Return last block from SocketInput when data is fully consumed

- This reduces Kestrel's memory usage for idle connections.

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/SocketInput.cs

@@ -52,6 +52,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
         }
 
         public MemoryPoolBlock IncomingStart()
+        {
+            lock (_sync)
             {
                 const int minimumSize = 2048;
 
@@ -66,6 +68,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
 
                 return _pinned;
             }
+        }
 
         public void IncomingComplete(int count, Exception error)
         {
@@ -112,6 +115,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
         {
             Debug.Assert(_pinned != null);
 
+            lock (_sync)
+            {
                 if (_pinned != null)
                 {
                     if (_pinned != _tail)
@@ -122,6 +127,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                     _pinned = null;
                 }
             }
+        }
 
         private void Complete()
         {
@@ -172,18 +178,31 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                         }
 
                         returnStart = _head;
+
+                        var consumedAll = !consumed.IsDefault && consumed.IsEnd;
+                        if (consumedAll && _pinned != _tail)
+                        {
+                            // Everything has been consumed and no data is being written to the
+                            // _tail block, so return all blocks between _head and _tail inclusive.
+                            returnEnd = null;
+                            _head = null;
+                            _tail = null;
+                        }
+                        else
+                        {
                             returnEnd = consumed.Block;
                             _head = consumed.Block;
                             _head.Start = consumed.Index;
+                        }
 
                         // Must call Subtract() after _head has been advanced, to avoid producer starting too early and growing
                         // buffer beyond max length.
                         _bufferSizeControl?.Subtract(lengthConsumed);
                     }
 
-                    if (!examined.IsDefault &&
+                    // If _head is null, everything has been consumed and examined.
-                        examined.IsEnd &&
+                    var examinedAll = (!examined.IsDefault && examined.IsEnd) || _head == null;
-                        ReadingInput)
+                    if (examinedAll && ReadingInput)
                     {
                         _manualResetEvent.Reset();