[Hosts] Backup Settings (#37778)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request

Add backup settings for the Hosts File Editor to allow users to
customize the existing hardcoded logic.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] **Closes:** #37666
- [ ] **Communication:** I've discussed this with core contributors
already. If work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end user facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [x] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here:
https://github.com/MicrosoftDocs/windows-dev-docs/pull/5342

<!-- Provide a more detailed description of the PR, other things fixed
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<img width="707" alt="image"
src="https://github.com/user-attachments/assets/e114431e-60e0-4b8c-bba7-df23f7af0182"
/>

<img width="707" alt="image"
src="https://github.com/user-attachments/assets/a02b591e-eb46-4964-bee7-548ec175b3aa"
/>

<img width="707" alt="image"
src="https://github.com/user-attachments/assets/6eb0ff21-74fa-4229-8832-df2df877b5cd"
/>

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

- Backup on: verified that backup isn't executed
- Backups off: Verified that only one backup is executed
- Verified that backup is located in the expected path
- Auto delete is set to "never": verified that no backups are deleted
- Auto delete is set to "based on count": verified that backups are
deleted according to count value
- Auto delete is set to "based on age and count": verified that backups
are deleted according to days and count values
- Verified that files without the backup pattern aren't deleted
- There is also adequate test coverage for these scenarios 🚀

---------

Co-authored-by: Gordon Lam (SH) <yeelam@microsoft.com>

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

@@ -0,0 +1,112 @@
+// Copyright (c) Microsoft Corporation
+// 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;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO.Abstractions;
+using System.Linq;
+using HostsUILib.Settings;
+
+namespace HostsUILib.Helpers
+{
+    public class BackupManager : IBackupManager
+    {
+        private const string BackupSuffix = "_PowerToysBackup_";
+        private readonly IFileSystem _fileSystem;
+        private readonly IUserSettings _userSettings;
+        private bool _backupDone;
+
+        public BackupManager(IFileSystem fileSystem, IUserSettings userSettings)
+        {
+            _fileSystem = fileSystem;
+            _userSettings = userSettings;
+        }
+
+        public void Create(string hostsFilePath)
+        {
+            if (_backupDone || !_userSettings.BackupHosts || !_fileSystem.File.Exists(hostsFilePath))
+            {
+                return;
+            }
+
+            try
+            {
+                if (!_fileSystem.Directory.Exists(_userSettings.BackupPath))
+                {
+                    _fileSystem.Directory.CreateDirectory(_userSettings.BackupPath);
+                }
+
+                var backupPath = _fileSystem.Path.Combine(_userSettings.BackupPath, $"hosts{BackupSuffix}{DateTime.Now.ToString("yyyyMMddHHmmss", CultureInfo.InvariantCulture)}");
+
+                _fileSystem.File.Copy(hostsFilePath, backupPath);
+                _backupDone = true;
+            }
+            catch (Exception ex)
+            {
+                LoggerInstance.Logger.LogError("Backup failed", ex);
+            }
+        }
+
+        public void Delete()
+        {
+            switch (_userSettings.DeleteBackupsMode)
+            {
+                case HostsDeleteBackupMode.Count:
+                    DeleteByCount(_userSettings.DeleteBackupsCount);
+                    break;
+                case HostsDeleteBackupMode.Age:
+                    DeleteByAge(_userSettings.DeleteBackupsDays, _userSettings.DeleteBackupsCount);
+                    break;
+            }
+        }
+
+        public void DeleteByCount(int count)
+        {
+            if (count < 1)
+            {
+                return;
+            }
+
+            var backups = GetAll().OrderByDescending(f => f.CreationTime).Skip(count).ToArray();
+            DeleteAll(backups);
+        }
+
+        public void DeleteByAge(int days, int count)
+        {
+            if (days < 1)
+            {
+                return;
+            }
+
+            var backupsEnumerable = GetAll();
+
+            if (count > 0)
+            {
+                backupsEnumerable = backupsEnumerable.OrderByDescending(f => f.CreationTime).Skip(count);
+            }
+
+            var backups = backupsEnumerable.Where(f => f.CreationTime < DateTime.Now.AddDays(-days)).ToArray();
+            DeleteAll(backups);
+        }
+
+        private IEnumerable<IFileInfo> GetAll()
+        {
+            if (!_fileSystem.Directory.Exists(_userSettings.BackupPath))
+            {
+                return [];
+            }
+
+            return _fileSystem.Directory.GetFiles(_userSettings.BackupPath, $"*{BackupSuffix}*").Select(_fileSystem.FileInfo.New);
+        }
+
+        private void DeleteAll(IFileInfo[] files)
+        {
+            foreach (var f in files)
+            {
+                _fileSystem.File.Delete(f.FullName);
+            }
+        }
+    }
+}

