Throw InvalidOperationException from IHttpUpgradeFeature.UpgradeAsync when request is not upgradable

src/Microsoft.AspNetCore.Server.Kestrel.Core/CoreStrings.resx

@@ -306,4 +306,7 @@
   <data name="RequestProcessingEndError" xml:space="preserve">
     <value>Connection processing ended abnormally.</value>
   </data>
+  <data name="CannotUpgradeNonUpgradableRequest" xml:space="preserve">
+    <value>Cannot upgrade a non-upgradable request. Check IHttpUpgradeFeature.IsUpgradableRequest to determine if a request can be upgraded.</value>
+  </data>
 </root>

src/Microsoft.AspNetCore.Server.Kestrel.Core/Internal/Http/Frame.FeatureCollection.cs

@@ -230,6 +230,11 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http
 
         async Task<Stream> IHttpUpgradeFeature.UpgradeAsync()
         {
+            if (!((IHttpUpgradeFeature)this).IsUpgradableRequest)
+            {
+                throw new InvalidOperationException(CoreStrings.CannotUpgradeNonUpgradableRequest);
+            }
+
             StatusCode = StatusCodes.Status101SwitchingProtocols;
             ReasonPhrase = "Switching Protocols";
             ResponseHeaders["Connection"] = "Upgrade";

src/Microsoft.AspNetCore.Server.Kestrel.Core/Properties/CoreStrings.Designer.cs

@@ -892,6 +892,20 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core
         internal static string FormatRequestProcessingEndError()
             => GetString("RequestProcessingEndError");
 
+        /// <summary>
+        /// Cannot upgrade a non-upgradable request. Check IHttpUpgradeFeature.IsUpgradableRequest to determine if a request can be upgraded.
+        /// </summary>
+        internal static string CannotUpgradeNonUpgradableRequest
+        {
+            get => GetString("CannotUpgradeNonUpgradableRequest");
+        }
+
+        /// <summary>
+        /// Cannot upgrade a non-upgradable request. Check IHttpUpgradeFeature.IsUpgradableRequest to determine if a request can be upgraded.
+        /// </summary>
+        internal static string FormatCannotUpgradeNonUpgradableRequest()
+            => GetString("CannotUpgradeNonUpgradableRequest");
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

test/Microsoft.AspNetCore.Server.Kestrel.FunctionalTests/KeepAliveTimeoutTests.cs

@@ -162,6 +162,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests
                 await connection.Send(
                     "GET /upgrade HTTP/1.1",
                     "Host:",
+                    "Connection: Upgrade",
                     "",
                     "");
                 await connection.Receive(

test/Microsoft.AspNetCore.Server.Kestrel.FunctionalTests/ResponseTests.cs

@@ -1775,6 +1775,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests
                     await connection.Send(
                         "GET / HTTP/1.1",
                         "Host:",
+                        "Connection: Upgrade",
                         "",
                         "");
                     await connection.ReceiveForcedEnd(
@@ -1789,7 +1790,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests
                 {
                     await connection.Send(
                         "GET / HTTP/1.0",
-                        "Connection: keep-alive",
+                        "Connection: keep-alive, Upgrade",
                         "",
                         "");
                     await connection.ReceiveForcedEnd(

test/Microsoft.AspNetCore.Server.Kestrel.FunctionalTests/UpgradeTests.cs

@@ -182,5 +182,41 @@ namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests
                     "");
             }
         }
+
+        [Fact]
+        public async Task ThrowsWhenUpgradingNonUpgradableRequest()
+        {
+            var upgradeTcs = new TaskCompletionSource<bool>();
+            using (var server = new TestServer(async context =>
+             {
+                 var feature = context.Features.Get<IHttpUpgradeFeature>();
+                 Assert.False(feature.IsUpgradableRequest);
+                 try
+                 {
+                     var stream = await feature.UpgradeAsync();
+                 }
+                 catch (Exception e)
+                 {
+                     upgradeTcs.TrySetException(e);
+                 }
+                 finally
+                 {
+                     upgradeTcs.TrySetResult(false);
+                 }
+             }))
+            {
+                using (var connection = server.CreateConnection())
+                {
+                    await connection.Send("GET / HTTP/1.1",
+                            "Host:",
+                            "",
+                            "");
+                    await connection.Receive("HTTP/1.1 200 OK");
+                }
+            }
+
+            var ex = await Assert.ThrowsAsync<InvalidOperationException>(async () => await upgradeTcs.Task).TimeoutAfter(TimeSpan.FromSeconds(15));
+            Assert.Equal(CoreStrings.CannotUpgradeNonUpgradableRequest, ex.Message);
+        }
     }
 }