Re: Changing set_session implementation

Regarding the below problem with readwrite autocommit
connections in default-readonly databases I have whittled
down a test case to the following (attached):

db = u'gnumed_v20' # a database configured "alter database %s set default_transaction_read_only to on"
user = 'gm-dbo' # a user with CREATE DATABASE powers

cmd_def_tx_ro = "SELECT upper(source), name, upper(setting) FROM pg_settings WHERE name = 'default_transaction_read_only'"
cmd_create_db = "create database %s_copy template %s" % (db, db)

import sys
import psycopg2

conn = psycopg2.connect(dbname = db, user = user)
print 'conn:', conn
print 'readonly:', conn.readonly
print 'autocommit:', conn.autocommit
conn.autocommit = False
conn.readonly = False # <========== comment out for success
conn.autocommit = True
print 'autocommit now:', conn.autocommit
conn.readonly = False
print 'readonly now:', conn.readonly
curs = conn.cursor()
curs.execute(cmd_def_tx_ro)
print 'should show DEFAULT_TRANSACTION_READ_ONLY set to ON'
print curs.fetchall()
curs.close()
conn.commit()
print 'the following CREATE DATABASE should fail'
curs = conn.cursor()
try:
curs.execute(cmd_create_db)
except psycopg2.InternalError as ex:
print 'SQL failed:'
print ex

print 'shutting down'

curs.close()
conn.rollback()
conn.close()

which shows on the console:

ncq(at)hermes:~$ python x-test-default_ro.py
conn: <connection object at 0xb674e6bc; dsn: 'dbname=gnumed_v20 user=gm-dbo', closed: 0>
readonly: None
autocommit: False
autocommit now: True
readonly now: False
should show DEFAULT_TRANSACTION_READ_ONLY set to ON
[('DATABASE', 'default_transaction_read_only', 'ON')]
the following CREATE DATABASE should fail
SQL failed:
cannot execute CREATE DATABASE in a read-only transaction

shutting down

The PostgreSQL log shows:

2017-08-03 21:00:19 GMT LOG: 00000: Verbindung empfangen: Host=[local]
2017-08-03 21:00:19 GMT ORT: BackendInitialize, postmaster.c:4135
2017-08-03 21:00:19 GMT LOG: 00000: Verbindung autorisiert: Benutzer=gm-dbo Datenbank=gnumed_v20
2017-08-03 21:00:19 GMT ORT: PerformAuthentication, postinit.c:272
2017-08-03 21:00:19 GMT LOG: 00000: statement: SELECT upper(source), name, upper(setting) FROM pg_settings WHERE name = 'default_transaction_read_only'
2017-08-03 21:00:19 GMT LOCATION: exec_simple_query, postgres.c:935
2017-08-03 21:00:19 GMT LOG: 00000: statement: create database gnumed_v20_copy template gnumed_v20
2017-08-03 21:00:19 GMT LOCATION: exec_simple_query, postgres.c:935
2017-08-03 21:00:19 GMT ERROR: 25006: cannot execute CREATE DATABASE in a read-only transaction
2017-08-03 21:00:19 GMT LOCATION: PreventCommandIfReadOnly, utility.c:236
2017-08-03 21:00:19 GMT STATEMENT: create database gnumed_v20_copy template gnumed_v20
2017-08-03 21:00:19 GMT LOG: 00000: disconnection: session time: 0:00:00.007 user=gm-dbo database=gnumed_v20 host=[local]
2017-08-03 21:00:19 GMT LOCATION: log_disconnections, postgres.c:4501

Commenting out the marked line makes it work and the log shows:

2017-08-03 21:01:24 GMT LOG: 00000: Verbindung empfangen: Host=[local]
2017-08-03 21:01:24 GMT ORT: BackendInitialize, postmaster.c:4135
2017-08-03 21:01:24 GMT LOG: 00000: Verbindung autorisiert: Benutzer=postgres Datenbank=postgres
2017-08-03 21:01:24 GMT ORT: PerformAuthentication, postinit.c:272
2017-08-03 21:01:25 GMT LOG: 00000: Anweisung: drop database gnumed_v20_copy ;
2017-08-03 21:01:25 GMT ORT: exec_simple_query, postgres.c:935
2017-08-03 21:01:26 GMT LOG: 00000: Verbindungsende: Sitzungszeit: 0:00:02.324 Benutzer=postgres Datenbank=postgres Host=[local]
2017-08-03 21:01:26 GMT ORT: log_disconnections, postgres.c:4501
2017-08-03 21:01:33 GMT LOG: 00000: Verbindung empfangen: Host=[local]
2017-08-03 21:01:33 GMT ORT: BackendInitialize, postmaster.c:4135
2017-08-03 21:01:33 GMT LOG: 00000: Verbindung autorisiert: Benutzer=gm-dbo Datenbank=gnumed_v20
2017-08-03 21:01:33 GMT ORT: PerformAuthentication, postinit.c:272
2017-08-03 21:01:33 GMT LOG: 00000: statement: SET default_transaction_read_only TO 'off'
2017-08-03 21:01:33 GMT LOCATION: exec_simple_query, postgres.c:935
2017-08-03 21:01:33 GMT LOG: 00000: statement: SELECT upper(source), name, upper(setting) FROM pg_settings WHERE name = 'default_transaction_read_only'
2017-08-03 21:01:33 GMT LOCATION: exec_simple_query, postgres.c:935
2017-08-03 21:01:33 GMT LOG: 00000: statement: create database gnumed_v20_copy template gnumed_v20
2017-08-03 21:01:33 GMT LOCATION: exec_simple_query, postgres.c:935
2017-08-03 21:01:37 GMT LOG: 00000: duration: 3447.360 ms
2017-08-03 21:01:37 GMT LOCATION: exec_simple_query, postgres.c:1164
2017-08-03 21:01:37 GMT LOG: 00000: disconnection: session time: 0:00:03.454 user=gm-dbo database=gnumed_v20 host=[local]
2017-08-03 21:01:37 GMT LOCATION: log_disconnections, postgres.c:4501