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

@@ -5,7 +5,6 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Globalization;
 using System.IO;
 using System.IO.Abstractions;
 using System.Linq;
@@ -23,16 +22,15 @@ namespace HostsUILib.Helpers
 {
     public partial class HostsService : IHostsService, IDisposable
     {
-        private const string _backupSuffix = $"_PowerToysBackup_";
-        private const int _defaultBufferSize = 4096; // From System.IO.File source code
+        private const int DefaultBufferSize = 4096; // From System.IO.File source code
 
         private readonly SemaphoreSlim _asyncLock = new SemaphoreSlim(1, 1);
         private readonly IFileSystem _fileSystem;
         private readonly IUserSettings _userSettings;
         private readonly IElevationHelper _elevationHelper;
         private readonly IFileSystemWatcher _fileSystemWatcher;
+        private readonly IBackupManager _backupManager;
         private readonly string _hostsFilePath;
-        private bool _backupDone;
         private bool _disposed;
 
         public string HostsFilePath => _hostsFilePath;
@@ -44,11 +42,13 @@ namespace HostsUILib.Helpers
         public HostsService(
             IFileSystem fileSystem,
             IUserSettings userSettings,
-            IElevationHelper elevationHelper)
+            IElevationHelper elevationHelper,
+            IBackupManager backupManager)
         {
             _fileSystem = fileSystem;
             _userSettings = userSettings;
             _elevationHelper = elevationHelper;
+            _backupManager = backupManager;
 
             _hostsFilePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Windows), @"System32\drivers\etc\hosts");
 
@@ -60,18 +60,13 @@ namespace HostsUILib.Helpers
             _fileSystemWatcher.EnableRaisingEvents = true;
         }
 
-        public bool Exists()
-        {
-            return _fileSystem.File.Exists(HostsFilePath);
-        }
-
         public async Task<HostsData> ReadAsync()
         {
             var entries = new List<Entry>();
             var unparsedBuilder = new StringBuilder();
             var splittedEntries = false;
 
-            if (!Exists())
+            if (!_fileSystem.File.Exists(HostsFilePath))
             {
                 return new HostsData(entries, unparsedBuilder.ToString(), false);
             }
@@ -192,15 +187,10 @@ namespace HostsUILib.Helpers
             {
                 await _asyncLock.WaitAsync();
                 _fileSystemWatcher.EnableRaisingEvents = false;
-
-                if (!_backupDone && Exists())
-                {
-                    _fileSystem.File.Copy(HostsFilePath, HostsFilePath + _backupSuffix + DateTime.Now.ToString("yyyyMMddHHmmss", CultureInfo.InvariantCulture));
-                    _backupDone = true;
-                }
+                _backupManager.Create(HostsFilePath);
 
                 // FileMode.OpenOrCreate is necessary to prevent UnauthorizedAccessException when the hosts file is hidden
-                using var stream = _fileSystem.FileStream.New(HostsFilePath, FileMode.OpenOrCreate, FileAccess.Write, FileShare.Read, _defaultBufferSize, FileOptions.Asynchronous);
+                using var stream = _fileSystem.FileStream.New(HostsFilePath, FileMode.OpenOrCreate, FileAccess.Write, FileShare.Read, DefaultBufferSize, FileOptions.Asynchronous);
                 using var writer = new StreamWriter(stream, Encoding);
                 foreach (var line in lines)
                 {
@@ -231,15 +221,6 @@ namespace HostsUILib.Helpers
             }
         }
 
-        public void CleanupBackup()
-        {
-            Directory.GetFiles(Path.GetDirectoryName(HostsFilePath), $"*{_backupSuffix}*")
-                .Select(f => new FileInfo(f))
-                .Where(f => f.CreationTime < DateTime.Now.AddDays(-15))
-                .ToList()
-                .ForEach(f => f.Delete());
-        }
-
         public void OpenHostsFile()
         {
             var notepadFallback = false;

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

@@ -0,0 +1,13 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace HostsUILib.Helpers
+{
+    public interface IBackupManager
+    {
+        void Create(string hostsFilePath);
+
+        void Delete();
+    }
+}

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

@@ -22,8 +22,6 @@ namespace HostsUILib.Helpers
 
         Task<bool> PingAsync(string address);
 
-        void CleanupBackup();
-
         void OpenHostsFile();
 
         void RemoveReadOnlyAttribute();

src/modules/Hosts/HostsUILib/Settings/HostsDeleteBackupMode.cs

@@ -0,0 +1,13 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace HostsUILib.Settings
+{
+    public enum HostsDeleteBackupMode
+    {
+        Never = 0,
+        Count = 1,
+        Age = 2,
+    }
+}

src/modules/Hosts/HostsUILib/Settings/IUserSettings.cs

@@ -16,6 +16,16 @@ namespace HostsUILib.Settings
 
         public HostsEncoding Encoding { get; }
 
+        public bool BackupHosts { get; }
+
+        public string BackupPath { get; }
+
+        public HostsDeleteBackupMode DeleteBackupsMode { get; }
+
+        public int DeleteBackupsDays { get; }
+
+        public int DeleteBackupsCount { get; }
+
         event EventHandler LoopbackDuplicatesChanged;
 
         public delegate void OpenSettingsFunction();