Allow file watcher to actively poll for changes

The ChangeToken.OnChange pattern that's commonly used by providers to listen to
changes requires IChangeToken to be active. The only two instances in the framework
that does not support are the PollingChangeToken.

This change makes the polling be active unless configured otherwise.

Fixes aspnet/Mvc#8173

src/FS.Physical/IPollingChangeToken.cs

@@ -0,0 +1,13 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading;
+using Microsoft.Extensions.Primitives;
+
+namespace Microsoft.Extensions.FileProviders
+{
+    internal interface IPollingChangeToken : IChangeToken
+    {
+        CancellationTokenSource CancellationTokenSource { get; }
+    }
+}

src/FS.Physical/PhysicalFileProvider.cs

@@ -2,7 +2,9 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Diagnostics;
 using System.IO;
+using System.Threading;
 using Microsoft.Extensions.FileProviders.Internal;
 using Microsoft.Extensions.FileProviders.Physical;
 using Microsoft.Extensions.FileProviders.Physical.Internal;
@@ -20,19 +22,25 @@ namespace Microsoft.Extensions.FileProviders
     public class PhysicalFileProvider : IFileProvider, IDisposable
     {
         private const string PollingEnvironmentKey = "DOTNET_USE_POLLING_FILE_WATCHER";
-
         private static readonly char[] _pathSeparators = new[]
             {Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar};
 
-        private readonly PhysicalFilesWatcher _filesWatcher;
         private readonly ExclusionFilters _filters;
 
+        private readonly Func<PhysicalFilesWatcher> _fileWatcherFactory;
+        private PhysicalFilesWatcher _fileWatcher;
+        private bool _fileWatcherInitialized;
+        private object _fileWatcherLock = new object();
+
+        private bool? _usePollingFileWatcher;
+        private bool? _useActivePolling;
+
         /// <summary>
         /// Initializes a new instance of a PhysicalFileProvider at the given root directory.
         /// </summary>
         /// <param name="root">The root directory. This should be an absolute path.</param>
         public PhysicalFileProvider(string root)
-            : this(root, CreateFileWatcher(root, ExclusionFilters.Sensitive), ExclusionFilters.Sensitive)
+            : this(root, ExclusionFilters.Sensitive)
         {
         }
 
@@ -42,20 +50,12 @@ public PhysicalFileProvider(string root)
         /// <param name="root">The root directory. This should be an absolute path.</param>
         /// <param name="filters">Specifies which files or directories are excluded.</param>
         public PhysicalFileProvider(string root, ExclusionFilters filters)
-            : this(root, CreateFileWatcher(root, filters), filters)
-        { }
-
-        // for testing
-        internal PhysicalFileProvider(string root, PhysicalFilesWatcher physicalFilesWatcher)
-            : this(root, physicalFilesWatcher, ExclusionFilters.Sensitive)
-        { }
-
-        private PhysicalFileProvider(string root, PhysicalFilesWatcher physicalFilesWatcher, ExclusionFilters filters)
         {
             if (!Path.IsPathRooted(root))
             {
                 throw new ArgumentException("The path must be absolute.", nameof(root));
             }
+
             var fullRoot = Path.GetFullPath(root);
             // When we do matches in GetFullPath, we want to only match full directory names.
             Root = PathUtils.EnsureTrailingSlash(fullRoot);
@@ -64,26 +64,115 @@ private PhysicalFileProvider(string root, PhysicalFilesWatcher physicalFilesWatc
                 throw new DirectoryNotFoundException(Root);
             }
 