So, under some circumstances psycopg2 does send the

2017-08-03 21:01:33 GMT LOG: 00000: statement: SET default_transaction_read_only TO 'off'

while sometimes it does not, despite both times being in
autocommit=True and readonly=False when the "create database"
is sent.

Of course, the

conn.autocommit = False
conn.readonly = False # <========== comment out for success
conn.autocommit = True
conn.readonly = False

seems redundant but that is the relevant sequence of events
condensed from a larger body of code. Redundant as it may
seem it should still work, no ?

I am not sure I fully grok what is happening...

Thanks for insights,
Karsten

On Wed, Aug 02, 2017 at 09:38:21PM +0200, Karsten Hilbert wrote:

> On Wed, Aug 02, 2017 at 03:10:15PM +0100, Daniele Varrazzo wrote:
>
> > If you want to be absolutely sure about what the adapter does I
> > suggest you to enable statements log on the server and check what
> > statements are produced by psycopg. If you find any behaviour
> > inconsistent with what documented please let us know.
>
> One thing I noticed is that it seems pycopg2 does not send a
> "set default_transaction_read_only to off" when
>
> - a database had been created and ALTERed to permanently
> be "default_transaction_read_only to ON"
>
> - a transaction had been opened on a connection with
>
> BEGIN ISOLATION LEVEL SERIALIZABLE READ WRITE
>
> in order to run various comments (such as setting
> default client timezone)
>
> - that transaction had been committed
>
> - the connection is switched to autocommit=true
> (for running DROP DATABASE et al)
>
> 2017-08-02 19:18:13 GMT LOG: 00000: Verbindung empfangen: Host=[local]
> 2017-08-02 19:18:13 GMT ORT: BackendInitialize, postmaster.c:4135
> 2017-08-02 19:18:13 GMT LOG: 00000: Verbindung autorisiert: Benutzer=postgres Datenbank=gnumed_v20
> 2017-08-02 19:18:13 GMT ORT: PerformAuthentication, postinit.c:272
> 2017-08-02 19:18:13 GMT LOG: 00000: statement: BEGIN ISOLATION LEVEL SERIALIZABLE READ WRITE
> 2017-08-02 19:18:13 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:13 GMT LOG: 00000: statement: set timezone to 'Europe/Madrid'
> 2017-08-02 19:18:13 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:13 GMT LOG: 00000: statement: COMMIT
> 2017-08-02 19:18:13 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:13 GMT LOG: 00000: statement: BEGIN ISOLATION LEVEL SERIALIZABLE READ WRITE
> 2017-08-02 19:18:13 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:13 GMT LOG: 00000: statement: set lc_messages to 'C'
> 2017-08-02 19:18:13 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:13 GMT LOG: 00000: statement: select md5(gm.concat_table_structure(20::integer)) as md5
> 2017-08-02 19:18:13 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:14 GMT LOG: 00000: duration: 1156.407 ms
> 2017-08-02 19:18:14 GMT LOCATION: exec_simple_query, postgres.c:1164
> 2017-08-02 19:18:14 GMT LOG: 00000: statement: SELECT datname FROM pg_database WHERE datname='gnumed_v21'
> 2017-08-02 19:18:14 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:14 GMT LOG: 00000: statement: COMMIT
> 2017-08-02 19:18:14 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:14 GMT LOG: 00000: statement: DROP DATABASE "gnumed_v21"
> 2017-08-02 19:18:14 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:14 GMT ERROR: 25006: cannot execute DROP DATABASE in a read-only transaction
> 2017-08-02 19:18:14 GMT LOCATION: PreventCommandIfReadOnly, utility.c:236
> 2017-08-02 19:18:14 GMT STATEMENT: DROP DATABASE "gnumed_v21"
> 2017-08-02 19:18:14 GMT LOG: 00000: statement: SET default_transaction_isolation TO DEFAULT
> 2017-08-02 19:18:14 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:14 GMT LOG: 00000: statement: SET default_transaction_read_only TO DEFAULT
> 2017-08-02 19:18:14 GMT LOCATION: exec_simple_query, postgres.c:935
> 2017-08-02 19:18:14 GMT LOG: 00000: disconnection: session time: 0:00:01.183 user=postgres database=gnumed_v20 host=[local]
> 2017-08-02 19:18:14 GMT LOCATION: log_disconnections, postgres.c:4501
>
> The only other indirect proof I have for this is: if I insert
> a manual "set default_transaction_read_only to off" right
> before the "drop database ..." (but after having been
> switched to autocommit) it works as expected.
>
> If the connection has NOT been used for any other
> transactions (that is, if it is switched to autocommit=true
> and then to readonly=off) it also works as expected -
> psycopg2 sends a "set default_transaction_read_only to off".
>
> Karsten
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