Comment on code

[Charles Yeomans]

On Dec 7, 2006, at 3:58 AM, Ruslan Zasukhin wrote:


> Hi Charles,
>
> Your test project have this function.
>
> -------------------------------------
> Sub Action()
>   dim connection as new VConnection("localhost", "sa", "sa")
>   connection.Open
>   dim db as new VDatabase(connection)
>   db.Open new FolderItem("pastefforts")
>
>   dim c as VCursor = db.SQLSelect("SELECT recID, * FROM Decisions  
> WHERE
> recid < 400", EVCursorLocation.kServerSide, EVLockType.kReadWrite,
> EVCursorDirection.kRandom)
>
>   if c.FirstRecord then
>     do
>       if not c.BlobField("notesStyleData").IsNull then
>         dim styleData as String = c.BlobField 
> ("notesStyleData").ReadRawData
>       end if
>     loop until not c.NextRecord
>   end if
>   c = nil
>   db.Close
>
> exception oops
>   break
> ---------------------------------------------------------
>
>
> This code contains at least 2 not obvious mistakes.
>
> 1) connection is never closed and destroyed.
>
>     so when I try reproduce your few-clicks issue,
>     I simply get in server log that connections are eaten to zero,
>     and then server refuse connection.
>
>     actually IT IS STRANGE that connection object not dies
>     on function exit. As I understand RB should kill connection  
> object.
>     right ?
>

RIght -- depending on your implementation.  Here, connection, db, and  
c are all local variables, so when the method exits, the objects to  
which they point will be destroyed.  I just did a little test --

Sub Action()
   dim connection as new VConnection("localhost", "sa", "sa")
   connection.Open

exception oops
   break
End Sub

The server log shows that the connection is closed when the object is  
destroyed.

But now, consider the following code.

   dim connection as new VConnection("localhost", "sa", "sa")
   connection.Open

   dim db as new VDatabase(connection)
   db.Open new FolderItem("testDB")
   db.Close

exception oops
   break


Executing this code repeatedly, I exhaust the available connections.   
V4Rb raises exception 533781, ERR_CONNECTION_BROKEN_RECEIVED_PACKET.   
But the server log shows Connection request rejected: ERROR #533761:  
Connection limit reached. I'll suspect a typo in the V4Rb client code  
explains the difference in errors.  The server log shows that the  
database is being closed on the server (even if I omit the db.Close  
call)

When I quit the app, all of the connections are released.

This code also closes the database and releases the connection.

   dim connection as new VConnection("localhost", "sa", "sa")
   connection.Open

   dim db as new VDatabase(connection)
   db.Open new FolderItem("testDB")

   connection.Close

exception oops
   break

So it appears that if I open a connection, then use that connection  
to open a database, then close the database, the connection object is  
not informed of the database being closed.  The result is that the  
connection is not closed when the VConnection object is destroyed.

In the short run, we can work around this by closing the connection  
explicitly.  But I don't want to do that, because I am lazy and  
prefer to load as much work onto the REALbasic framework as possible.



>
>
> 2) this code is not safe from point of view of exception throw.
>     look.
>
> If in loop any method will throw exception, then you will go into  
> last 2
> lines of code. But lines
>
>   c = nil
>   db.Close
>   connection.close
>   connection = nil
>
> Never will be executed. You see?
>

Yes.  But from my point of view it is exception-safe to the extent  
that I trust the REALbasic runtime framework, because DB.Close and  
connection.Close should be called in the object destructor.  Indeed  
it may be better, because exception handling is a little broken right  
now in Mach-O builds.


>
> SAFE code must have such finalization code in both
>
>         normal branch
>         exception branch
>
> May be realbasic have like Java keyword:   'final' ?
>

REALbasic has a Finally block that allows you to do this.


Charles Yeomans