Checking postgres database existense using django - throws OS error .

Question

I have a django application running in a virtual environment using supervisor . Supervisor is run by root and application is run by user ubuntu .

I want to check whether a database exist in postgres or not. My function below works well in normal python shell (Repl) and even when I run my application using python migrate.py runserver and even in django shell .

However as soon as I start the application using supervisor and that code block executes , I get the following exception -

Exception Type: OSError Exception Value:[Errno 2] No such file or directory

Here's the function which is

def database_exists(database_name):
    try:
         db= "anydbname"
         c1 = "psql -U postgres -lqt"
         c2 = "cut -d | -f 1"
         c3 = "grep -w " + db              

         ps1 = subprocess.Popen(c1.split(),stdout=subprocess.PIPE)
         ps2 = subprocess.Popen(c2.split(),stdin=ps1.stdout, stdout=subprocess.PIPE)
         ps3 = subprocess.Popen(c3.split(),stdin=ps2.stdout , stdout=subprocess.PIPE)
         result = ps3.communicate()[0]  # if empty , db not found 
         result = result.strip()
         if result == "" :
             return False
         else :
             return True 
    except Exception, e:
        raise e
        return False, str(e)

I failed to understand what is the exact directory or file it wants to find.Is it any kind of permission issue ? But even the normal shells are run using ubuntu user so doesn't seem to be permission error . How to debug it which file not found when run in supervisor ? I added the logs in supervisor but it only shows <request url > HTTP/1.0" 500 so no clues about it .

Here's the supervisor config

[program:myprog]
environment=PATH="/home/ubuntu/.virtualenvs/myvirtualenv/bin"
command=python /var/www/myapp/manage.py rungevent 127.0.0.1:8010 250
directory=/var/www/myapp
autostart=true
autorestart=true
redirect_stderr=True
killasgroup=true
stopasgroup=true
user=ubuntu
stdout_logfile = /var/log/supervisor/myapp.log
stderr_logfile = /var/log/supervisor/myapp-err.log

This could be a case where its not able to find psql command. Try giving complete path where psql is present. — redoc
– redoc, Commented May 2, 2016 at 6:11
hey redoc ! you nailed it man . Exactly this was the reason. This code was a part of a big system . we did few parts using pyscopg2 as suggested by @Sergey below however you saved it man. — Ankit Kulkarni
– Ankit Kulkarni, Commented May 2, 2016 at 7:30
I got the reason of it . We reassigned the PATH variable of system while in supervisor so its not able to find path — Ankit Kulkarni
– Ankit Kulkarni, Commented May 2, 2016 at 7:31

Sergey Gornostaev · Accepted Answer · 2016-05-02 06:11:47Z

1

You don't need so sophisticated way to test database existence. Try this:

import psycopg2

def database_exists(database_name):
    con = None
    res = None
    try:
        con = psycopg2.connect(database="test", user="test", password="abcd", host="127.0.0.1") #It's better to get the parameters from settings
        cur = con.cursor()
        cur.execute("SELECT exists(SELECT 1 from pg_catalog.pg_database where datname = %s)", (database_name,))
        res = cur.fetchone()[0]
    except psycopg2.DatabaseError as e:
        res = False
    finally:
        if con:
            con.close()
    return res

edited May 2, 2016 at 6:11

answered May 2, 2016 at 6:06

Sergey Gornostaev

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3 Comments

Ankit Kulkarni Over a year ago

Thanks for the answer . This seems to be a better way to do query rather the psql . 100% agreed . Thanks a lot .

Ankit Kulkarni Over a year ago

I upvoted the answer however couldn't accept it as the solution to my problem was with supervisor config

Sergey Gornostaev Over a year ago

Regarding the supervisor, try to use absolute paths to executables - psql, cut and grep.

Ankit Kulkarni · Accepted Answer · 2016-05-02 08:36:51Z

0

The problem was with supervisor config . It was trying to find psql and not getting path of it . Reason was the wrong path variable in supervisor conf . Here's the correct setting which worked well .

Change : Removed PATH and specified the full executable path of virtual env's python .

[program:myprog]
command=/home/ubuntu/.virtualenvs/myvirtualenv/bin/python /var/www/myapp/manage.py rungevent 127.0.0.1:8010 250
directory=/var/www/myapp
autostart=true
autorestart=true
redirect_stderr=True
killasgroup=true
stopasgroup=true
user=ubuntu
stdout_logfile = /var/log/supervisor/myapp.log
stderr_logfile = /var/log/supervisor/myapp-err.log

edited May 2, 2016 at 8:36

answered May 2, 2016 at 7:57

Ankit Kulkarni

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