Avoid recursive FileSystemWatcher when not required in PhysicalFilesWatcher

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

@@ -34,6 +34,14 @@ public class PhysicalFilesWatcher : IDisposable
         private readonly object _fileWatcherLock = new();
         private readonly string _root;
         private readonly ExclusionFilters _filters;
+        // True when the FileSystemWatcher watches a strict ancestor of _root (rather than _root
+        // itself). In that case IncludeSubdirectories must always be true so that events occurring
+        // inside _root (which is below the FSW's watched path) are observed.
+        private readonly bool _fileWatcherIsAboveRoot;
+        // Number of currently registered tokens whose pattern requires watching subdirectories.
+        // Maintained as tokens are added and removed so we don't iterate the lookups when
+        // re-evaluating IncludeSubdirectories.
+        private int _subdirectoryRequiringTokenCount;
 
         // A single non-recursive watcher used when _root does not exist.
         // Watches for _root to be created, then enables the main FileSystemWatcher.
@@ -111,10 +119,15 @@ public PhysicalFilesWatcher(
                     {
                         throw new ArgumentException(SR.Format(SR.FileSystemWatcherPathError, watcherFullPath, _root), nameof(fileSystemWatcher));
                     }
+
+                    // If the FSW watches an ancestor of _root, every event of interest occurs
+                    // in a subdirectory from the FSW's perspective, so subdirectory watching
+                    // is required to observe any of them.
+                    _fileWatcherIsAboveRoot = !watcherFullPath.Equals(_root, StringComparison.OrdinalIgnoreCase) &&
+                        _root.StartsWith(watcherFullPath, StringComparison.OrdinalIgnoreCase);
                 }
 
                 _fileWatcher = fileSystemWatcher;
-                _fileWatcher.IncludeSubdirectories = true;
                 _fileWatcher.Created += OnChanged;
                 _fileWatcher.Changed += OnChanged;
                 _fileWatcher.Renamed += OnRenamed;
@@ -189,8 +202,16 @@ internal IChangeToken GetOrAddFilePathChangeToken(string filePath)
             {
                 var cancellationTokenSource = new CancellationTokenSource();
                 var cancellationChangeToken = new CancellationChangeToken(cancellationTokenSource.Token);
-                tokenInfo = new ChangeTokenInfo(cancellationTokenSource, cancellationChangeToken);
-                tokenInfo = _filePathTokenLookup.GetOrAdd(filePath, tokenInfo);
+                var newTokenInfo = new ChangeTokenInfo(cancellationTokenSource, cancellationChangeToken);
+                tokenInfo = _filePathTokenLookup.GetOrAdd(filePath, newTokenInfo);
+
+                // GetOrAdd may not have actually added our entry if another thread won the race.
+                // Compare by reference to detect whether our entry was the one stored.
+                if (ReferenceEquals(tokenInfo.TokenSource, cancellationTokenSource) &&
+                    FilePathRequiresSubdirectories(filePath))
+                {
+                    Interlocked.Increment(ref _subdirectoryRequiringTokenCount);
+                }
             }
 
             IChangeToken changeToken = tokenInfo.ChangeToken;
@@ -227,8 +248,14 @@ internal IChangeToken GetOrAddWildcardChangeToken(string pattern)
                 var cancellationChangeToken = new CancellationChangeToken(cancellationTokenSource.Token);
                 var matcher = new Matcher(StringComparison.OrdinalIgnoreCase);
                 matcher.AddInclude(pattern);
-                tokenInfo = new ChangeTokenInfo(cancellationTokenSource, cancellationChangeToken, matcher);
-                tokenInfo = _wildcardTokenLookup.GetOrAdd(pattern, tokenInfo);
+                var newTokenInfo = new ChangeTokenInfo(cancellationTokenSource, cancellationChangeToken, matcher);
+                tokenInfo = _wildcardTokenLookup.GetOrAdd(pattern, newTokenInfo);
+
+                if (ReferenceEquals(tokenInfo.TokenSource, cancellationTokenSource) &&
+                    WildcardRequiresSubdirectories(pattern))
+                {
+                    Interlocked.Increment(ref _subdirectoryRequiringTokenCount);
+                }
             }
 
