Fast Shutdown and Transactions

I'm implementing failover for streaming replication using HAProxy. I'm  
trying to implement soft failover, (or failback), that will give  
applications in the midst of transaction a clear indication of the  
state of their transaction.

My question is, if you use:

pg_ctl -D [DIR] stop -m fast

and the application has issue a commit if a transaction and the  
transaction has completed, will postmaster send an affirmative message  
before shutting down? Or will it just close the socket?

I can see that stopping fast while a transaction is open in psql will  
not generate a message until the next statement.

I'm concerned about the case where an application detects that the  
connection has been terminated and assumes that the transaction  
failed, so it reattempts the transaction and creates duplicates. I'd  
like applications to be able to assume that a dropped connection is a  
failed transaction.

Is there some other, better way to check to see if your transaction  
was successful or not? One that can be performed across a subsequent  
connection?

--
Alan Gutierrez - http://twitter.com/bigeasy - http://github.com/bigeasy