Le lundi 21 mars 2016 13:35:19 UTC+1, Aymeric Augustin a écrit : > > > The problem is to determine what “safe” means here :-) I’m afraid this > won’t be possible in general (at least not in a remotely efficient fashion) > because Django inevitably depends on the guarantees of the underlying > database, and those guarantees are weak at lower isolation levels. >
Sure, some of it is left to interpretation. There are some tricky cases, like two conflicting updates happening at the same time. Would safe mean that they should they both succeed, even if there would be a lost update? Or would it mean that one of should them fail? But here, we have "two non-conflicting updates (no-ops, even) causes data to be lost". I bet no one would call this safe. The guarantees offered by the database may not be perfect, but my point is that those guarantees are used badly in Django. Consistent read is a "feature", it can be used to make things "safer", but in that bug, it's simply used incorrectly because of false assumptions. I reckon that given those same guarantees, and knowing how they may be different from backend to backend, they can be used to have a "safer" behavior than this. That's also why I don't think lowering the isolation level is a good idea. The higher the isolation level, the more "guarantees" to build upon there are. This bug is an unfortunate case where an assumption that is true in a lower isolation level is not anymore in a higher one. That doesn't mean that the assumption was safe to make in the first place. I applied the patch proposed by Shai (thanks!) in the ticket, that replaces the SELECT by a SELECT FOR UPDATE. This effectively fixes the bug. I think this is the right kind of fix. Fix the code, do not change the isolation level because it fixes the code by chance. -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-developers+unsubscr...@googlegroups.com. To post to this group, send email to django-developers@googlegroups.com. Visit this group at https://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/08fed447-1ad9-4e38-a23c-e170d255cb07%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
