Attempt to mitigate locked machine.config

Author: tyrielv

GVFS/GVFS.Common/Http/HttpRequestor.cs

@@ -8,6 +8,7 @@
 using System.Net;
 using System.Net.Http;
 using System.Net.Http.Headers;
+using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
@@ -27,9 +28,15 @@ public abstract class HttpRequestor : IDisposable
 
         static HttpRequestor()
         {
-            ServicePointManager.SecurityProtocol = ServicePointManager.SecurityProtocol | SecurityProtocolType.Tls12;
-            ServicePointManager.DefaultConnectionLimit = Environment.ProcessorCount;
-            availableConnections = new SemaphoreSlim(ServicePointManager.DefaultConnectionLimit);
+            /* If machine.config is locked, then initializing ServicePointManager will fail and be unrecoverable.
+             * Machine.config locking is typically very brief (~1ms by the antivirus scanner) so we can attempt to lock
+             * it ourselves (by opening it for read) *beforehand and briefly wait if it's locked */
+            using (var machineConfigLock = GetMachineConfigLock())
+            {
+                ServicePointManager.SecurityProtocol = ServicePointManager.SecurityProtocol | SecurityProtocolType.Tls12;
+                ServicePointManager.DefaultConnectionLimit = Environment.ProcessorCount;
+                availableConnections = new SemaphoreSlim(ServicePointManager.DefaultConnectionLimit);
+            }
         }
 
         protected HttpRequestor(ITracer tracer, RetryConfig retryConfig, Enlistment enlistment)
@@ -329,5 +336,28 @@ private static bool TryGetResponseMessageFromHttpRequestException(HttpRequestExc
             return true;
 
         }
+
+        private static FileStream GetMachineConfigLock()
+        {
+            var machineConfigLocation = RuntimeEnvironment.SystemConfigurationFile;
+            var tries = 0;
+            var maxTries = 3;
+            while (tries++ < maxTries)
+            {
+                try
+                {
+                    /* Opening with FileShare.Read will fail if another process (eg antivirus) has opened the file for write,
+                     but will still let ServicePointManager read the file.*/
+                    FileStream stream = File.Open(machineConfigLocation, FileMode.Open, FileAccess.Read, FileShare.Read);
+                    return stream;
+                }
+                catch (IOException e) when ((uint)e.HResult == 0x80070020) // SHARING_VIOLATION
+                {
+                    Thread.Sleep(10);
+                }
+            }
+            /* Couldn't get the lock - the process will likely fail. */
+            return null;
+        }
     }
 }

GVFS/GitHooksLoader/GitHooksLoader.cpp

@@ -117,6 +117,18 @@ int ExecuteHook(const std::wstring &applicationName, wchar_t *hookName, int argc
     si.dwFlags = STARTF_USESTDHANDLES;
 
     ZeroMemory(&pi, sizeof(pi));
+
+    /* The child process will inherit ErrorMode from this process.
+     * SEM_FAILCRITICALERRORS will prevent the .NET runtime from
+     * creating a dialog box for critical errors - in particular
+     * if antivirus has locked the machine.config file.
+     * Disabling the dialog box lets the child process (typically GVFS.Hooks.exe)
+     * continue trying to run, and if it still needs machine.config then it
+     * can handle the exception at that time (whereas the dialog box would
+     * hang the app until clicked, and is not handleable by our code).
+     */
+    UINT previousErrorMode = SetErrorMode(SEM_FAILCRITICALERRORS);
+
     if (!CreateProcess(
         NULL,           // Application name
         const_cast<LPWSTR>(commandLine.c_str()),
@@ -131,8 +143,10 @@ int ExecuteHook(const std::wstring &applicationName, wchar_t *hookName, int argc
         )
     {
         fwprintf(stderr, L"Could not execute '%s'. CreateProcess error (%d).\n", applicationName.c_str(), GetLastError());
+        SetErrorMode(previousErrorMode);
         exit(3);
     }
+    SetErrorMode(previousErrorMode);
 
     // Wait until child process exits.
     WaitForSingleObject(pi.hProcess, INFINITE);