//------------------------------------------------------------------------------
// <copyright file="SqlDependencyUtils.cs" company="Microsoft">
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>
// <owner current="true" primary="true">[....]</owner>
// <owner current="true" primary="true">[....]</owner>
// <owner current="false" primary="false">[....]</owner>
//------------------------------------------------------------------------------

namespace System.Data.SqlClient {
    using System;
    using System.Collections;
    using System.Collections.Generic;
    using System.Data.Common;
    using System.Diagnostics;
    using System.Security.Principal;
    using System.Security.AccessControl;
    using System.Text;
    using System.Threading;

    // This is a singleton instance per AppDomain that acts as the notification dispatcher for
    // that AppDomain.  It receives calls from the SqlDependencyProcessDispatcher with an ID or a server name
    // to invalidate matching dependencies in the given AppDomain.

    internal class SqlDependencyPerAppDomainDispatcher : MarshalByRefObject { // MBR, since ref'ed by ProcessDispatcher.

        // ----------------
        // Instance members
        // ----------------

        internal static readonly SqlDependencyPerAppDomainDispatcher
                 SingletonInstance = new SqlDependencyPerAppDomainDispatcher(); // singleton object

        // Dependency ID -> Dependency hashtable.  1 -> 1 mapping.
        // 1) Used for ASP.Net to map from ID to dependency.  
        // 2) Used to enumerate dependencies to invalidate based on server.
        private Dictionary<string, SqlDependency>       _dependencyIdToDependencyHash;

        // holds dependencies list per notification and the command hash from which this notification was generated
        // command hash is needed to remove its entry from _commandHashToNotificationId when the notification is removed
        sealed class DependencyList : List<SqlDependency> {
            public readonly string CommandHash;

            internal DependencyList(string commandHash) {
                this.CommandHash = commandHash;
            }
        }

        // notificationId -> Dependencies hashtable:  1 -> N mapping.  notificationId == appDomainKey + commandHash.
        // More than one dependency can be using the same command hash values resulting in a hash to the same value.
        // We use this to cache mapping between command to dependencies such that we may reduce the notification
        // resource effect on SQL Server.  The Guid identifier is sent to the server during notification enlistment,
        // and returned during the notification event.  Dependencies look up existing Guids, if one exists, to ensure
        // they are re-using notification ids.
        private Dictionary<string, DependencyList> _notificationIdToDependenciesHash;

        // CommandHash value -> notificationId associated with it:  1->1 mapping. This map is used to quickly find if we need to create
        // new notification or hookup into existing one.
        // CommandHash is built from connection string, command text and parameters
        private Dictionary<string, string> _commandHashToNotificationId;

        // TIMEOUT LOGIC DESCRIPTION
        //
        // Every time we add a dependency we compute the next, earlier timeout.
        //
        // We setup a timer to get a callback every 15 seconds. In the call back:
        // - If there are no active dependencies, we just return.
        // - If there are dependencies but none of them timed-out (compared to the "next timeout"),
        //   we just return.
        // - Otherwise we Invalidate() those that timed-out.
        //
        // So the client-generated timeouts have a granularity of 15 seconds. This allows
        // for a simple and low-resource-consumption implementation.
        //
        // LOCKS: don't update _nextTimeout outside of the _dependencyHash.SyncRoot lock.

		private bool     _SqlDependencyTimeOutTimerStarted = false;
        // Next timeout for any of the dependencies in the dependency table.
        private DateTime _nextTimeout;
        // Timer to periodically check the dependencies in the table and see if anyone needs
        // a timeout.  We'll enable this only on demand.
        private Timer    _timeoutTimer;

        // -----------
        // BID members
        // -----------

        private readonly int _objectID        = System.Threading.Interlocked.Increment(ref _objectTypeCount);
        private static   int _objectTypeCount; // Bid counter
        internal int ObjectID {
            get {
                return _objectID;
            }
        }

        private SqlDependencyPerAppDomainDispatcher() {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher|DEP> %d#", ObjectID);
            try {
                _dependencyIdToDependencyHash = new Dictionary<string, SqlDependency>();
                _notificationIdToDependenciesHash    = new Dictionary<string, DependencyList>();
                _commandHashToNotificationId = new Dictionary<string, string>();

                _timeoutTimer = new Timer(new TimerCallback(TimeoutTimerCallback), null, Timeout.Infinite, Timeout.Infinite);

