[V4RB] Open/Close trick Revisted

[keatk at comcast.net]

It turns out I did have to rely on LockFiles on the network at work to use it. 

I need to be Xplatform so to get fully atomic operations I likely would have had resort to declares on each platform and I did not have the time to figure out how to do that...

So i tried to make it almost foolproof in RB code... Not sure if I succeeded, because with the low current usage collisons are likely to be rare for awhile. Anyway here is my lock file checking code. any comments world be appreciated:

Protected Function LockFileCheck() As Boolean
   'At App Starup :
   '    Lockfile FolderItem is created
   '    AppGlobals.NetworkProcessID Is assigned
   '    AppGlobals.NetworkProcessID is created as follows:
   '      If System.NetworkInterfaceCount = 0 then
   '        NetworkProcessID = Format(Microseconds,"#.0000000000000000000000e")
   '      Else
   '        NetworkProcessID = System.GetNetworkInterface(0).MACAddress + Format(Microseconds,"#.0000000000000000000000e")
   '      End if
    
   Dim Now As New Date
   Dim OutStream as TextOutputStream, InStream as TextInputStream
   Dim LockID, MyID AS String
   
   If AppGlobals.NetworkProcessID = "" Then AppGlobals.MakeNetworkProcessID
   
   MyID = AppGlobals.NetworkProcessID
   
   If LockFile.Exists Then 
      If (Now.TotalSeconds - LockFile.CreationDate.TotalSeconds) > MaxLockFileAge_Sec Then 
         LockFile.Delete
      Else ' then try to read it
         InStream = LockFile.OpenAsTextFile
         LockID = InStream.ReadLine
         InStream.Close
         If MyID <> LockID Then Return False  'Another process owns it
         LockFile.Delete
      End if
   End if
   
   If Not LockFile.Exists Then ' Create it
      OutStream = LockFile.CreateTextFile
      OutStream.WriteLine MyID
      OutStream.Close
   Else  'Another process beat this one to the punch
      Return False
   End if
   
   ' Try to read from the one just created just in case another process snuck in
   
   InStream = LockFile.OpenAsTextFile
   LockID = InStream.ReadLine
   InStream.Close
   Return MyID = LockID
   
Exception ' File system does not allow simultaneous acess or file deleted after creation but before reading
   Return false
End Function

------------------------------------
And I use it like so: from my VDatabase subclass
------------------------------------

Function Open() As Boolean
   If  Me.isOpen Then Return false
   
   Dim theErrNo as integer 
   theErrNo = me.TryToOpen 
   
   If me.isOpen Then 
      TryingToOpen = False
      Return true
   End If
   
   dbBusyDialog.ShowDialog(me, Location.Name, theErrNo) ' this creates a timer that calls 
'                                                                                       TryToOpen
   
   If me.isOpen Then 
      Return True
   Else
      Quit
   End if
End Function

---------------------

Protected Function TryToOpen() As Integer
   Dim DidOpen as boolean, Err as Integer
   
   If Not TryingToOpen Then dbOpeningDialog.Display(Location.Name)
   
   isOpen = False
   
   If me.LockFileCheck Then
       DidOpen= Super.Open(Location)
      
      if DidOpen and me.ErrNumber = 0 Then
         isOpen = True
         err = 0
      Elseif Not DidOpen AND me.ErrNumber = 0 then
         Err = -666
      Else
         err = me.ErrNumber
      End if
      
   Else
      Err = -999
   End if
   
   If Not TryingToOpen Then dbOpeningDialog.Close
   Return Err
   
End Function