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Durable decorator
import functools
from django.conf import settings
from django.db import transaction, utils
def durable(func):
"""
Decorator to ensure that a function is not being called within an atomic block.
Usage:
@durable
def some_function(...):
with transaction.atomic():
...
Code decorated like this is guaranteed to be *durable* - that is, not at risk of being rolled
back due to an exception that happens after this function has completed.
This is achieved by enforcing that:
1. The function does not begin in the context of a currently open transaction.
2. The function does not leave work uncommitted.
Warning: This may not work with SQLite (which requires workarounds for bugs in the stdlib
sqlite3 module).
Disabling this behaviour in tests
---------------------------------
This behaviour doesn't play well with tests that are wrapped in transactions.
It can be disabled by setting DISABLE_DURABILITY_CHECKING to True in your Django settings.
"""
@functools.wraps(func)
def wrapper(*args, **kwargs):
durability_checking_enabled = not getattr(
settings, "DISABLE_DURABILITY_CHECKING", False
)
if durability_checking_enabled and _is_in_atomic_block():
raise utils.ProgrammingError(
"A durable function must not be called within a database transaction."
)
return_value = func(*args, **kwargs)
if durability_checking_enabled and _db_may_have_uncommitted_work():
# Clean up first, otherwise we may see errors later that will mask this one.
transaction.rollback()
raise utils.ProgrammingError(
"A durable function must not leave work uncommitted."
)
return return_value
return wrapper
def _is_in_atomic_block():
return not transaction.get_autocommit()
def _db_may_have_uncommitted_work():
# Django doesn't seem to provide an API to tell this, but practically speaking there shouldn't
# be any uncommitted work if we're not in an atomic block, and autocommit is turned on.
return not transaction.get_autocommit()
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