Remove proxy settings detection behavior in "default proxy mode." (#1188)

* Remove proxy settings detection behavior in "default proxy mode."

This commit partially reverts eb108ad in order to work around a reliability problem with WinHttpGetProxyForUrl. That function hangs unless there is an available thread pool thread to complete the WPAD request. As a result, if a customer issued ~512 concurrent HTTP requests, or otherwise needed that many thread pool threads, there would not be a thread available for WinHTTP to complete the operation, and the program would deadlock.

Moreover this call to WinHttpGetDefaultProxyConfiguration is extremely expensive, taking ~20% of overall CPU for the entire program for some Azure Storage SDK customers.

The function WinHttpGetProxyForUrlEx is supposed to help with this problem by being asynchronous, but that function was added in Windows 8, so we can't use it unconditionally. And on Windows 8.1 we already are using WINHTTP_ACCESS_TYPE_AUTOMATIC_PROXY instead of trying to do proxy autodetect ourselves.

Author: BillyONeal

Release/include/cpprest/details/web_utilities.h

@@ -10,10 +10,6 @@
  ****/
 #pragma once
 
-#ifdef _WIN32
-#include <Windows.h>
-#endif // _WIN32
-
 #include "cpprest/asyncrt_utils.h"
 #include "cpprest/uri.h"
 

Release/src/http/client/http_client_winhttp.cpp

@@ -243,6 +243,24 @@ enum msg_body_type
     transfer_encoding_chunked
 };
 
+static DWORD WinHttpDefaultProxyConstant() CPPREST_NOEXCEPT
+{
+#if _WIN32_WINNT >= _WIN32_WINNT_VISTA
+#if _WIN32_WINNT < _WIN32_WINNT_WINBLUE
+    if (!IsWindows8Point1OrGreater())
+    {
+        // Not Windows 8.1 or later, use the default proxy setting
+        return WINHTTP_ACCESS_TYPE_DEFAULT_PROXY;
+    }
+#endif // _WIN32_WINNT < _WIN32_WINNT_WINBLUE
+
+    // Windows 8.1 or later, use the automatic proxy setting
+    return WINHTTP_ACCESS_TYPE_AUTOMATIC_PROXY;
+#else  // ^^^ _WIN32_WINNT >= _WIN32_WINNT_VISTA ^^^ // vvv _WIN32_WINNT < _WIN32_WINNT_VISTA vvv
+    return WINHTTP_ACCESS_TYPE_DEFAULT_PROXY;
+#endif // _WIN32_WINNT >= _WIN32_WINNT_VISTA
+}
+
 // Additional information necessary to track a WinHTTP request.
 class winhttp_request_context final : public request_context
 {
@@ -818,38 +836,30 @@ class winhttp_client final : public _http_client_communicator
         ie_proxy_config proxyIE;
 
         DWORD access_type;
-        LPCWSTR proxy_name;
+        LPCWSTR proxy_name = WINHTTP_NO_PROXY_NAME;
         LPCWSTR proxy_bypass = WINHTTP_NO_PROXY_BYPASS;
+        m_proxy_auto_config = false;
         utility::string_t proxy_str;
         http::uri uri;
 
         const auto& config = client_config();
-
-        if (config.proxy().is_disabled())
+        const auto& proxy = config.proxy();
+        if (proxy.is_default())
+        {
+            access_type = WinHttpDefaultProxyConstant();
+        }
+        else if (proxy.is_disabled())
         {
             access_type = WINHTTP_ACCESS_TYPE_NO_PROXY;
-            proxy_name = WINHTTP_NO_PROXY_NAME;
         }
-        else if (config.proxy().is_default() || config.proxy().is_auto_discovery())
+        else if (proxy.is_auto_discovery())
         {
-            // Use the default WinHTTP proxy by default.
-            access_type = WINHTTP_ACCESS_TYPE_DEFAULT_PROXY;
-            proxy_name = WINHTTP_NO_PROXY_NAME;
-
-#if _WIN32_WINNT < _WIN32_WINNT_VISTA
-            if (config.proxy().is_auto_discovery())
+            access_type = WinHttpDefaultProxyConstant();
+            if (access_type != WINHTTP_ACCESS_TYPE_AUTOMATIC_PROXY)
             {
+                // Windows 8 or earlier, do proxy autodetection ourselves
                 m_proxy_auto_config = true;
-            }
-#else  // ^^^ _WIN32_WINNT < _WIN32_WINNT_VISTA ^^^ // vvv _WIN32_WINNT >= _WIN32_WINNT_VISTA vvv
-            if (IsWindows8Point1OrGreater())
-            {
-                // Windows 8.1 and newer supports automatic proxy discovery and auto-fallback to IE proxy settings
-                access_type = WINHTTP_ACCESS_TYPE_AUTOMATIC_PROXY;
-            }
-            else
-            {
-                // However, if it is not configured...
+
                 proxy_info proxyDefault;
                 if (!WinHttpGetDefaultProxyConfiguration(&proxyDefault) ||
                     proxyDefault.dwAccessType == WINHTTP_ACCESS_TYPE_NO_PROXY)
@@ -881,13 +891,7 @@ class winhttp_client final : public _http_client_communicator
                         }
                     }
                 }
-
-                if (config.proxy().is_auto_discovery())
-                {
-                    m_proxy_auto_config = true;
-                }
             }
-#endif // _WIN32_WINNT < _WIN32_WINNT_VISTA
         }
         else
         {
@@ -1007,9 +1011,7 @@ class winhttp_client final : public _http_client_communicator
 
         if (m_proxy_auto_config)
         {
-            WINHTTP_AUTOPROXY_OPTIONS autoproxy_options;
-            memset(&autoproxy_options, 0, sizeof(WINHTTP_AUTOPROXY_OPTIONS));
-
+            WINHTTP_AUTOPROXY_OPTIONS autoproxy_options {};
             if (m_proxy_auto_config_url.empty())
             {
                 autoproxy_options.dwFlags = WINHTTP_AUTOPROXY_AUTO_DETECT;