[racket] racket/db exception handling and unmanaged transactions with postgres

According to the manual,
http://docs.racket-lang.org/db/query-api.html#%28part._transactions%29,
 section
3.5,  postgres transactions can be issued directly in SQL (so called
'unmanaged').   Here is a short code segment that fails doing unmanaged
transactions (and nothing else).

My question here is whether this code is doing unmanaged transactions
correctly, and they are not working as advertised, or if there is something
left out like an explicit reset to the connection after it throws an
exception.

For purposes of illustration, I run the following query here in the psql
shell to show the the error message that it correctly issues:

=> insert into x_unique_counters values ('table_author','1');
ERROR:  duplicate key value violates unique constraint
"x_unique_counters_pkey"
DETAIL:  Key (name)=(table_author) already exists.


Now the racket program is ready for this kind of problem, here 'the-query'
is the same query just shown above, but this time issued from this code


       (begin
            (semaphore-wait (current-db-semaphore))
            (query-exec pgc  "begin")
            (begin0
             (with-handlers
               (
                 [(lambda (v) #t) ; this handler catches anything
                   (lambda (v)
                     (query-exec pgc "rollback")
                     (semaphore-post (current-db-semaphore))
                     (raise v)
                     )
                   ]
                 )

               (query-exec pgc the-query) ; this throws an exception

               )
              (db-exec "commit")
              (semaphore-post (current-db-semaphore))
            ))
        ]
      ))


So when query-exec throws an exception, the handler grabs it.  Then the
handler executes an SQL rollback to cancel the transaction.  All appears to
be good, but then bang!  we get an exception in the handler while doing the
rollback:

>(db-exec* '("rollback"))
query-exec: current transaction is invalid


All attempts to do any further queries on the connection fail with the same
error.   It appears to be foo-bar and a new connection must be made before
any further SQL can be issued against the database.

Hmm, what is going on here?  Unmanaged connections not working as
advertised .. or some switch that must be flipped to tell the connection to
work again?

-Thomas
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20150120/59ee3243/attachment.html>