[pgAdmin III] #12: pgScript engine is not threadsafe

#12: pgScript engine is not threadsafe
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Reporter: Andras Fabian | Owner: dpage
Type: bug | Status: new
Priority: major | Milestone:
Component: pgadmin | Version:
Keywords: | Platform:
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We are users of your great tool pgAdmin since we started migrating
projects to PostgreSQl. But just two days ago we noticed a very nasty
bug in the otherwise very great little tool pgScript.

One of our developers needed to fire a lot of updates for a specific
range of values, so he wrote a neat little pgScript code to do it.
Essentially only a loop with some variables which are incremented and
passed to an UPDATE statement. But some days ago he also noticed a very
nasty behavior while running two pgScript in two different query windows
at the same time (he needed to run it against many diferent DBs). As he
had some PRINT outputs to monitor the advance of his script he noticed,
that one script got some absolutely wrong values for one of his
variables.

The I wrote a simplified version of the loop - even left out the SQL
code (!) - to look what happens. And as soon as I run them it TWO query
windows (but started from the same pgAdmin process!) I could notice the
weirdest behaviors one could expect. The most usual was early
termination of the loop - without error messages - but there were also
loops which then run much longer as their target value would have
suggested. In one case, we even got an "unhandled exception" which
completely nuked pgAdmin.

As soon as we did not run in TWO threads in ONE process, but instead run
the Query in two different processes (started pgAdmin twice and opened
one Query window in each), everything was fine. We could run the script
as often as we wanted (and the same time as the other was running) and
nothing went wrong. The script went just trough deterministically.

So, from this observations I would dare to say, that this looks like
some very basic "thread safety" precautions were omitted in the pgScript
code.

I would also say, that this is a dangerous BUG! Think about a similar
script like ours, which has to delete data from a specific range. Now,
what if it deletes more, because the LOOP runs longer?`

And you might also ask, why the hell we would run more than one similar
scripts at the same time? Well, if you need to do a maintenance task
(the same task) for many DBs or Servers, you might easily come up with
the idea to do it from many query windows.

Finally, I would - of course - share the simple script, which helps to
reproduce the odd behavior. With it, I hope, you can quickly pinpoint
the issue and fix it. Before that I would recommend to issue a warning
to users, to never run more then one pgScript rom the same process.

Here is the Script, start it from two query windows (from one process)
at the same time, and you should easily see what I mean:
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DECLARE @v_package;
DECLARE @v_purchaseidmin;
DECLARE @v_purchaseidmax;

DECLARE @v_to;
DECLARE @v_from;
DECLARE @v_i;

SET @v_package = 10;
SET @v_purchaseidmin = 0;
SET @v_purchaseidmax = 120000000;

SET @v_from = @v_purchaseidmin;
SET @v_to = (@v_purchaseidmin + @v_package);
SET @v_i = 1;

WHILE ( @v_from <= @v_purchaseidmax ) BEGIN

PRINT 'from: ' + CAST(@v_from AS STRING);
PRINT 'to: ' + CAST(@v_to AS STRING);
PRINT 'i: ' + CAST(@v_i AS STRING);

-- some SQL code could be executed here

SET @v_from = (@v_from + @v_package);
SET @v_to = (@v_to + @v_package);
SET @v_i = @v_i+1;
END
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Ticket URL: <http://code.pgadmin.org/trac/ticket/12>
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