private void StartForeignRequest( TrackerStateHolder trackerStateHolder )
{
LocationRequest locationRequest = PrepareLocationRequest( trackerStateHolder );
if ( locationRequest == null )
return;
try
{
if ( !HolderRwLock.TryEnterWriteLock( ) )
{
Log.ErrorFormat( "Can't enter write mode in BeginRequest for {0}", trackerStateHolder.ForeignId );
return;
}
try
{
trackerStateHolder.CurrentRequest = locationRequest;
trackerStateHolder.RequestStartTime = TimeService.Now;
}
finally
{
HolderRwLock.ExitWriteLock( );
}
// Despite TrackerStateHolder having CurrentRequest field, locationRequest still need to
// be passed as an implicit parameter because there is no guarantee that at the moment when
// OnEndReadLocation is called there will be no other request started. E.g. this request
// can be stuck, aborted by timeout, and then suddenly pop in again in OnEndReadLocation.
// It's unlikely but still possible. So pass locationRequest to avoid any chance that
// EndReadLocation is called on wrong locationRequest.
Tuple<TrackerStateHolder, LocationRequest> paramTuple =
new Tuple<TrackerStateHolder, LocationRequest>( trackerStateHolder, locationRequest );
DateTime requestStart = TimeService.Now;
locationRequest.BeginReadLocation( OnEndReadLocation, paramTuple );
this.statistics.AddRequestStartEvent( trackerStateHolder.ForeignId, requestStart );
}
catch
{
this.statistics.AddRequestErrorEvent(trackerStateHolder.ForeignId.Type);
trackerStateHolder.CurrentRequest = null; // no-good => no CurrentRequest, no setting ScheduledTime to null.
throw;
}
Thread.MemoryBarrier( ); // set ScheduledTime only _after_ successful BeginReadLocation.
trackerStateHolder.ScheduledTime = null;
}
private static int CompareHolders( TrackerStateHolder x, TrackerStateHolder y )
{
bool areEqual;
TrackerStateHolder moreStale = Scheduler.GetMoreStaleTracker( x, y, out areEqual );
if ( areEqual )
return string.Compare( x.ForeignId.Id, y.ForeignId.Id, StringComparison.InvariantCultureIgnoreCase );
if ( ReferenceEquals( moreStale, x ) )
return -1;
return 1;
}
private LocationRequest PrepareLocationRequest( TrackerStateHolder trackerStateHolder )
{
LocationRequestFactory requestFactory =
ForeignAccessCentral.LocationRequestFactories[trackerStateHolder.ForeignId.Type];
RequestParams requestParams;
requestParams.Id = trackerStateHolder.ForeignId.Id;
// Read lock needed to read trackerStateHolder.Snapshot.Position
if ( !HolderRwLock.TryEnterReadLock( ) )
{
Log.ErrorFormat( "Can't enter read mode in PrepareLocationRequest for {0}", trackerStateHolder.ForeignId );
return null;
}
try
{
// Potentially, the snapshot with the existing track might change while the new data is being requested.
// But it's not a problem because the old track is valid anyway - to make sure that, the requesting
// algorithm will check if the old track's starting section matches new data loaded from the foreign
// system. So existing track (most probably its trailing points) would be in use anyway.
if ( trackerStateHolder.Snapshot == null ||
trackerStateHolder.Snapshot.Position == null )
{
requestParams.ExistingTrack = null;
}
else
{
requestParams.ExistingTrack = trackerStateHolder.Snapshot.Position.FullTrack;
}
}
finally
{
HolderRwLock.ExitReadLock( );
}
return requestFactory.CreateRequest( requestParams );
}