Support request streams in classic mode in framework services (#550)

Fixes #549

Author: twsouthwick

src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/RequestExtensions.cs

@@ -0,0 +1,37 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Web;
+
+namespace Microsoft.AspNetCore.SystemWebAdapters;
+
+internal static class RequestExtensions
+{
+    /// <summary>
+    /// Gets the request stream. If it has already been retrieved, it will be in whatever <see cref="HttpRequest.ReadEntityBodyMode"/>
+    /// it was already accessed. Otherwise, it will get a bufferless version via <see cref="HttpRequest.GetBufferlessInputStream()"/>.
+    /// </summary>
+    /// <param name="request">The request</param>
+    /// <returns></returns>
+    public static Stream GetInputStream(this HttpRequestBase request)
+    {
+        // If a call has been made to InputStream, we must use that. It is seekable, so we need to rewind it to the beginning
+        if (request.ReadEntityBodyMode == ReadEntityBodyMode.Classic)
+        {
+            var stream = request.InputStream;
+            stream.Position = 0;
+            return stream;
+        }
+
+        // If a call has been made to GetBufferedInputStream, we must use that. It is seekable, so we need to rewind it to the beginning
+        if (request.ReadEntityBodyMode == ReadEntityBodyMode.Buffered)
+        {
+            var stream = request.GetBufferedInputStream();
+            stream.Position = 0;
+            return stream;
+        }
+
+        // Otherwise, let's attempt to get a bufferless version since we don't need it buffered
+        return request.GetBufferlessInputStream();
+    }
+}

src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/SessionState/RemoteSession/StoreSessionStateHandler.cs

@@ -43,7 +43,7 @@ public async Task ProcessRequestAsync(HttpContextBase context)
         }
         else
         {
-            using var content = context.Request.GetBufferlessInputStream();
+            using var content = context.Request.GetInputStream();
             var result = await _cache.SaveAsync(sessionId, content, context.Response.ClientDisconnectedToken);
 
             if (result is SessionSaveResult.Success)