Make PhysicalFilesWatcher exclusively use polling when pollForChanges=true

src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFileProvider.cs

@@ -159,7 +159,10 @@ internal PhysicalFilesWatcher FileWatcher
         internal PhysicalFilesWatcher CreateFileWatcher()
         {
             string root = PathUtils.EnsureTrailingSlash(Path.GetFullPath(Root));
-            return new PhysicalFilesWatcher(root, new FileSystemWatcher(root), UsePollingFileWatcher, _filters)
+
+            // When both UsePollingFileWatcher & UseActivePolling are set, we won't use a FileSystemWatcher.
+            FileSystemWatcher watcher = UsePollingFileWatcher && UseActivePolling ? null : new FileSystemWatcher(root);
+            return new PhysicalFilesWatcher(root, watcher, UsePollingFileWatcher, _filters)
             {
                 UseActivePolling = UseActivePolling,
             };

src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFilesWatcher.cs

@@ -79,14 +79,23 @@ public PhysicalFilesWatcher(
             bool pollForChanges,
             ExclusionFilters filters)
         {
+            if (fileSystemWatcher == null && !pollForChanges)
+            {
+                throw new ArgumentNullException(nameof(fileSystemWatcher), SR.Error_FileSystemWatcherRequiredWithoutPolling);
+            }
+
             _root = root;
-            _fileWatcher = fileSystemWatcher;
-            _fileWatcher.IncludeSubdirectories = true;
-            _fileWatcher.Created += OnChanged;
-            _fileWatcher.Changed += OnChanged;
-            _fileWatcher.Renamed += OnRenamed;
-            _fileWatcher.Deleted += OnChanged;
-            _fileWatcher.Error += OnError;
+
+            if (fileSystemWatcher != null)
+            {
+                _fileWatcher = fileSystemWatcher;
+                _fileWatcher.IncludeSubdirectories = true;
+                _fileWatcher.Created += OnChanged;
+                _fileWatcher.Changed += OnChanged;
+                _fileWatcher.Renamed += OnRenamed;
+                _fileWatcher.Deleted += OnChanged;
+                _fileWatcher.Error += OnError;
+            }
 
             PollForChanges = pollForChanges;
             _filters = filters;
@@ -361,27 +370,33 @@ private void ReportChangeForMatchedEntries(string path)
 
         private void TryDisableFileSystemWatcher()
         {
-            lock (_fileWatcherLock)
+            if (_fileWatcher != null)
             {
-                if (_filePathTokenLookup.IsEmpty &&
-                    _wildcardTokenLookup.IsEmpty &&
-                    _fileWatcher.EnableRaisingEvents)
+                lock (_fileWatcherLock)
                 {
-                    // Perf: Turn off the file monitoring if no files to monitor.
-                    _fileWatcher.EnableRaisingEvents = false;
+                    if (_filePathTokenLookup.IsEmpty &&
+                        _wildcardTokenLookup.IsEmpty &&
+                        _fileWatcher.EnableRaisingEvents)
+                    {
+                        // Perf: Turn off the file monitoring if no files to monitor.
+                        _fileWatcher.EnableRaisingEvents = false;
+                    }
                 }
             }
         }
 
         private void TryEnableFileSystemWatcher()
         {
-            lock (_fileWatcherLock)
+            if (_fileWatcher != null)
             {
-                if ((!_filePathTokenLookup.IsEmpty || !_wildcardTokenLookup.IsEmpty) &&
-                    !_fileWatcher.EnableRaisingEvents)
+                lock (_fileWatcherLock)
                 {
-                    // Perf: Turn off the file monitoring if no files to monitor.
-                    _fileWatcher.EnableRaisingEvents = true;
+                    if ((!_filePathTokenLookup.IsEmpty || !_wildcardTokenLookup.IsEmpty) &&
+                        !_fileWatcher.EnableRaisingEvents)
+                    {
+                        // Perf: Turn off the file monitoring if no files to monitor.
+                        _fileWatcher.EnableRaisingEvents = true;
+                    }
                 }
             }
         }

src/libraries/Microsoft.Extensions.FileProviders.Physical/src/Resources/Strings.resx

