[System.Web] Add optional shared FileSystemWatcher's to CacheDependency

Right now we create a separate FileSystemWatcher for each file in
CacheDependency. On systems where inotify handles are limited this
leads to problems. As a workaround we can share the FSW which watches
the same directory. Since this wasn't extensively tested, hide it behind
and environment variable.

We enable this in the Helix environment only for now.

mcs/class/System.Web/System.Web.Caching/CacheDependency.cs

@@ -27,6 +27,7 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 using System.Collections;
+using System.Collections.Generic;
 using System.ComponentModel;
 using System.IO;
 using System.Security.Permissions;
@@ -45,10 +46,12 @@ namespace System.Web.Caching
 		DateTime start;
 		Cache cache;
 		FileSystemWatcher[] watchers;
+		static readonly bool useSharedWatchers = Environment.GetEnvironmentVariable ("MONO_SYSTEMWEB_CACHEDEPENDENCY_SHARED_FSW") != null;
+		static readonly Dictionary<string, FileSystemWatcher> sharedWatchers = new Dictionary<string, FileSystemWatcher> ();
 		bool hasChanged;
 		bool used;
 		DateTime utcLastModified;
-		object locker = new object ();
+		static readonly object locker = new object ();
 		EventHandlerList events = new EventHandlerList ();
 		
 		internal event EventHandler DependencyChanged {
@@ -102,25 +105,47 @@ namespace System.Web.Caching
 					filename = filenames [n];
 					if (String.IsNullOrEmpty (filename))
 						continue;
-					
-					FileSystemWatcher watcher = new FileSystemWatcher ();
+					string path = null;
+					string filter = null;
 					if (Directory.Exists (filename))
-						watcher.Path = filename;
+						path = filename;
 					else {
 						string parentPath = Path.GetDirectoryName (filename);
 						if (parentPath != null && Directory.Exists (parentPath)) {
-							watcher.Path = parentPath;
-							watcher.Filter = Path.GetFileName (filename);
+							path = parentPath;
+							filter = Path.GetFileName (filename);
 						} else
 							continue;
 					}
-					watcher.NotifyFilter |= NotifyFilters.Size;
-					watcher.Created += new FileSystemEventHandler (OnChanged);
-					watcher.Changed += new FileSystemEventHandler (OnChanged);
-					watcher.Deleted += new FileSystemEventHandler (OnChanged);
-					watcher.Renamed += new RenamedEventHandler (OnChanged);
-					watcher.EnableRaisingEvents = true;
-					watchers [n] = watcher;
+
+					lock (locker) {
+						FileSystemWatcher watcher;
+						if (useSharedWatchers) {
+							if (!sharedWatchers.TryGetValue (path, out watcher)) {
+								watcher = new FileSystemWatcher ();
+								watcher.Path = path;
+								watcher.NotifyFilter |= NotifyFilters.Size;
+								watcher.Created += new FileSystemEventHandler ((s, e) => { if (filter == null || e.Name == filter) OnChanged (s, e); });
+								watcher.Changed += new FileSystemEventHandler ((s, e) => { if (filter == null || e.Name == filter) OnChanged (s, e); });
+								watcher.Deleted += new FileSystemEventHandler ((s, e) => { if (filter == null || e.Name == filter) OnChanged (s, e); });
+								watcher.Renamed += new RenamedEventHandler ((s, e) => { if (filter == null || e.OldName == filter) OnChanged (s, e); });
+								watcher.EnableRaisingEvents = true;
+								sharedWatchers [path] = watcher;
+							}
+						} else {
+							watcher = new FileSystemWatcher ();
+							watcher.Path = path;
+							if (filter != null)
+								watcher.Filter = filter;
+							watcher.NotifyFilter |= NotifyFilters.Size;
+							watcher.Created += new FileSystemEventHandler (OnChanged);
+							watcher.Changed += new FileSystemEventHandler (OnChanged);
+							watcher.Deleted += new FileSystemEventHandler (OnChanged);
+							watcher.Renamed += new RenamedEventHandler (OnChanged);
+							watcher.EnableRaisingEvents = true;
+							watchers [n] = watcher;
+						}
+					}
 				}
 			}
 			this.cachekeys = cachekeys;
@@ -137,8 +162,17 @@ namespace System.Web.Caching
 			var sb = new StringBuilder ();
 			
 			lock (locker) {
-				if (watchers != null)
-					foreach (FileSystemWatcher fsw in watchers)
+				FileSystemWatcher[] watcherList;
+				
+				if (useSharedWatchers) {
+					watcherList = new FileSystemWatcher[sharedWatchers.Count];
+					sharedWatchers.Values.CopyTo (watcherList, 0);
+				} else {
+					watcherList = watchers;
+				}
+
+				if (watcherList != null)
+					foreach (FileSystemWatcher fsw in watcherList)
 						if (fsw != null && fsw.Path != null && fsw.Path.Length != 0)
 							sb.Append ("_" + fsw.Path);
 			}
@@ -172,6 +206,8 @@ namespace System.Web.Caching
 		
 		void DisposeWatchers ()
 		{
+			if (useSharedWatchers) return;
+
 			lock (locker) {
 				if (watchers != null) {
 					foreach (FileSystemWatcher w in watchers)

runtime/mono-test.sh

@@ -49,6 +49,7 @@ fi
 if [ "$test_suite" = "--nunit" ]; then
     cd net_4_x || exit 1
     export MONO_PATH="$r/net_4_x/tests:$MONO_PATH"
+    export MONO_SYSTEMWEB_CACHEDEPENDENCY_SHARED_FSW=1
     case "$test_argument_1" in
         *"Microsoft.Build"*)
             export TESTING_MONO=a