[Hosts] Handle hidden hosts file (#34308)

* handle hidden hosts file

* don't remove hidden attribute

src/modules/Hosts/Hosts.Tests/HostsServiceTest.cs

@@ -2,6 +2,7 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.IO;
 using System.IO.Abstractions.TestingHelpers;
 using System.Linq;
 using System.Threading.Tasks;
@@ -248,25 +249,53 @@ namespace Hosts.Tests
         {
             var fileSystem = new CustomMockFileSystem();
             var service = new HostsService(fileSystem, _userSettings.Object, _elevationHelper.Object);
-            var hostsFile = new MockFileData(string.Empty);
+
-            hostsFile.Attributes = System.IO.FileAttributes.ReadOnly;
+            var hostsFile = new MockFileData(string.Empty)
+            {
+                Attributes = FileAttributes.ReadOnly,
+            };
+
             fileSystem.AddFile(service.HostsFilePath, hostsFile);
 
             await Assert.ThrowsExceptionAsync<ReadOnlyHostsException>(async () => await service.WriteAsync("# Empty hosts file", Enumerable.Empty<Entry>()));
         }
 
         [TestMethod]
-        public void Remove_ReadOnly()
+        public void Remove_ReadOnly_Attribute()
         {
             var fileSystem = new CustomMockFileSystem();
             var service = new HostsService(fileSystem, _userSettings.Object, _elevationHelper.Object);
-            var hostsFile = new MockFileData(string.Empty);
+
-            hostsFile.Attributes = System.IO.FileAttributes.ReadOnly;
+            var hostsFile = new MockFileData(string.Empty)
+            {
+                Attributes = FileAttributes.ReadOnly,
+            };
+
             fileSystem.AddFile(service.HostsFilePath, hostsFile);
 
-            service.RemoveReadOnly();
+            service.RemoveReadOnlyAttribute();
-            var readOnly = fileSystem.FileInfo.FromFileName(service.HostsFilePath).Attributes.HasFlag(System.IO.FileAttributes.ReadOnly);
+
+            var readOnly = fileSystem.FileInfo.FromFileName(service.HostsFilePath).Attributes.HasFlag(FileAttributes.ReadOnly);
             Assert.IsFalse(readOnly);
         }
+
+        [TestMethod]
+        public async Task Save_Hidden_Hosts()
+        {
+            var fileSystem = new CustomMockFileSystem();
+            var service = new HostsService(fileSystem, _userSettings.Object, _elevationHelper.Object);
+
+            var hostsFile = new MockFileData(string.Empty)
+            {
+                Attributes = FileAttributes.Hidden,
+            };
+
+            fileSystem.AddFile(service.HostsFilePath, hostsFile);
+
+            await service.WriteAsync("# Empty hosts file", Enumerable.Empty<Entry>());
+
+            var hidden = fileSystem.FileInfo.FromFileName(service.HostsFilePath).Attributes.HasFlag(FileAttributes.Hidden);
+            Assert.IsTrue(hidden);
+        }
     }
 }

src/modules/Hosts/HostsUILib/Helpers/HostsService.cs

@@ -23,6 +23,7 @@ namespace HostsUILib.Helpers
     public class HostsService : IHostsService, IDisposable
     {
         private const string _backupSuffix = $"_PowerToysBackup_";
+        private const int _defaultBufferSize = 4096; // From System.IO.File source code
 
         private readonly SemaphoreSlim _asyncLock = new SemaphoreSlim(1, 1);
         private readonly IFileSystem _fileSystem;
@@ -197,7 +198,16 @@ namespace HostsUILib.Helpers
                     _backupDone = true;
                 }
 
-                await _fileSystem.File.WriteAllLinesAsync(HostsFilePath, lines, Encoding);
+                // FileMode.OpenOrCreate is necessary to prevent UnauthorizedAccessException when the hosts file is hidden
+                using var stream = _fileSystem.FileStream.Create(HostsFilePath, FileMode.OpenOrCreate, FileAccess.Write, FileShare.Read, _defaultBufferSize, FileOptions.Asynchronous);
+                using var writer = new StreamWriter(stream, Encoding);
+                foreach (var line in lines)
+                {
+                    await writer.WriteLineAsync(line.AsMemory());
+                }
+
+                stream.SetLength(stream.Position);
+                await writer.FlushAsync();
             }
             finally
             {
@@ -292,7 +302,7 @@ namespace HostsUILib.Helpers
             }
         }
 
-        public void RemoveReadOnly()
+        public void RemoveReadOnlyAttribute()
         {
             var fileInfo = _fileSystem.FileInfo.FromFileName(HostsFilePath);
             if (fileInfo.IsReadOnly)

src/modules/Hosts/HostsUILib/Helpers/IHostsService.cs

@@ -25,6 +25,6 @@ namespace HostsUILib.Helpers
 
         void OpenHostsFile();
 
-        void RemoveReadOnly();
+        void RemoveReadOnlyAttribute();
     }
 }

src/modules/Hosts/HostsUILib/ViewModels/MainViewModel.cs

@@ -283,7 +283,7 @@ namespace HostsUILib.ViewModels
         [RelayCommand]
         public void OverwriteHosts()
         {
-            _hostsService.RemoveReadOnly();
+            _hostsService.RemoveReadOnlyAttribute();
             _ = Task.Run(SaveAsync);
         }