             IChangeToken changeToken = tokenInfo.ChangeToken;
@@ -347,17 +374,22 @@ private void OnChanged(object sender, FileSystemEventArgs e)
         private void OnError(object sender, ErrorEventArgs e)
         {
             // Notify all cache entries on error.
-            CancelAll(_filePathTokenLookup);
-            CancelAll(_wildcardTokenLookup);
+            CancelAll(_filePathTokenLookup, FilePathRequiresSubdirectories);
+            CancelAll(_wildcardTokenLookup, WildcardRequiresSubdirectories);
 
             TryDisableFileSystemWatcher();
 
-            static void CancelAll(ConcurrentDictionary<string, ChangeTokenInfo> tokens)
+            void CancelAll(ConcurrentDictionary<string, ChangeTokenInfo> tokens, Func<string, bool> requiresSubdirectories)
             {
                 foreach (KeyValuePair<string, ChangeTokenInfo> entry in tokens)
                 {
                     if (tokens.TryRemove(entry.Key, out ChangeTokenInfo matchInfo))
                     {
+                        if (requiresSubdirectories(entry.Key))
+                        {
+                            Interlocked.Decrement(ref _subdirectoryRequiringTokenCount);
+                        }
+
                         CancelToken(matchInfo);
                     }
                 }
@@ -418,6 +450,11 @@ private void ReportChangeForMatchedEntries(string path)
             bool matched = false;
             if (_filePathTokenLookup.TryRemove(path, out ChangeTokenInfo matchInfo))
             {
+                if (FilePathRequiresSubdirectories(path))
+                {
+                    Interlocked.Decrement(ref _subdirectoryRequiringTokenCount);
+                }
+
                 CancelToken(matchInfo);
                 matched = true;
             }
@@ -428,6 +465,11 @@ private void ReportChangeForMatchedEntries(string path)
                 if (matchResult.HasMatches &&
                     _wildcardTokenLookup.TryRemove(wildCardEntry.Key, out matchInfo))
                 {
+                    if (WildcardRequiresSubdirectories(wildCardEntry.Key))
+                    {
+                        Interlocked.Decrement(ref _subdirectoryRequiringTokenCount);
+                    }
+
                     CancelToken(matchInfo);
                     matched = true;
                 }
@@ -491,6 +533,14 @@ private void TryDisableFileSystemWatcher()
                         // Perf: Turn off the file monitoring if no files or directories to monitor.
                         _fileWatcher.EnableRaisingEvents = false;
                     }
+                    else if (_fileWatcher.IncludeSubdirectories &&
+                        !_fileWatcherIsAboveRoot &&
+                        Volatile.Read(ref _subdirectoryRequiringTokenCount) == 0)
+                    {
+                        // Perf: Some tokens were removed and none of the remaining ones require
+                        // subdirectory watching, so we can stop recursing.
+                        _fileWatcher.IncludeSubdirectories = false;
+                    }
                 }
             }
 
@@ -529,6 +579,17 @@ private void TryEnableFileSystemWatcher()
                 {
                     bool rootExists = Directory.Exists(_root);
 
+                    // Only enable recursive subdirectory watching when at least one registered
+                    // pattern actually references a subdirectory. This avoids creating an inotify
+                    // watch descriptor on every descendant directory on Linux when only root-level files
+                    // (e.g. appsettings.json) are being monitored.
+                    bool needsSubdirectories = _fileWatcherIsAboveRoot ||
+                        Volatile.Read(ref _subdirectoryRequiringTokenCount) > 0;
+                    if (_fileWatcher.IncludeSubdirectories != needsSubdirectories)
+                    {
+                        _fileWatcher.IncludeSubdirectories = needsSubdirectories;
+                    }
+
                     // On some platforms (e.g., Linux), FileSystemWatcher currently does not
                     // invoke OnError when the watched directory is deleted, so we don't disable
                     // the FSW and start root watcher at that point.
@@ -655,6 +716,18 @@ private void EnsureRootCreationWatcher()
             }
         }
 
