ScheduleWrite();
             }
 
             _tasksCompleted.Clear();
@@ -399,7 +399,7 @@ private void PoolWriteContext(WriteContext writeContext)
             }
         }
 
-        public void Dispose()
+        private void Dispose()
         {
             lock (_contextLock)
             {
@@ -471,6 +471,11 @@ public void DoWriteIfNeeded()
                     return;
                 }
 
+                // Sample values locally in case write completes inline
+                // to allow block to be Reset and still complete this function
+                var lockedEndBlock = _lockedEnd.Block;
+                var lockedEndIndex = _lockedEnd.Index;
+
                 _writeReq.Write(Self._socket, _lockedStart, _lockedEnd, _bufferCount, (_writeReq, status, error, state) =>
                 {
                     var _this = (WriteContext)state;
@@ -480,12 +485,8 @@ public void DoWriteIfNeeded()
                     _this.DoShutdownIfNeeded();
                 }, this);
 
-                Self._head = _lockedEnd.Block;
-                if (Self._head != null)
-                {
-                    // Avoid shutdown race
-                    Self._head.Start = _lockedEnd.Index;
-                }
+                Self._head = lockedEndBlock;
+                Self._head.Start = lockedEndIndex;
             }
 
             /// <summary>