from django.db import connection
from django.conf import settings

#
# NOTE ABOUT LOCKING
#
# This decorator is needed to prevent concurrent access to
# data in code sections that have to be serialized. One option
# is to set default isolation level of your database management
# system to SERIALIZABLE which has a performance drawback. There
# is another option, namely to set transaction isolation level
# at beginning of transaction via SET TRANSACTION ISOLATION
# LEVEL SERIALIZABLE or similar statement, but this is not
# possible in Django with TransactionMiddleware where transaction
# is opened at the start of a HTTP request.
#
# Therefore this method implements EXCLUSIVE table locks to
# acquire exclusive access for code sections where needed.
#

# Check for database drivers, this is not done in the decorator
# so this check is executed only once upon module load
if settings.DATABASES['default']['ENGINE'].find('postgresql') != -1:
    # READ COMMITTED is the default isolation level in PostgreSQL so
    # we add explicit locking to support default isolation level.
    LOCK_TYPE = "postgresql"
elif settings.DATABASES['default']['ENGINE'].find('mysql') != -1:
    # MySQL InnoDB default isolation level is REPEATABLE READ so we
    # add explicit locking to support default isolation level.
    LOCK_TYPE = "mysql"
elif settings.DATABASES['default']['ENGINE'].find('sqlite') != -1:
    # Locking is not necessary for SQLite as default isolation
    # level for SQLite is SERIALIZABLE. (This means concurrent
    # transactions can fail and users have to retry their requests.)
    # You should probably add locking if you change isolation level.
    LOCK_TYPE = None
else:
    LOCK_TYPE = None

def require_lock(*models):
    tables = [model._meta.db_table for model in models]
    
    def _lock(func):
        def _do_lock(*args,**kws):
            cursor = connection.cursor()

            if LOCK_TYPE == "postgresql":
                cursor.execute("LOCK TABLE %s IN ROW EXCLUSIVE MODE" % ', '.join(tables))
            elif LOCK_TYPE == "mysql":
                cursor.execute("LOCK TABLES %s" % ', '.join(["%s WRITE" % x for x in tables]))
            
            try:
                return func(*args,**kws)
            finally:
                if LOCK_TYPE == "mysql":
                    cursor.execute("UNLOCK TABLES")
                
                if cursor:
                    cursor.close()
        return _do_lock
    return _lock

def model_lock(model):
    cursor = connection.cursor()
    
    if LOCK_TYPE == "postgresql":
        cursor.execute("SELECT 1 FROM %s WHERE %s = '%s' FOR UPDATE" % (model._meta.db_table, model._meta.pk.name, model.pk))