+        // Patterns are normalized to use forward slashes. A file path token references a single
+        // file, so it requires subdirectory watching only when the path is in a subdirectory
+        // (i.e. contains '/').
+        private static bool FilePathRequiresSubdirectories(string normalizedFilePath) =>
+            normalizedFilePath.Contains('/');
+
+        // A wildcard pattern requires subdirectory watching when it explicitly references a
+        // subdirectory (contains '/') or uses the recursive globbing wildcard '**'. A simple
+        // wildcard like '*' or '*.json' only matches entries directly in the root directory.
+        private static bool WildcardRequiresSubdirectories(string normalizedPattern) =>
+            normalizedPattern.Contains('/') || normalizedPattern.Contains("**");
+
         private static string NormalizePath(string filter) => filter.Replace('\\', '/');
 
         private static bool IsDirectoryPath(string path)

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

@@ -69,6 +69,140 @@ private static async Task WhenChanged(IChangeToken token, bool withTimeout = tru
             }
         }
 
+        [Theory]
+        [InlineData(new[] { "appsettings.json" }, false)]
+        [InlineData(new[] { "appsettings.json", "appsettings.Production.json" }, false)]
+        [InlineData(new[] { "*.json" }, false)]
+        [InlineData(new[] { "*" }, false)]
+        [InlineData(new[] { "sub/file.json" }, true)]
+        [InlineData(new[] { "sub/*.json" }, true)]
+        [InlineData(new[] { "**/appsettings.json" }, true)]
+        [InlineData(new[] { "**" }, true)]
+        [InlineData(new[] { "sub/" }, true)]
+        [InlineData(new[] { "appsettings.json", "sub/file.json" }, true)]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
+        public void IncludeSubdirectories_OnlyEnabledWhenAPatternRequiresIt(string[] patterns, bool expectedIncludeSubdirectories)
+        {
+            using var root = new TempDirectory(GetTestFilePath());
+            using var fileSystemWatcher = new MockFileSystemWatcher(root.Path);
+            using var physicalFilesWatcher = new PhysicalFilesWatcher(root.Path, fileSystemWatcher, pollForChanges: false);
+
+            foreach (string pattern in patterns)
+            {
+                physicalFilesWatcher.CreateFileChangeToken(pattern);
+            }
+
+            Assert.Equal(expectedIncludeSubdirectories, fileSystemWatcher.IncludeSubdirectories);
+            Assert.True(fileSystemWatcher.EnableRaisingEvents);
+        }
+
+        [Fact]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
+        public void IncludeSubdirectories_UpgradedWhenSubdirectoryPatternAddedLater()
+        {
+            using var root = new TempDirectory(GetTestFilePath());
+            using var fileSystemWatcher = new MockFileSystemWatcher(root.Path);
+            using var physicalFilesWatcher = new PhysicalFilesWatcher(root.Path, fileSystemWatcher, pollForChanges: false);
+
+            physicalFilesWatcher.CreateFileChangeToken("appsettings.json");
+            Assert.False(fileSystemWatcher.IncludeSubdirectories);
+
+            physicalFilesWatcher.CreateFileChangeToken("sub/file.json");
+            Assert.True(fileSystemWatcher.IncludeSubdirectories);
+        }
+
+        [Fact]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
+        public void IncludeSubdirectories_DowngradedWhenSubdirectoryPatternRemoved()
+        {
+            using var root = new TempDirectory(GetTestFilePath());
+            using var fileSystemWatcher = new MockFileSystemWatcher(root.Path);
+            using var physicalFilesWatcher = new PhysicalFilesWatcher(root.Path, fileSystemWatcher, pollForChanges: false);
+
+            physicalFilesWatcher.CreateFileChangeToken("appsettings.json");
+            physicalFilesWatcher.CreateFileChangeToken("sub/file.json");
+            Assert.True(fileSystemWatcher.IncludeSubdirectories);
+
+            // Fire an event matching the subdirectory token to remove it from the lookup.
+            // The remaining root-only token does not require subdirectory watching, IncludeSubdirectories should be downgraded.
+            fileSystemWatcher.CallOnChanged(new FileSystemEventArgs(WatcherChangeTypes.Changed, root.Path, "sub/file.json"));
+            Assert.False(fileSystemWatcher.IncludeSubdirectories);
