Make PhysicalFilesWatcher exclusively use polling when pollForChanges=true

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

@@ -130,7 +130,10 @@ public IChangeToken CreateFileChangeToken(string filter)
             }
 
             IChangeToken changeToken = GetOrAddChangeToken(filter);
-            TryEnableFileSystemWatcher();
+            if (!PollForChanges)
+            {
+                TryEnableFileSystemWatcher();
+            }
 
             return changeToken;
         }

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;
@@ -87,6 +89,48 @@ public void GetFileInfoReturnsNotFoundFileInfoForIllegalPathWithLeadingSlashes_U
             GetFileInfoReturnsNotFoundFileInfoForIllegalPathWithLeadingSlashes(path);
         }
 
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Linux)] 
+        public async void PollingFileProviderShouldntConsumeINotifyInstances()
+        {
+            List<IDisposable> disposables = new List<IDisposable>();
+            using var root = new DisposableFileSystem();
+
+            int callbacks = 0;
+            int maxInstances = int.Parse(File.ReadAllText("/proc/sys/fs/inotify/max_user_instances"));
+            int instances = maxInstances + 16;
+            try
+            {
+                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(
+                        o => Interlocked.Increment(ref callbacks),
+                        i));
+                }
+
+                // trigger an event
+                root.CreateFile("test.txt");
+
+                // let some events fire
+                await Task.Delay(WaitTimeForTokenToFire * 16);
+
+                Assert.NotEqual(0, callbacks);
+            }
+            finally
+            {
+                foreach (var disposable in disposables)
+                {
+                    disposable.Dispose();
+                }
+            }
+        }
+
         private void GetFileInfoReturnsNotFoundFileInfoForIllegalPathWithLeadingSlashes(string path)
         {
             using (var provider = new PhysicalFileProvider(Path.GetTempPath()))