                // If rude abort - we'll leak.  This is acceptable for now.  
                AppDomain.CurrentDomain.DomainUnload += new EventHandler(this.UnloadEventHandler);
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // SQL Hotfix 236
        //  When remoted across appdomains, MarshalByRefObject links by default time out if there is no activity 
        //  within a few minutes.  Add this override to prevent marshaled links from timing out.
        public override object InitializeLifetimeService() {
            return null;
        }

        // ------
        // Events
        // ------

        private void UnloadEventHandler(object sender, EventArgs e) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.UnloadEventHandler|DEP> %d#", ObjectID);
            try {
                // Make non-blocking call to ProcessDispatcher to ThreadPool.QueueUserWorkItem to complete 
                // stopping of all start calls in this AppDomain.  For containers shared among various AppDomains,
                // this will just be a ref-count subtract.  For non-shared containers, we will close the container
                // and clean-up.
                SqlDependencyProcessDispatcher dispatcher = SqlDependency.ProcessDispatcher;
                if (null != dispatcher) {
                    dispatcher.QueueAppDomainUnloading(SqlDependency.AppDomainKey);
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // ----------------------------------------------------
        // Methods for dependency hash manipulation and firing.
        // ----------------------------------------------------

        // This method is called upon SqlDependency constructor.
        internal void AddDependencyEntry(SqlDependency dep) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.AddDependencyEntry|DEP> %d#, SqlDependency: %d#", ObjectID, dep.ObjectID);
            try {
                lock (this) {
                    _dependencyIdToDependencyHash.Add(dep.Id, dep);
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // This method is called upon Execute of a command associated with a SqlDependency object.
        internal string AddCommandEntry(string commandHash, SqlDependency dep) {
            IntPtr hscp;
            string notificationId = string.Empty;

            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.AddCommandEntry|DEP> %d#, commandHash: '%ls', SqlDependency: %d#", ObjectID, commandHash, dep.ObjectID);
            try {
                lock (this) {
                    if (!_dependencyIdToDependencyHash.ContainsKey(dep.Id)) { // Determine if depId->dep hashtable contains dependency.  If not, it's been invalidated.
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.AddCommandEntry|DEP> Dependency not present in depId->dep hash, must have been invalidated.\n");
                    }
                    else {
                        // check if we already have notification associated with given command hash
                        if (_commandHashToNotificationId.TryGetValue(commandHash, out notificationId)) {
                            // we have one or more SqlDependency instances with same command hash

                            DependencyList dependencyList = null;
                            if (!_notificationIdToDependenciesHash.TryGetValue(notificationId, out dependencyList))
                            {
                                // this should not happen since _commandHashToNotificationId and _notificationIdToDependenciesHash are always
                                // updated together
                                Debug.Assert(false, "_commandHashToNotificationId has entries that were removed from _notificationIdToDependenciesHash. Remember to keep them in [....]");
                                throw ADP.InternalError(ADP.InternalErrorCode.SqlDependencyCommandHashIsNotAssociatedWithNotification);
                            }

                            // join the new dependency to the list
                            if (!dependencyList.Contains(dep)) {
                                Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.AddCommandEntry|DEP> Dependency not present for commandHash, adding.\n");
                                dependencyList.Add(dep);
                            }
                            else {
                                Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.AddCommandEntry|DEP> Dependency already present for commandHash.\n");
                            }
                        }
                        else {
                            // we did not find notification ID with the same app domain and command hash, create a new one
                            // use unique guid to avoid duplicate IDs
                            // prepend app domain ID to the key - SqlConnectionContainer::ProcessNotificationResults (SqlDependencyListener.cs)
                            // uses this app domain ID to route the message back to the app domain in which this SqlDependency was created
                            notificationId = string.Format(System.Globalization.CultureInfo.InvariantCulture,
                                "{0};{1}",
                                SqlDependency.AppDomainKey, // must be first
                                Guid.NewGuid().ToString("D", System.Globalization.CultureInfo.InvariantCulture)
                                );

                            Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.AddCommandEntry|DEP> Creating new Dependencies list for commandHash.\n");
                            DependencyList dependencyList = new DependencyList(commandHash);
                            dependencyList.Add(dep);

                            // map command hash to notification we just created to reuse it for the next client
                            // do it inside finally block to avoid ThreadAbort exception interrupt this operation
                            try {}
                            finally {
                                _commandHashToNotificationId.Add(commandHash, notificationId);
                                _notificationIdToDependenciesHash.Add(notificationId, dependencyList);
                            }
                        }


