Optimize proxy PAC file discovery process for SocketsHttpHandler (#29050)

This PR will reduce the calls to WinHTTP GetProxyForUrl() if a previous call recently discovered
no PAC file within a certain timeframe. We call this API when "Automatically detect settings"
is turned on. The default configuration on Windows has this proxy setting always turned on. This
is ironic given that most network environments don't have a PAC file on the network. So, it's
important to try to optimize this common case on customers' networks.

Initial testing shows at least a 2x speed improvement with these changes.

I did not use any locking mechanisms since those would drag down performance. In the worst case,
any race condition would mean we might call GetProxyForUrl() an extra time. But since we're only
filtering on ERROR_WINHTTP_AUTODETECTION_FAILED, it is unlikely to actually change over time.
That particular error means that the WPAD protocol was able to complete on the network, but the
answer discovered is that there is no PAC file registered on the network.

Fixes #28543

Author: davidsh

src/System.Net.Http.WinHttpHandler/src/System/Net/Http/WinInetProxyHelper.cs

@@ -14,6 +14,9 @@ namespace System.Net.Http
     internal class WinInetProxyHelper
     {
         private bool _useProxy = false;
+        private bool _autoDetectionFailed;
+        private int _lastTimeAutoDetectionFailed; // Environment.TickCount units (milliseconds).
+        private const int _recentAutoDetectionInterval = 120_000; // 2 minutes in milliseconds.
 
         public WinInetProxyHelper()
         {
@@ -79,6 +82,10 @@ public bool ManualSettingsOnly
 
         public string ProxyBypass { get; set; }
 
+        public bool RecentAutoDetectionFailure =>
+            _autoDetectionFailed &&
+            Environment.TickCount - _lastTimeAutoDetectionFailed <= _recentAutoDetectionInterval;
+
         public bool GetProxyForUrl(
             SafeWinHttpHandle sessionHandle,
             Uri uri,
@@ -123,6 +130,7 @@ public bool GetProxyForUrl(
             var repeat = false;
             do
             {
+                _autoDetectionFailed = false;
                 if (Interop.WinHttp.WinHttpGetProxyForUrl(
                     sessionHandle,
                     uri.AbsoluteUri,
@@ -131,7 +139,7 @@ public bool GetProxyForUrl(
                 {
                     WinHttpTraceHelper.Trace("WinInetProxyHelper.GetProxyForUrl: Using autoconfig proxy settings");
                     useProxy = true;
-                    
+
                     break;
                 }
                 else
@@ -154,6 +162,12 @@ public bool GetProxyForUrl(
                     }
                     else
                     {
+                        if (lastError == Interop.WinHttp.ERROR_WINHTTP_AUTODETECTION_FAILED)
+                        {
+                            _autoDetectionFailed = true;
+                            _lastTimeAutoDetectionFailed = Environment.TickCount;
+                        }
+
                         break;
                     }
                 }

src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpSystemProxy.cs

@@ -326,20 +326,25 @@ public Uri GetProxy(Uri uri)
                 return (uri.Scheme == UriScheme.Https || uri.Scheme == UriScheme.Wss) ? _secureProxyUri : _insecureProxyUri;
             }
 
-            // For anything else ask WinHTTP.
-            var proxyInfo = new Interop.WinHttp.WINHTTP_PROXY_INFO();
-            try
+            // For anything else ask WinHTTP. To improve performance, we don't call into
+            // WinHTTP if there was a recent failure to detect a PAC file on the network.
+            if (!_proxyHelper.RecentAutoDetectionFailure)
             {
-                if (_proxyHelper.GetProxyForUrl(_sessionHandle, uri, out proxyInfo))
+                var proxyInfo = new Interop.WinHttp.WINHTTP_PROXY_INFO();
+                try
                 {
-                    return GetUriFromString(Marshal.PtrToStringUni(proxyInfo.Proxy));
+                    if (_proxyHelper.GetProxyForUrl(_sessionHandle, uri, out proxyInfo))
+                    {
+                        return GetUriFromString(Marshal.PtrToStringUni(proxyInfo.Proxy));
+                    }
+                }
+                finally
+                {
+                    Marshal.FreeHGlobal(proxyInfo.Proxy);
+                    Marshal.FreeHGlobal(proxyInfo.ProxyBypass);
                 }
             }
-            finally
-            {
-                Marshal.FreeHGlobal(proxyInfo.Proxy);
-                Marshal.FreeHGlobal(proxyInfo.ProxyBypass);
-            }
+
             return null;
         }