#20571: Using savepoints within transaction.atomic() can result in the entire
transaction being incorrectly and silently rolled back
---------------------------------+---------------------------------------
Reporter: lamby | Owner: nobody
Type: Bug | Status: new
Component: Documentation | Version: 1.6-alpha-1
Severity: Release blocker | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by akaariai):
I was wrong in the above example, the second transaction.atomic() doesn't
create any savepoints as the whole transaction needs to be rolled back
anyways. Thus the needs_rollback flag isn't cleared.
I updated the
https://github.com/akaariai/django/compare/manual_savepoint_atomic patch.
Now you can't create savepoints in failed blocks, so s2 in the example of
comment:4 would fail. This seems good enough, there really isn't any point
in creating a savepoint which is going to be rolled back anyways.
As for do we need the ability to use manual savepoints? If possible, yes.
Some things become nasty to code if your only option is using exceptions.
Say, you call a method, and if that method returns false you will need to
rollback the current savepoint. Options:
{{{
def myf():
sp = savepoint()
if somemethod():
savepoint_commit()
else:
savepoint_rollback()
def myf():
try:
with atomic():
if somemethod():
return
else:
raise FakeException # This will now rollback the
savepoint.
except FakeException:
return
}}}
Another situation where atomic() isn't easy is if you have different paths
for enter and exit. Then naturally any with statement can't be used. An
example is TransactionMiddleware.
--
Ticket URL: <https://code.djangoproject.com/ticket/20571#comment:7>
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