Index: django/db/models/sql/compiler.py =================================================================== --- django/db/models/sql/compiler.py (revision 13301) +++ django/db/models/sql/compiler.py (working copy) @@ -114,6 +114,10 @@ result.append('LIMIT %d' % val) result.append('OFFSET %d' % self.query.low_mark) + if self.query.select_for_update and self.connection.features.has_select_for_update: + nowait = self.query.select_for_update_nowait and self.connection.features.has_select_for_update + result.append("%s" % self.connection.ops.for_update_sql(nowait=nowait)) + return ' '.join(result), tuple(params) def as_nested_sql(self): Index: django/db/models/sql/query.py =================================================================== --- django/db/models/sql/query.py (revision 13301) +++ django/db/models/sql/query.py (working copy) @@ -11,7 +11,7 @@ from django.utils.tree import Node from django.utils.datastructures import SortedDict from django.utils.encoding import force_unicode -from django.db import connections, DEFAULT_DB_ALIAS +from django.db import connections, DEFAULT_DB_ALIAS, DatabaseError from django.db.models import signals from django.db.models.fields import FieldDoesNotExist from django.db.models.query_utils import select_related_descend, InvalidQuery @@ -23,8 +23,16 @@ ExtraWhere, AND, OR) from django.core.exceptions import FieldError -__all__ = ['Query', 'RawQuery'] +__all__ = ['Query', 'RawQuery', 'LockNotAvailable'] + +class LockNotAvailable(DatabaseError): + ''' + Raised when a query fails because a lock was not available. + ''' + pass + + class RawQuery(object): """ A single raw SQL query @@ -84,9 +92,13 @@ def _execute_query(self): self.cursor = connections[self.using].cursor() - self.cursor.execute(self.sql, self.params) + try: + self.cursor.execute(self.sql, self.params) + except DatabaseError, e: + if self.connection.features.has_select_for_update_nowait and self.connection.ops.signals_lock_not_available(e): + raise LockNotAvailable(*e.args) + raise - class Query(object): """ A single SQL query. @@ -131,6 +143,8 @@ self.order_by = [] self.low_mark, self.high_mark = 0, None # Used for offset/limit self.distinct = False + self.select_for_update = False + self.select_for_update_nowait = False self.select_related = False self.related_select_cols = [] @@ -259,6 +273,8 @@ obj.order_by = self.order_by[:] obj.low_mark, obj.high_mark = self.low_mark, self.high_mark obj.distinct = self.distinct + obj.select_for_update = self.select_for_update + obj.select_for_update_nowait = self.select_for_update_nowait obj.select_related = self.select_related obj.related_select_cols = [] obj.aggregates = deepcopy(self.aggregates, memo=memo) @@ -359,6 +375,7 @@ query.clear_ordering(True) query.clear_limits() + query.select_for_update = False query.select_related = False query.related_select_cols = [] query.related_select_fields = [] Index: django/db/models/manager.py =================================================================== --- django/db/models/manager.py (revision 13301) +++ django/db/models/manager.py (working copy) @@ -164,6 +164,9 @@ def order_by(self, *args, **kwargs): return self.get_query_set().order_by(*args, **kwargs) + def select_for_update(self, *args, **kwargs): + return self.get_query_set().select_for_update(*args, **kwargs) + def select_related(self, *args, **kwargs): return self.get_query_set().select_related(*args, **kwargs) Index: django/db/models/__init__.py =================================================================== --- django/db/models/__init__.py (revision 13301) +++ django/db/models/__init__.py (working copy) @@ -11,6 +11,7 @@ from django.db.models.fields.subclassing import SubfieldBase from django.db.models.fields.files import FileField, ImageField from django.db.models.fields.related import ForeignKey, OneToOneField, ManyToManyField, ManyToOneRel, ManyToManyRel, OneToOneRel +from django.db.models.sql.query import LockNotAvailable from django.db.models import signals # Admin stages. Index: django/db/models/query.py =================================================================== --- django/db/models/query.py (revision 13301) +++ django/db/models/query.py (working copy) @@ -424,6 +424,7 @@ del_query._for_write = True # Disable non-supported fields. + del_query.query.select_for_update = False del_query.query.select_related = False del_query.query.clear_ordering() @@ -585,6 +586,18 @@ else: return self._filter_or_exclude(None, **filter_obj) + def select_for_update(self, **kwargs): + """ + Returns a new QuerySet instance that will select objects with a + FOR UPDATE lock. + """ + # Default to false for nowait + nowait = kwargs.pop('nowait', False) + obj = self._clone() + obj.query.select_for_update = True + obj.query.select_for_update_nowait = nowait + return obj + def select_related(self, *fields, **kwargs): """ Returns a new QuerySet instance that will select related objects. Index: django/db/backends/mysql/base.py =================================================================== --- django/db/backends/mysql/base.py (revision 13301) +++ django/db/backends/mysql/base.py (working copy) @@ -124,9 +124,12 @@ allows_group_by_pk = True related_fields_match_type = True allow_sliced_subqueries = False + has_select_for_update = True + has_select_for_update_nowait = False class DatabaseOperations(BaseDatabaseOperations): compiler_module = "django.db.backends.mysql.compiler" + signals_deadlock = lambda self, e: e.args[0] == ER.LOCK_DEADLOCK def date_extract_sql(self, lookup_type, field_name): # http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html Index: django/db/backends/oracle/base.py =================================================================== --- django/db/backends/oracle/base.py (revision 13301) +++ django/db/backends/oracle/base.py (working copy) @@ -50,6 +50,8 @@ uses_savepoints = True can_return_id_from_insert = True allow_sliced_subqueries = False + has_select_for_update = True + has_select_for_update_nowait = True class DatabaseOperations(BaseDatabaseOperations): @@ -279,6 +281,12 @@ 'column': column_name}) return output + def signals_deadlock(self, exception): + return exception.args[0].code == 60 + + def signals_lock_not_available(self, exception): + return exception.args[0].code == 54 + def start_transaction_sql(self): return '' Index: django/db/backends/__init__.py =================================================================== --- django/db/backends/__init__.py (revision 13301) +++ django/db/backends/__init__.py (working copy) @@ -96,6 +96,8 @@ # integer primary keys. related_fields_match_type = False allow_sliced_subqueries = True + has_select_for_update = False + has_select_for_update_nowait = False class BaseDatabaseOperations(object): """ @@ -185,6 +187,15 @@ ordering. """ return [] + def for_update_sql(self, nowait=False): + """ + Return FOR UPDATE SQL clause to lock row for update + """ + if nowait: + nowaitstr = ' NOWAIT' + else: + nowaitstr = '' + return 'FOR UPDATE' + nowaitstr def fulltext_search_sql(self, field_name): """ Index: django/db/backends/postgresql_psycopg2/base.py =================================================================== --- django/db/backends/postgresql_psycopg2/base.py (revision 13301) +++ django/db/backends/postgresql_psycopg2/base.py (working copy) @@ -19,6 +19,7 @@ try: import psycopg2 as Database import psycopg2.extensions + from psycopg2 import errorcodes except ImportError, e: from django.core.exceptions import ImproperlyConfigured raise ImproperlyConfigured("Error loading psycopg2 module: %s" % e) @@ -67,8 +68,12 @@ class DatabaseFeatures(BaseDatabaseFeatures): needs_datetime_string_cast = False can_return_id_from_insert = False + has_select_for_update = True + has_select_for_update_nowait = True class DatabaseOperations(PostgresqlDatabaseOperations): + signals_deadlock = lambda self, e: e.pgcode == errorcodes.DEADLOCK_DETECTED + signals_lock_not_available = lambda self, e: e.pgcode == errorcodes.LOCK_NOT_AVAILABLE def last_executed_query(self, cursor, sql, params): # With psycopg2, cursor objects have a "query" attribute that is the # exact query sent to the database. See docs here: Index: docs/ref/models/querysets.txt =================================================================== --- docs/ref/models/querysets.txt (revision 13301) +++ docs/ref/models/querysets.txt (working copy) @@ -963,6 +963,53 @@ # queries the database with the 'backup' alias >>> Entry.objects.using('backup') +``select_for_update(nowait=False)`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Returns a queryset that will lock rows until the end of the transaction, +generating a SELECT ... FOR UPDATE statement on supported databases. + +For example:: + + entries = Entry.objects.select_for_update().filter(author=request.user) + +All matched entries will be locked until the end of the transaction block, +meaning that other transactions will be prevented from changing or acquiring +locks on them. + +Usually, if another transaction has already acquired a lock on one of the +selected rows, the query will block until the lock is released. If this is +not the behaviour you want, call ``select_for_update(nowait=True)``. This will +make the call non-blocking. If a conflicting lock is already acquired by +another transaction, ``django.db.models.LockNotAvailable`` will be raised when +the queryset is evaluated. + +Using blocking locks on a database can lead to deadlocks. This occurs when two +concurrent transactions are both waiting on a lock the other transaction +already holds. To deal with deadlocks, wrap your views that use +``select_for_update(nowait=False)`` with the +``django.views.decorators.deadlock.handle_deadlocks`` decorator. + +For example:: + + from django.db import transaction + from django.views.decorators.deadlock import handle_deadlocks + + @handle_deadlocks(max_retries=2) + @transaction.commit_on_success + def my_view(request): + ... + +If the database engine detects a deadlock involving ``my_view`` and decides +to abort its transaction, it will be automatically retried. If deadlocks keep +occurring after two repeated attempts, +``django.views.decorators.DeadlockError`` will be raised, which can be +propagated to the user or handled in a middleware. + +Currently the ``postgresql_psycopg2``, ``oracle``, and ``mysql`` +database backends support ``select_for_update()`` but MySQL has no +support for the ``nowait`` argument. Other backends will simply +generate queries as if ``select_for_update()`` had not been used. QuerySet methods that do not return QuerySets --------------------------------------------- Index: docs/ref/databases.txt =================================================================== --- docs/ref/databases.txt (revision 13301) +++ docs/ref/databases.txt (working copy) @@ -345,6 +345,15 @@ column types have a maximum length restriction of 255 characters, regardless of whether ``unique=True`` is specified or not. +Row locking with ``QuerySet.select_for_update()`` +------------------------------------------------- + +MySQL does not support the NOWAIT option to the SELECT ... FOR UPDATE +statement. However, you may call the ``select_for_update()`` method of a +queryset with ``nowait=True``. In that case, the argument will be silently +discarded and the generated query will block until the requested lock can be +acquired. + .. _sqlite-notes: SQLite notes