[New-bugs-announce] [issue27334] pysqlite3 context manager not performing rollback when a database is locked elsewhere for non-DML statements
report at bugs.python.org
Thu Jun 16 12:45:50 EDT 2016
New submission from Luca Citi:
I have reported this bug to the pysqlite module for python2 ( https://github.com/ghaering/pysqlite/issues/103 ) but I also report it here because it applies to python3 too.
The pysqlite3 context manager does not perform a rollback when a transaction fails because the database is locked by some other process performing non-DML statements (e.g. during the sqlite3 command line .dump method).
To reproduce the problem, open a terminal and run the following:
sqlite3 /tmp/test.db 'drop table person; create table person (id integer primary key, firstname varchar)'
echo -e 'begin transaction;\nselect * from person;\n.system sleep 1000\nrollback;' | sqlite3 /tmp/test.db
Leave this shell running and run the python3 interpreter from a different shell, then type:
con = sqlite3.connect('/tmp/test.db')
con.execute("insert into person(firstname) values (?)", ("Jan",))
You should receive the following:
1 with con:
2 con.execute("insert into person(firstname) values (?)", ("Jan",))
----> 3 pass
OperationalError: database is locked
Without exiting python, switch back to the first shell and kill the `'echo ... | sqlite3'` process. Then run:
sqlite3 /tmp/test.db .dump
you should get:
/**** ERROR: (5) database is locked *****/
ROLLBACK; -- due to errors
This means that the python process never executed a `rollback` and is still holding the lock. To release the lock one can exit python (clearly, this is not the intended behaviour of the context manager).
I believe the reason for this problem is that the exception happened in the implicit `commit` that is run on exiting the context manager, rather than inside it. In fact the exception is in the `pass` line rather than in the `execute` line. This exception did not trigger a `rollback` because the it happened after `pysqlite_connection_exit` checks for exceptions.
The expected behaviour (pysqlite3 rolling back and releasing the lock) is recovered if the initial blocking process is a Data Modification Language (DML) statement, e.g.:
echo -e 'begin transaction; insert into person(firstname) values ("James");\n.system sleep 1000\nrollback;' | sqlite3 /tmp/test.db
because this raises an exception at the `execute` time rather than at `commit` time.
To fix this problem, I think the `pysqlite_connection_exit` function in src/connection.c should handle the case when the commit itself raises an exception, and invoke a rollback. Please see patch attached.
components: Extension Modules
title: pysqlite3 context manager not performing rollback when a database is locked elsewhere for non-DML statements
versions: Python 3.6
Added file: http://bugs.python.org/file43420/fix_pysqlite_connection_exit.patch
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