-            _filesWatcher = physicalFilesWatcher;
             _filters = filters;
+            _fileWatcherFactory = () => CreateFileWatcher();
+        }
+
+        /// <summary>
+        /// Gets or sets a value that determines if this instance of <see cref="PhysicalFileProvider"/>
+        /// uses polling to determine file changes.
+        /// <para>
+        /// By default, <see cref="PhysicalFileProvider"/>  uses <see cref="FileSystemWatcher"/> to listen to file change events
+        /// for <see cref="Watch(string)"/>. <see cref="FileSystemWatcher"/> is ineffective in some scenarios such as mounted drives.
+        /// Polling is required to effectively watch for file changes.
+        /// </para>
+        /// <seealso cref="UseActivePolling"/>.
+        /// </summary>
+        /// <value>
+        /// Defaults to <see langword="true"/>, if the environment variable <c>DOTNET_USE_POLLING_FILE_WATCHER</c> is <c>true</c> or <c>1</c>.
+        /// </value>
+        public bool UsePollingFileWatcher
+        {
+            get
+            {
+                if (_fileWatcher != null)
+                {
+                    throw new InvalidOperationException($"Cannot modify {nameof(UsePollingFileWatcher)} once file watcher has been initialized.");
+                }
+
+                if (_usePollingFileWatcher == null)
+                {
+                    ReadPollingEnvironmentVariables();
+                }
+
+                return _usePollingFileWatcher.Value;
+            }
+            set => _usePollingFileWatcher = value;
+        }
+
+        /// <summary>
+        /// Gets or sets a value that determines if this instance of <see cref="PhysicalFileProvider"/>
+        /// actively polls for file changes.
+        /// <para>
+        /// When <see langword="true"/>, <see cref="IChangeToken"/> returned by <see cref="Watch(string)"/> will actively poll for file changes
+        /// (<see cref="IChangeToken.ActiveChangeCallbacks"/> will be <see langword="true"/>) instead of being passive.
+        /// </para>
+        /// <para>
+        /// This property is only effective when <see cref="UsePollingFileWatcher"/> is set.
+        /// </para>
+        /// </summary>
+        /// <value>
+        /// Defaults to <see langword="true"/>, if the environment variable <c>DOTNET_USE_POLLING_FILE_WATCHER</c> is <c>true</c> or <c>1</c>.
+        /// </value>
+        public bool UseActivePolling
+        {
+            get
+            {
+                if (_useActivePolling == null)
+                {
+                    ReadPollingEnvironmentVariables();
+                }
+
+                return _useActivePolling.Value;
+            }
+
+            set => _useActivePolling = value;
+        }
+
+        internal PhysicalFilesWatcher FileWatcher
+        {
+            get
+            {
+                return LazyInitializer.EnsureInitialized(
+                    ref _fileWatcher,
+                    ref _fileWatcherInitialized,
+                    ref _fileWatcherLock,
+                    _fileWatcherFactory);
+            }
+            set
+            {
+                Debug.Assert(!_fileWatcherInitialized);
+
+                _fileWatcherInitialized = true;
+                _fileWatcher = value;
+            }
+        }
+
+        internal PhysicalFilesWatcher CreateFileWatcher()
+        {
+            var root = PathUtils.EnsureTrailingSlash(Path.GetFullPath(Root));
+            return new PhysicalFilesWatcher(root, new FileSystemWatcher(root), UsePollingFileWatcher, _filters)
+            {
+                UseActivePolling = UseActivePolling,
+            };
         }
 
-        private static PhysicalFilesWatcher CreateFileWatcher(string root, ExclusionFilters filters)
+        private void ReadPollingEnvironmentVariables()
         {
             var environmentValue = Environment.GetEnvironmentVariable(PollingEnvironmentKey);
             var pollForChanges = string.Equals(environmentValue, "1", StringComparison.Ordinal) ||
-                                 string.Equals(environmentValue, "true", StringComparison.OrdinalIgnoreCase);
+                string.Equals(environmentValue, "true", StringComparison.OrdinalIgnoreCase);
 
-            root = PathUtils.EnsureTrailingSlash(Path.GetFullPath(root));
-            return new PhysicalFilesWatcher(root, new FileSystemWatcher(root), pollForChanges, filters);
+            _usePollingFileWatcher = pollForChanges;
+            _useActivePolling = pollForChanges;
         }
 
         /// <summary>
         /// Disposes the provider. Change tokens may not trigger after the provider is disposed.
         /// </summary>
         public void Dispose()
         {
-            _filesWatcher.Dispose();
+            _fileWatcher?.Dispose();
         }
 
         /// <summary>
@@ -225,7 +314,7 @@ public IChangeToken Watch(string filter)
             // Relative paths starting with leading slashes are okay
             filter = filter.TrimStart(_pathSeparators);
 
-            return _filesWatcher.CreateFileChangeToken(filter);
+            return FileWatcher.CreateFileChangeToken(filter);
         }
     }
 }