                        Debug.Assert(_notificationIdToDependenciesHash.Count == _commandHashToNotificationId.Count, "always keep these maps in [....]!");
                    }
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }

            return notificationId;
        }

        // This method is called by the ProcessDispatcher upon a notification for this AppDomain.
        internal void InvalidateCommandID(SqlNotification sqlNotification) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.InvalidateCommandID|DEP> %d#, commandHash: '%ls'", ObjectID, sqlNotification.Key);
            try {
                List<SqlDependency> dependencyList = null;

                lock (this) {
                    dependencyList = LookupCommandEntryWithRemove(sqlNotification.Key);

                    if (null != dependencyList) {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.InvalidateCommandID|DEP> commandHash found in hashtable.\n");

                        foreach (SqlDependency dependency in dependencyList) {
                            // Ensure we remove from process static app domain hash for dependency initiated invalidates.
                            LookupDependencyEntryWithRemove(dependency.Id);

                            // Completely remove Dependency from commandToDependenciesHash.
                            RemoveDependencyFromCommandToDependenciesHash(dependency);  
                        }
                    }
                    else {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.InvalidateCommandID|DEP> commandHash NOT found in hashtable.\n");
                    }
                }

                if (null != dependencyList) {
                    // After removal from hashtables, invalidate.
                    foreach (SqlDependency dependency in dependencyList) {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.InvalidateCommandID|DEP> Dependency found in commandHash dependency ArrayList - calling invalidate.\n");

                        try {
                            dependency.Invalidate(sqlNotification.Type, sqlNotification.Info, sqlNotification.Source);
                        }
                        catch (Exception e) {
                            // Since we are looping over dependencies, do not allow one Invalidate
                            // that results in a throw prevent us from invalidating all dependencies
                            // related to this server.
                            if (!ADP.IsCatchableExceptionType(e)) {
                                throw;
                            }
                            ADP.TraceExceptionWithoutRethrow(e);
                        }
                    }
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // This method is called when a connection goes down or other unknown error occurs in the ProcessDispatcher.
        internal void InvalidateServer(string server, SqlNotification sqlNotification) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.Invalidate|DEP> %d#, server: '%ls'", ObjectID, server);
            try {
                List<SqlDependency> dependencies = new List<SqlDependency>();

                lock (this) { // Copy inside of lock, but invalidate outside of lock.
                    foreach (KeyValuePair<string, SqlDependency> entry in _dependencyIdToDependencyHash) {
                        SqlDependency dependency = entry.Value;
                        if (dependency.ContainsServer(server)) { 
                            dependencies.Add(dependency);
                        }
                    }

                    foreach (SqlDependency dependency in dependencies) { // Iterate over resulting list removing from our hashes.
                        // Ensure we remove from process static app domain hash for dependency initiated invalidates.
                        LookupDependencyEntryWithRemove(dependency.Id);

                        // Completely remove Dependency from commandToDependenciesHash.
                        RemoveDependencyFromCommandToDependenciesHash(dependency);  
                    }
                }
                
                foreach (SqlDependency dependency in dependencies) { // Iterate and invalidate.
                    try {
                        dependency.Invalidate(sqlNotification.Type, sqlNotification.Info, sqlNotification.Source);
                    }
                    catch (Exception e) {
                        // Since we are looping over dependencies, do not allow one Invalidate
                        // that results in a throw prevent us from invalidating all dependencies
                        // related to this server.
                        if (!ADP.IsCatchableExceptionType(e)) {
                            throw;
                        }
                        ADP.TraceExceptionWithoutRethrow(e);
                    }
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // This method is called by SqlCommand to enable ASP.Net scenarios - map from ID to Dependency.
        internal SqlDependency LookupDependencyEntry(string id) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.LookupDependencyEntry|DEP> %d#, Key: '%ls'", ObjectID, id);
            try {
                if (null == id) {
                    throw ADP.ArgumentNull("id");
                }
                if (ADP.IsEmpty(id)) {
                    throw SQL.SqlDependencyIdMismatch();
                }

                SqlDependency entry = null;

                lock (this) {
                    if (_dependencyIdToDependencyHash.ContainsKey(id)) {
                        entry = _dependencyIdToDependencyHash[id];
                    }
                    else {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.LookupDependencyEntry|DEP|ERR> ERROR - dependency ID mismatch - not throwing.\n");
                    }
                }

                return entry;
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // Remove the dependency from the hashtable with the passed id.
        private void LookupDependencyEntryWithRemove(string id) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.LookupDependencyEntryWithRemove|DEP> %d#, id: '%ls'", ObjectID, id);
            try {
                lock (this) {
                    if (_dependencyIdToDependencyHash.ContainsKey(id)) {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.LookupDependencyEntryWithRemove|DEP> Entry found in hashtable - removing.\n");
                        _dependencyIdToDependencyHash.Remove(id);