+        }
+
+        [Fact]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
+        public void IncludeSubdirectories_NotDowngradedWhileSubdirectoryPatternRemains()
+        {
+            using var root = new TempDirectory(GetTestFilePath());
+            using var fileSystemWatcher = new MockFileSystemWatcher(root.Path);
+            using var physicalFilesWatcher = new PhysicalFilesWatcher(root.Path, fileSystemWatcher, pollForChanges: false);
+
+            physicalFilesWatcher.CreateFileChangeToken("sub/file.json");
+            physicalFilesWatcher.CreateFileChangeToken("appsettings.json");
+            Assert.True(fileSystemWatcher.IncludeSubdirectories);
+
+            // Remove only the root-level token; the subdirectory token must keep subdirectories enabled.
+            fileSystemWatcher.CallOnChanged(new FileSystemEventArgs(WatcherChangeTypes.Changed, root.Path, "appsettings.json"));
+            Assert.True(fileSystemWatcher.IncludeSubdirectories);
+        }
+
+        [Fact]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
+        public void IncludeSubdirectories_NotDowngradedWhenWildcardSubdirectoryPatternRemains()
+        {
+            using var root = new TempDirectory(GetTestFilePath());
+            using var fileSystemWatcher = new MockFileSystemWatcher(root.Path);
+            using var physicalFilesWatcher = new PhysicalFilesWatcher(root.Path, fileSystemWatcher, pollForChanges: false);
+
+            physicalFilesWatcher.CreateFileChangeToken("sub/*.json");
+            physicalFilesWatcher.CreateFileChangeToken("appsettings.json");
+            Assert.True(fileSystemWatcher.IncludeSubdirectories);
+
+            // Remove the root-level token; the recursive wildcard token must keep subdirectories enabled.
+            fileSystemWatcher.CallOnChanged(new FileSystemEventArgs(WatcherChangeTypes.Changed, root.Path, "appsettings.json"));
+            Assert.True(fileSystemWatcher.IncludeSubdirectories);
+        }
+
+        [Fact]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
+        public void IncludeSubdirectories_AlwaysTrueWhenWatcherIsAboveRoot()
+        {
+            using var tempDir = new TempDirectory(GetTestFilePath());
+            string rootDir = Path.Combine(tempDir.Path, "rootDir");
+            Directory.CreateDirectory(rootDir);
+
+            using var fsw = new FileSystemWatcher(tempDir.Path);
+            using var physicalFilesWatcher = new PhysicalFilesWatcher(rootDir, fsw, pollForChanges: false);
+
+            // Even with only a root-only pattern, IncludeSubdirectories must be true because
+            // the FSW watches an ancestor of _root.
+            physicalFilesWatcher.CreateFileChangeToken("appsettings.json");
+            Assert.True(fsw.IncludeSubdirectories);
+        }
+
+        [Fact]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
+        public async Task IncludeSubdirectories_StarPatternIsNotRecursive_DoesNotMatchSubdirectoryFile()
+        {
+            using var root = new TempDirectory(GetTestFilePath());
+            using var fileSystemWatcher = new MockFileSystemWatcher(root.Path);
+            using var physicalFilesWatcher = new PhysicalFilesWatcher(root.Path, fileSystemWatcher, pollForChanges: false);
+
+            IChangeToken token = physicalFilesWatcher.CreateFileChangeToken("*.json");
+            Assert.False(fileSystemWatcher.IncludeSubdirectories);
+
+            Task changed = WhenChanged(token);
+
+            // A '*' pattern should not match files in subdirectories.
+            fileSystemWatcher.CallOnChanged(new FileSystemEventArgs(WatcherChangeTypes.Changed, root.Path, "sub/foo.json"));
+            await Task.Delay(WaitTimeForTokenToFire);
+            Assert.False(changed.IsCompleted, "Token must not fire for an event in a subdirectory.");
+
+            // Sanity check: a root-level event does fire the token.
+            fileSystemWatcher.CallOnChanged(new FileSystemEventArgs(WatcherChangeTypes.Changed, root.Path, "foo.json"));
+            await changed;
+        }
+
         [Fact]
         [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.iOS | TestPlatforms.tvOS, "System.IO.FileSystem.Watcher is not supported on Browser/iOS/tvOS")]
         public void CreateFileChangeToken_DoesNotAllowPathsAboveRoot()