@@ -0,0 +1,123 @@
+<?xml version="1.0" encoding="utf-8"?>
+<root>
+  <!-- 
+    Microsoft ResX Schema 
+
+    Version 2.0
+
+    The primary goals of this format is to allow a simple XML format 
+    that is mostly human readable. The generation and parsing of the 
+    various data types are done through the TypeConverter classes 
+    associated with the data types.
+
+    Example:
+
+    ... ado.net/XML headers & schema ...
+    <resheader name="resmimetype">text/microsoft-resx</resheader>
+    <resheader name="version">2.0</resheader>
+    <resheader name="reader">System.Resources.ResXResourceReader, System.Windows.Forms, ...</resheader>
+    <resheader name="writer">System.Resources.ResXResourceWriter, System.Windows.Forms, ...</resheader>
+    <data name="Name1"><value>this is my long string</value><comment>this is a comment</comment></data>
+    <data name="Color1" type="System.Drawing.Color, System.Drawing">Blue</data>
+    <data name="Bitmap1" mimetype="application/x-microsoft.net.object.binary.base64">
+        <value>[base64 mime encoded serialized .NET Framework object]</value>
+    </data>
+    <data name="Icon1" type="System.Drawing.Icon, System.Drawing" mimetype="application/x-microsoft.net.object.bytearray.base64">
+        <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
+        <comment>This is a comment</comment>
+    </data>
+
+    There are any number of "resheader" rows that contain simple 
+    name/value pairs.
+
+    Each data row contains a name, and value. The row also contains a 
+    type or mimetype. Type corresponds to a .NET class that support 
+    text/value conversion through the TypeConverter architecture. 
+    Classes that don't support this are serialized and stored with the 
+    mimetype set.
+
+    The mimetype is used for serialized objects, and tells the 
+    ResXResourceReader how to depersist the object. This is currently not 
+    extensible. For a given mimetype the value must be set accordingly:
+
+    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    that the ResXResourceWriter will generate, however the reader can 
+    read any of the formats listed below.
+
+    mimetype: application/x-microsoft.net.object.binary.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
+            : and then encoded with base64 encoding.
+
+    mimetype: application/x-microsoft.net.object.soap.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
+            : and then encoded with base64 encoding.
+
+    mimetype: application/x-microsoft.net.object.bytearray.base64
+    value   : The object must be serialized into a byte array 
+            : using a System.ComponentModel.TypeConverter
+            : and then encoded with base64 encoding.
+    -->
+  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
+    <xsd:import namespace="http://www.w3.org/XML/1998/namespace" />
+    <xsd:element name="root" msdata:IsDataSet="true">
+      <xsd:complexType>
+        <xsd:choice maxOccurs="unbounded">
+          <xsd:element name="metadata">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" />
+              </xsd:sequence>
+              <xsd:attribute name="name" use="required" type="xsd:string" />
+              <xsd:attribute name="type" type="xsd:string" />
+              <xsd:attribute name="mimetype" type="xsd:string" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="assembly">
+            <xsd:complexType>
+              <xsd:attribute name="alias" type="xsd:string" />
+              <xsd:attribute name="name" type="xsd:string" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="data">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+                <xsd:element name="comment" type="xsd:string" minOccurs="0" msdata:Ordinal="2" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" msdata:Ordinal="1" />
+              <xsd:attribute name="type" type="xsd:string" msdata:Ordinal="3" />
+              <xsd:attribute name="mimetype" type="xsd:string" msdata:Ordinal="4" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="resheader">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" />
+            </xsd:complexType>
+          </xsd:element>
+        </xsd:choice>
+      </xsd:complexType>
+    </xsd:element>
+  </xsd:schema>
+  <resheader name="resmimetype">
+    <value>text/microsoft-resx</value>
+  </resheader>
+  <resheader name="version">
+    <value>2.0</value>
+  </resheader>
+  <resheader name="reader">
+    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <resheader name="writer">
+    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <data name="Error_FileSystemWatcherRequiredWithoutPolling" xml:space="preserve">
+    <value>The fileSystemWatcher parameter must be non-null when pollForChanges is false.</value>
+  </data>
+</root>

src/libraries/Microsoft.Extensions.FileProviders.Physical/tests/PhysicalFileProviderTests.cs

