[release/5.0] make async name resolution working from impersonificated context

src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs

@@ -464,15 +464,22 @@ private static Task GetHostEntryOrAddressesCoreAsync(string hostName, bool justR
             {
                 ValidateHostName(hostName);
 
+                Task? t;
                 if (NameResolutionTelemetry.Log.IsEnabled())
                 {
-                    return justAddresses
-                        ? (Task)GetAddrInfoWithTelemetryAsync<IPAddress[]>(hostName, justAddresses)
-                        : (Task)GetAddrInfoWithTelemetryAsync<IPHostEntry>(hostName, justAddresses);
+                    t = justAddresses
+                        ? (Task?)GetAddrInfoWithTelemetryAsync<IPAddress[]>(hostName, justAddresses)
+                        : (Task?)GetAddrInfoWithTelemetryAsync<IPHostEntry>(hostName, justAddresses);
                 }
                 else
                 {
-                    return NameResolutionPal.GetAddrInfoAsync(hostName, justAddresses);
+                    t = NameResolutionPal.GetAddrInfoAsync(hostName, justAddresses);
+                }
+
+                // If async resolution started, return task to user. otherwise fall back to sync API on threadpool.
+                if (t != null)
+                {
+                    return t;
                 }
             }
 
@@ -481,20 +488,34 @@ private static Task GetHostEntryOrAddressesCoreAsync(string hostName, bool justR
                 RunAsync(s => GetHostEntryCore((string)s), hostName);
         }
 
-        private static async Task<T> GetAddrInfoWithTelemetryAsync<T>(string hostName, bool justAddresses)
+        private static Task<T>? GetAddrInfoWithTelemetryAsync<T>(string hostName, bool justAddresses)
             where T : class
         {
-            ValueStopwatch stopwatch = NameResolutionTelemetry.Log.BeforeResolution(hostName);
+            ValueStopwatch stopwatch = ValueStopwatch.StartNew();
+            Task? task = NameResolutionPal.GetAddrInfoAsync(hostName, justAddresses);
 
-            T? result = null;
-            try
+            if (task != null)
             {
-                result = await ((Task<T>)NameResolutionPal.GetAddrInfoAsync(hostName, justAddresses)).ConfigureAwait(false);
-                return result;
+                return CompleteAsync(task, hostName, stopwatch);
             }
-            finally
+
+            // If resolution even did not start don't bother with telemetry.
+            // We will retry on thread-pool.
+            return null;
+
+            static async Task<T> CompleteAsync(Task task, string hostName, ValueStopwatch stopwatch)
             {
-                NameResolutionTelemetry.Log.AfterResolution(stopwatch, successful: result is not null);
+                _ = NameResolutionTelemetry.Log.BeforeResolution(hostName);
+                T? result = null;
+                try
+                {
+                    result = await ((Task<T>)task).ConfigureAwait(false);
+                    return result;
+                }
+                finally
+                {
+                    NameResolutionTelemetry.Log.AfterResolution(stopwatch, successful: result is not null);
+                }
             }
         }
 

src/libraries/System.Net.NameResolution/src/System/Net/NameResolutionPal.Windows.cs

@@ -141,7 +141,7 @@ public static unsafe string GetHostName()
             return new string((sbyte*)buffer);
         }
 
-        public static unsafe Task GetAddrInfoAsync(string hostName, bool justAddresses)
+        public static unsafe Task? GetAddrInfoAsync(string hostName, bool justAddresses)
         {
             GetAddrInfoExContext* context = GetAddrInfoExContext.AllocateContext();
 
@@ -166,7 +166,18 @@ public static unsafe Task GetAddrInfoAsync(string hostName, bool justAddresses)
             SocketError errorCode = (SocketError)Interop.Winsock.GetAddrInfoExW(
                 hostName, null, Interop.Winsock.NS_ALL, IntPtr.Zero, &hints, &context->Result, IntPtr.Zero, &context->Overlapped, s_getAddrInfoExCallback, &context->CancelHandle);
 
-            if (errorCode != SocketError.IOPending)
+
+            if (errorCode == SocketError.TryAgain)
+            {
+                // WSATRY_AGAIN indicates possible problem with reachability according to docs.
+                // However, if servers are really unreachable, we would still get IOPending here
+                // and final result would be posted via overlapped IO.
+                // synchronous failure here may signal issue when GetAddrInfoExW does not work from
+                // impersonated context.
+                GetAddrInfoExContext.FreeContext(context);
+                return null;
+            }
+            else if (errorCode != SocketError.IOPending)
             {
                 ProcessResult(errorCode, context);
             }