                        // if there are no more dependencies then we can dispose the timer.
                        if (0 == _dependencyIdToDependencyHash.Count) {
                            _timeoutTimer.Change(Timeout.Infinite, Timeout.Infinite);
                            _SqlDependencyTimeOutTimerStarted = false;                        
                        }
                    }
                    else {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.LookupDependencyEntryWithRemove|DEP> Entry NOT found in hashtable.\n");
                    }
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // Find and return arraylist, and remove passed hash value.
        private List<SqlDependency> LookupCommandEntryWithRemove(string notificationId) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.LookupCommandEntryWithRemove|DEP> %d#, commandHash: '%ls'", ObjectID, notificationId);
            try {
                DependencyList entry = null;

                lock (this) {
                    if (_notificationIdToDependenciesHash.TryGetValue(notificationId, out entry)) {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.LookupDependencyEntriesWithRemove|DEP> Entries found in hashtable - removing.\n");

                        // update the tables - do it inside finally block to avoid ThreadAbort exception interrupt this operation
                        try { }
                        finally {
                            _notificationIdToDependenciesHash.Remove(notificationId);
                            // VSTS 216991: cleanup the map between the command hash and associated notification ID
                            _commandHashToNotificationId.Remove(entry.CommandHash);
                        }
                    }
                    else {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.LookupDependencyEntriesWithRemove|DEP> Entries NOT found in hashtable.\n");
                    }

                    Debug.Assert(_notificationIdToDependenciesHash.Count == _commandHashToNotificationId.Count, "always keep these maps in [....]!");
                }

                return entry; // DependencyList inherits from List<SqlDependency>
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // Remove from commandToDependenciesHash all references to the passed dependency.
        private void RemoveDependencyFromCommandToDependenciesHash(SqlDependency dependency) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.RemoveDependencyFromCommandToDependenciesHash|DEP> %d#, SqlDependency: %d#", ObjectID, dependency.ObjectID);
            try {
                lock (this) {
                    List<string> notificationIdsToRemove = new List<string>();
                    List<string> commandHashesToRemove = new List<string>();

                    foreach (KeyValuePair<string, DependencyList> entry in _notificationIdToDependenciesHash) {
                        DependencyList dependencies = entry.Value;
                        if (dependencies.Remove(dependency)) {
                            Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.RemoveDependencyFromCommandToDependenciesHash|DEP> Removed SqlDependency: %d#, with ID: '%ls'.\n", dependency.ObjectID, dependency.Id);
                            if (dependencies.Count == 0) {
                                // this dependency was the last associated with this notification ID, remove the entry
                                // note: cannot do it inside foreach over dictionary
                                notificationIdsToRemove.Add(entry.Key);
                                commandHashesToRemove.Add(entry.Value.CommandHash);
                            }
                        }

                        // same SqlDependency can be associated with more than one command, so we have to continue till the end...
                    }

                    Debug.Assert(commandHashesToRemove.Count == notificationIdsToRemove.Count, "maps should be kept in [....]");
                    for (int i = 0; i < notificationIdsToRemove.Count; i++ ) {
                        // cleanup the entry outside of foreach
                        // do it inside finally block to avoid ThreadAbort exception interrupt this operation
                        try { }
                        finally {
                            _notificationIdToDependenciesHash.Remove(notificationIdsToRemove[i]);
                            // VSTS 216991: cleanup the map between the command hash and associated notification ID
                            _commandHashToNotificationId.Remove(commandHashesToRemove[i]);
                        }
                    }

                    Debug.Assert(_notificationIdToDependenciesHash.Count == _commandHashToNotificationId.Count, "always keep these maps in [....]!");
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        // -----------------------------------------
        // Methods for Timer maintenance and firing.
        // -----------------------------------------

        internal void StartTimer(SqlDependency dep) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.StartTimer|DEP> %d#, SqlDependency: %d#", ObjectID, dep.ObjectID);
            try {
                // If this dependency expires sooner than the current next timeout, change
                // the timeout and enable timer callback as needed.  Note that we change _nextTimeout
                // only inside the hashtable syncroot.
                lock (this) {
                    // Enable the timer if needed (disable when empty, enable on the first addition).
                    if (!_SqlDependencyTimeOutTimerStarted) {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.StartTimer|DEP> Timer not yet started, starting.\n");

                        _timeoutTimer.Change(15000 /* 15 secs */, 15000 /* 15 secs */);

