On Tue, Mar 5, 2019 at 6:36 AM Khushboo Vashi < khushboo.va...@enterprisedb.com> wrote:
> > > On Mon, Mar 4, 2019 at 4:05 PM Khushboo Vashi < > khushboo.va...@enterprisedb.com> wrote: > >> >> >> On Mon, Mar 4, 2019 at 4:00 PM Dave Page <dp...@pgadmin.org> wrote: >> >>> >>> >>> On Mon, Mar 4, 2019 at 10:18 AM Khushboo Vashi < >>> khushboo.va...@enterprisedb.com> wrote: >>> >>>> >>>> >>>> On Mon, Mar 4, 2019 at 3:43 PM Dave Page <dp...@pgadmin.org> wrote: >>>> >>>>> Hi >>>>> >>>>> On Mon, Mar 4, 2019 at 5:43 AM Khushboo Vashi < >>>>> khushboo.va...@enterprisedb.com> wrote: >>>>> >>>>>> >>>>>> >>>>>> On Fri, Mar 1, 2019 at 10:01 PM Dave Page <dp...@pgadmin.org> wrote: >>>>>> >>>>>>> In investigating #3656 I found the initial problem to be that when >>>>>>> running in a container, Gunicorn will kill the worker process if a >>>>>>> thread >>>>>>> doesn't respond for 30 seconds by default. I fixed that by making the >>>>>>> timeout match the application session timeout, but it revealed another >>>>>>> issue. >>>>>>> >>>>>>> Given the function below (from the ticket), if you open the query >>>>>>> tool and run: >>>>>>> >>>>>>> SELECT 1; SELECT fails_after(30); >>>>>>> >>>>>>> the async query actually blocks for 30 seconds in the cur.execute() >>>>>>> call in execute_async() in connection.py (line 968). This causes the >>>>>>> entire >>>>>>> app to hang (watch the dashboard requests pile up in pending state in >>>>>>> the >>>>>>> network tab of the browser dev tools). >>>>>>> >>>>>>> If you run just the second SELECT, it works as expected, as does >>>>>>> running something like: >>>>>>> >>>>>>> SELECT 1; SELECT pg_sleep(30); >>>>>>> >>>>>>> Anyone have any idea what's going on? >>>>>>> >>>>>> >>>>>> Connection.poll() blocking the call here. ( connection.py - >>>>>> _wait_timeout >>>>>> function line #1378 state = conn.poll() ) >>>>>> In the asynchronous connection, after executing the query, the >>>>>> conn.poll() is being called to fetch the connection status. >>>>>> It gives the status accordingly but in this case, it is blocking and >>>>>> not giving the status. >>>>>> >>>>> >>>>> If I put a breakpoint on the _wait_timeout call (line 969 in >>>>> execute_async), it hits it *after* the 30 seconds has passed (during which >>>>> time all other queries for cancel or dashboards and status etc don't get >>>>> processed). >>>>> >>>>> Put the breakpoint in the _wait_timeout function itself. The line no >>>> 1378 (state = conn.poll()) is blocking the execution 30 seconds. >>>> >>>>> If I walk down the call stack, it returns from the _cursor.execute >>>>> call in cursor.py and then hangs right before it goes into _wait_timeout. >>>>> >>>> >>> Hmm, yeah - my debugger is doing some weird off-by-one thing. If I put >>> the break point on the while 1, it blocks there! If I put it on the poll() >>> call, then it does do as you describe. >>> >> It's a psycopg2 call, it should return the specific status but it >> doesn't, so right now I am digging the Psycopg2 code. >> > > If I remove RAISE statements and then execute SELECT 1; SELECT > fails_after(30); it works as expected. > > As per the documentation, (Ref: > http://initd.org/psycopg/docs/connection.html) > Connection.poll returns one of the constants defined in Poll constants > <http://initd.org/psycopg/docs/extensions.html#poll-constants>. If it > returns POLL_OK > <http://initd.org/psycopg/docs/extensions.html#psycopg2.extensions.POLL_OK> > then > the connection has been established or the query results are available on > the client. Otherwise wait until the file descriptor returned by fileno() > <http://initd.org/psycopg/docs/connection.html#connection.fileno> is > ready to read or to write. > > So, in case of Raise no file descriptor to read or write, and so before > raising the notice, It will block the connection till the pg_sleep > completes. > Thanks - confirmed, and created a test case for a bug report: https://github.com/psycopg/psycopg2/issues/856 -- Dave Page Blog: http://pgsnake.blogspot.com Twitter: @pgsnake EnterpriseDB UK: http://www.enterprisedb.com The Enterprise PostgreSQL Company
