[Qt-interest] Postgresql-8.3 blocking with qt 3.3.3

[John A. Sullivan III]

Hello, all.  I'm having a bizarre problem with postgresql-8.3 and qt
3.3.3 on Ubuntu 8.0.4 fully patched.  That's my troubleshooting
environment but we originally saw this problem on RHEL 5.2.  We are
using the QPSQL7 driver.  The database has a public schema for shared
production use and each administrator creates a copy of the public
schema for edits and then merges those edits into production at the end
of their session.  This is for the open source network security
management project ISCS (http://iscs.sourceforge.net).

During the update of the security gateways, we spawn several threads to
apply the updates.  When the thread terminates, it creates a custom
event in the main event loop.  The event handler reads the database
version from the thread and updates the public (production) schema with
the value.  The update is made using a QSqlQuery object with very simple
SQL.  The actual query does not seem to matter.  As soon as this query
executes, the public schema seems to block.  It query returns true, the
code logic progresses, but the change has not been made (it is a single
change not bounded by a transaction) and any subsequent action upon the
public schema fails.  Actions upon the edit schema appear to work fine.
Previous updates using the same object work perfectly fine.  Only this
one fails.  The only unique characteristic is it is called within the
custom event handler upon thread termination. The threads open their own
QSqlDatabase and QSqlQuery objects.

I've tracked it down to this specific query by running an independent
instance of psql from a terminal.  As I step through the code, line by
line, I begin a transaction, do a simple update to the public schema and
then rollback the transaction.  It works fine up until I execute the
QSqlQuery.  The query in the code is very simple:

update peps set dbversion=6, current=true where pep='WessonSGGW';

I can run it within a transaction in psql by simply cutting and pasting
from the debugger.  I even coded an even simpler update in its place:

"update peps set comment='fudge' where pep='" + PEPDB + "';"

and it fails just the same.

If I change the update to a select statement, it does not block.  If I
leave the test update from psql running (blocked), and then exit the
ISCS application, the update completes.  This also implies it is not
just blocking within the application; it is blocking the entire public
schema from any access, e.g., command line psql.  Even stranger, the
update blocks itself, i.e., it never completes even though
QSqlQuery::exec() returns true!

I have no idea of what to do next.  Here is the pertinent code snippet:

void SPMMainForm::customEvent( QCustomEvent * ce )
{
. . .
	if (ce->type() == 10002) { // This is a PEPUpdateThread termination event
		QListViewItem * LVI;
		int MaxThreads;
		bool success, NoErrors, SQTempExists = (so->SQEdit != 0);
		QSqlQuery SQTemp( QString::null, SQTempExists ? so->DbEdit : so->DbProd );
//		SQTemp.setForwardOnly(true); // Awaiting a bug fix for ForwardOnly using MySQL
		UpdateThread * puth = (UpdateThread *)ce->data();
		puth = static_cast<UpdateThread *>(ce->data());
		if ( puth->running()) {
			puth->wait();
		}
		QString PEPDB;
		PEPDB = QDeepCopy<QString>(puth->PEP);
		PEPDB = PEPDB.replace("'", "''");
		NoErrors = (puth->ErrorCode.isEmpty() || (puth->ErrorCode.left(20) == "[PEP Update Warning]"));
		if (NoErrors || puth->Retry) { // either there was no error or we errored after a retry
			
//			so->ThreadCounter->lock();
			so->OpenUpdateThreads--; // keeping track of open threads so we know when we are done
//			so->ThreadCounter->unlock();
			
			// Update the dbVersion in the peps database table
			if ( ! so->SQProd->exec("update peps set dbversion=" + QString::number(puth->PEPDbVersion) + ", current=" + (puth->ErrorCode.isEmpty() ? so->True : so->False) + " where pep='" + PEPDB + "';")) {
				success = false;


Can anyone please point me in the right direction? I was delighted to
port this application from MySQL and really do not want to revert back
to MySQL only.  Thanks - John
-- 
John A. Sullivan III
Open Source Development Corporation
+1 207-985-7880
jsullivan at opensourcedevel.com

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