                        // Save this as the earlier timeout to come.
                        _nextTimeout = dep.ExpirationTime;
                        _SqlDependencyTimeOutTimerStarted = true;
                    }
                    else if(_nextTimeout > dep.ExpirationTime) {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.StartTimer|DEP> Timer already started, resetting time.\n");

                        // Save this as the earlier timeout to come.
                        _nextTimeout = dep.ExpirationTime;
                    }
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }

        private static void TimeoutTimerCallback(object state) {
            IntPtr hscp;
            Bid.NotificationsScopeEnter(out hscp, "<sc.SqlDependencyPerAppDomainDispatcher.TimeoutTimerCallback|DEP> AppDomainKey: '%ls'", SqlDependency.AppDomainKey);
            try {
                SqlDependency[] dependencies;

                // Only take the lock for checking whether there is work to do
                // if we do have work, we'll copy the hashtable and scan it after releasing
                // the lock.
                lock (SingletonInstance) {
                    if (0 == SingletonInstance._dependencyIdToDependencyHash.Count) {
                        // Nothing to check.
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.TimeoutTimerCallback|DEP> No dependencies, exiting.\n");
                        return;
                    }
                    if (SingletonInstance._nextTimeout > DateTime.UtcNow)  {
                        Bid.NotificationsTrace("<sc.SqlDependencyPerAppDomainDispatcher.TimeoutTimerCallback|DEP> No timeouts expired, exiting.\n");
                        // No dependency timed-out yet.
                        return;
                    }

                    // If at least one dependency timed-out do a scan of the table.
                    // NOTE: we could keep a shadow table sorted by expiration time, but
                    // given the number of typical simultaneously alive dependencies it's
                    // probably not worth the optimization.
                    dependencies = new SqlDependency[SingletonInstance._dependencyIdToDependencyHash.Count];
                    SingletonInstance._dependencyIdToDependencyHash.Values.CopyTo(dependencies, 0);
                }

                // Scan the active dependencies if needed.
                DateTime now            = DateTime.UtcNow;
                DateTime newNextTimeout = DateTime.MaxValue;

                for (int i=0; i < dependencies.Length; i++) {
                    // If expired fire the change notification.
                    if(dependencies[i].ExpirationTime <= now) {
                        try {
                            // This invokes user-code which may throw exceptions.
                            // NOTE: this is intentionally outside of the lock, we don't want
                            // to invoke user-code while holding an internal lock.
                            dependencies[i].Invalidate(SqlNotificationType.Change, SqlNotificationInfo.Error, SqlNotificationSource.Timeout);
                        }
                        catch(Exception e) {
                            if (!ADP.IsCatchableExceptionType(e)) {
                                throw;
                            }

                            // This is an exception in user code, and we're in a thread-pool thread
                            // without user's code up in the stack, no much we can do other than
                            // eating the exception.
                            ADP.TraceExceptionWithoutRethrow(e);
                        }
                    }
                    else {
                        if (dependencies[i].ExpirationTime < newNextTimeout) {
                            newNextTimeout = dependencies[i].ExpirationTime; // Track the next earlier timeout.
                        }
                        dependencies[i] = null; // Null means "don't remove it from the hashtable" in the loop below.
                    }
                }

                // Remove timed-out dependencies from the hashtable.
                lock (SingletonInstance) {
                    for (int i=0; i < dependencies.Length; i++) {
                        if (null != dependencies[i]) {
                            SingletonInstance._dependencyIdToDependencyHash.Remove(dependencies[i].Id);
                        }
                    }
                    if (newNextTimeout < SingletonInstance._nextTimeout) {
                        SingletonInstance._nextTimeout = newNextTimeout; // We're inside the lock so ok to update.
                    }
                }
            }
            finally {
                Bid.ScopeLeave(ref hscp);
            }
        }
    }

    // Simple class used to encapsulate all data in a notification.
    internal class SqlNotification : MarshalByRefObject {
        // This class could be Serializable rather than MBR...

        private readonly SqlNotificationInfo   _info;
        private readonly SqlNotificationSource _source;
        private readonly SqlNotificationType   _type;
        private readonly string                _key;

        internal SqlNotification(SqlNotificationInfo info, SqlNotificationSource source, SqlNotificationType type, string key) {
            _info    = info;
            _source  = source;
            _type    = type;
            _key     = key;
        }

        internal SqlNotificationInfo Info {
            get {
                return _info;
            }
        }

        internal string Key {
            get {
                return _key;
            }
        }

        internal SqlNotificationSource Source {
            get {
                return _source;
            }
        }

        internal SqlNotificationType Type {
            get {
                return _type;
            }
        }
    }
}