@@ -2,9 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
+using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.FileProviders.Internal;
 using Microsoft.Extensions.FileProviders.Physical;
@@ -16,6 +18,7 @@ namespace Microsoft.Extensions.FileProviders
     public class PhysicalFileProviderTests
     {
         private const int WaitTimeForTokenToFire = 500;
+        private const int WaitTimeForTokenCallback = 10000;
 
         [Fact]
         public void GetFileInfoReturnsNotFoundFileInfoForNullPath()
@@ -87,6 +90,54 @@ public void GetFileInfoReturnsNotFoundFileInfoForIllegalPathWithLeadingSlashes_U
             GetFileInfoReturnsNotFoundFileInfoForIllegalPathWithLeadingSlashes(path);
         }
 
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Linux)] 
+        public void PollingFileProviderShouldntConsumeINotifyInstances()
+        {
+            List<IDisposable> disposables = new List<IDisposable>();
+            using (var root = new DisposableFileSystem())
+            {
+                string maxInstancesFile = "/proc/sys/fs/inotify/max_user_instances";
+                Assert.True(File.Exists(maxInstancesFile));
+                int maxInstances = int.Parse(File.ReadAllText(maxInstancesFile));
+
+                // choose an arbitrary number that exceeds max
+                int instances = maxInstances + 16;
+
+                AutoResetEvent are = new AutoResetEvent(false);
+
+                var oldPollingInterval = PhysicalFilesWatcher.DefaultPollingInterval;
+                try
+                {
+                    PhysicalFilesWatcher.DefaultPollingInterval = TimeSpan.FromMilliseconds(WaitTimeForTokenToFire);
+                    for (int i = 0; i < instances; i++)
+                    {
+                        PhysicalFileProvider pfp = new PhysicalFileProvider(root.RootPath)
+                        {
+                            UsePollingFileWatcher = true,
+                            UseActivePolling = true
+                        };
+                        disposables.Add(pfp);
+                        disposables.Add(pfp.Watch("*").RegisterChangeCallback(_ => are.Set(), null));
+                    }
+
+                    // trigger an event
+                    root.CreateFile("test.txt");
+
+                    // wait for at least one event.
+                    Assert.True(are.WaitOne(WaitTimeForTokenCallback));
+                }
+                finally
+                {
+                    PhysicalFilesWatcher.DefaultPollingInterval = oldPollingInterval;
+                    foreach (var disposable in disposables)
+                    {
+                        disposable.Dispose();
+                    }
+                }
+            }
+        }
+
         private void GetFileInfoReturnsNotFoundFileInfoForIllegalPathWithLeadingSlashes(string path)
         {
             using (var provider = new PhysicalFileProvider(Path.GetTempPath()))
@@ -1479,5 +1530,30 @@ public void CreateFileWatcher_CreatesWatcherWithPollingAndActiveFlags()
                 Assert.True(fileWatcher.UseActivePolling);
             }
         }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34580", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
+        public async Task CanDeleteWatchedDirectory(bool useActivePolling)
+        {
+            using (var root = new DisposableFileSystem())
+            using (var provider = new PhysicalFileProvider(root.RootPath))
+            {
+                var fileName = Path.GetRandomFileName();
+                PollingFileChangeToken.PollingInterval = TimeSpan.FromMilliseconds(10);
+
+                provider.UsePollingFileWatcher = true;  // We must use polling due to https://github.com/dotnet/runtime/issues/44484
+                provider.UseActivePolling = useActivePolling;
+
+                root.CreateFile(fileName);
+                var token = provider.Watch(fileName);
+                Directory.Delete(root.RootPath, true);
+
+                await Task.Delay(WaitTimeForTokenToFire).ConfigureAwait(false);
+
+                Assert.True(token.HasChanged);
+            }
+        }
     }
 }