psycopg2 weirdness

[Yang Zhang]

For posterity: the problem turned out to be a second request being made 
in quick succession by the client-side Javascript, causing the web.py 
request handler to run in multiple threads concurrently.  The request 
handlers don't create their own Postgresql connections, but instead 
share one across all sessions.  The absence of any synchronization 
protecting this connection resulted in myriad errors and crashes in the 
C extension module (in both pygresql and psycopg2).

Neha Gupta wrote:
> Hey,
> 
> I only have little experience with web.py and psycopg2 and am running
> into a weird problem, I'd appreciate any help I can get with debugging
> it.
> 
> I wrote a simple program that works and I don't see any crash:
> ----
> import psycopg2
> 
> try:
> 	database_conn = psycopg2.connect("dbname='dbname' user='username'
> host='hostname'");
> except:
> 	print "Unable to connect to the  database!"
> 
> database_conn.set_isolation_level(0)
> cur = database_conn.cursor();
> 
> while True:
>     query = "SELECT avg(dep_delay), extract(hour from crs_dep_time) as
> crs_dep_hour, origin from flightdata where date = '01-05-2007' group
> by origin, crs_dep_hour order by origin, crs_dep_hour";
>     cur.execute(query)
>     rows = cur.fetchall()
>     print rows
> -----
> 
> However, I have a small website built using web.py framework which has
> a date picker that lets the user pick a date and it takes the user to
> a new url such as: localhost:8080/departures/01-05-2007. I issue a
> query to my database for the date selected by the user and retrieve
> the results.  The problem now is that if I select different dates
> directly by changing the url then everything works but as soon as I
> pick a date from date picker the server crashes. I removed the date
> picker and made it just a text box but as soon as I hit the submit
> button, server crashes so I know it is not the date picker that
> causing trouble.
> ---
> class departures:
> 	def buildDepartureTableHtml(self, date):
>         	web.debug('date', date)
> 		# Issue the query.
> 		# select avg(dep_delay), extract(hour from crs_dep_time) as
> crs_dep_hour, origin from flightdata where date = '2007-02-15' group
> by origin, crs_dep
> 		# _hour order by origin, crs_dep_hour;
> 		try:
>                       web.debug("About to issue query")
> 		#	query = "SELECT avg(dep_delay), extract(hour from crs_dep_time) as
> crs_dep_hour, origin from flightdata where date = '" + date + "' group
> by origin, crs_dep_hour order by origin, crs_dep_hour";
> 			query = "SELECT avg(dep_delay), extract(hour from crs_dep_time) as
> crs_dep_hour, origin from flightdata where date = '01-05-2007' group
> by origin, crs_dep_hour order by origin, crs_dep_hour";
> 			cur.execute(query)
> 			web.debug('query executed!')
> 			rows = cur.fetchall()
> 			web.debug('rows fetched!')
> 			web.debug(rows)
> 		except Exception, e:
> 			print repr(e)
> 			database_conn.rollback()
> 			return "<div id='welcome-text'>Invalid Date</div>"
> --
> // JS code
> function submitForm() {
>       var date = ($("date").value).replace(/\//g,"-");
>      window.location = "http://" + window.location.host + "/
> departures/" + date;
> }
> 
> You can see above that I even ignored the date passed from the form
> and I have hardcoded '01-05-2007'. The message "About to issue query"
> gets printed as well as the right date chosen from the date picker but
> then I see the following:
> 
> Assertion failed: (str != NULL), function PyString_FromString, file
> Objects/stringobject.c, line 107.
> Abort trap
> 
> with a pop that says: "The application Python quit unexpectedly. The
> problem may have been caused by the _psycopg.so plug-in".
> --
> I don't understand the error message above. The date did get passed
> correctly and am now not even using it, I use the hard coded date. So
> what is going on?
> 
> Any help would be great.
> 
> Thank you!
> Neha
> --
> http://mail.python.org/mailman/listinfo/python-list


-- 
Yang Zhang
http://www.mit.edu/~y_z/