A cautionary tale - mgo asserts

[roger peppe]

This is also relevant (but probably only for larger documents):

https://jeremywsherman.com/blog/2013/04/23/key-reordering-ruins-mongodb/

Another reason to avoid entire-subdocument matches.

On 8 June 2016 at 10:42, Menno Smits <menno.smits at canonical.com> wrote:
>
>
> On 8 June 2016 at 21:05, Tim Penhey <tim.penhey at canonical.com> wrote:
>>
>> Hi folks,
>>
>> tl;dr: not use structs in transaction asserts
>>
>> ...
>>
>> The solution is to not use a field struct equality, even though it is easy
>> to write, but to use the dotted field notation to check the embedded field
>> values.
>
>
>
> To give a more concrete example, asserting on a embedded document field like
> this is problematic:
>
>   ops := []txn.Op{{
>       C: "collection",
>       Id: ...,
>       Assert: bson.D{{"some-field", Thing{A: "foo", B: 99}}},
>       Update: ...
>   }
>
> Due to the way mgo works[1], the document the transaction operation is
> asserting against may have been written with A and B in reverse order, or
> the Thing struct in the Assert may have A and B swapped by the time it's
> used. Either way, the assertion will fail randomly.
>
> The correct approach is to express the assertion like this:
>
>   ops := []txn.Op{{
>       C: "collection",
>       Id: ...,
>       Assert: bson.D{
>           {"some-field.A", "foo"},
>           {"some-field.B", 99},
>       },
>       Update: ...
>   }
>
> or this:
>
>   ops := []txn.Op{{
>       C: "collection",
>       Id: ...,
>       Assert: bson.M{
>           "some-field.A": "foo",
>           "some-field.B": 99,
>       },
>       Update: ...
>   }
>
>>
>> Yet another thing to add to the list of things to check when doing
>> reviews.
>
>
> I think we can go a bit further and error on attempts to use structs for
> comparison in txn.Op asserts in Juju's txn layers in state. Just as we
> already do some munging and checking of database operations to ensure
> correct multi-model behaviour, we should be able to do this same for this
> issue and prevent it from happening again.
>
> - Menno
>
> [1] If transaction operations are loaded and used from the DB (more likely
> under load when multiple runners are acting concurrently), the Insert,
> Update and Assert fields are loaded as bson.M (this is what the bson
> Unmarshaller does for interface{} typed fields). Once this happens field
> ordering is lost.
>
>
>
>
> --
> Juju-dev mailing list
> Juju-dev at lists.ubuntu.com
> Modify settings or unsubscribe at:
> https://lists.ubuntu.com/mailman/listinfo/juju-dev
>