A cautionary tale - mgo asserts

[Menno Smits]

On 9 June 2016 at 03:44, Gustavo Niemeyer <gustavo.niemeyer at canonical.com>
wrote:

> Is it mgo/txn that is internally unmarahalling onto that?
>
> Let's get that fixed at its heart.
>

That would be ideal. The root of the problem is that the Assert, Insert and
Update fields of txn.Op are of type interface{} and the bson unmarshalling
uses bson.M for these. This means when a transaction is loaded from the
txns collection the contents of these fields are loaded into bson.M and
field ordering is lost.

It looks trivial to change the bson unmarshalling code to default to bson.D
but naively changing this will likely break existing users of the bson
package. That's probably not the right solution here. Perhaps transactions
which are written to/loaded from the database by mgo/txn should use a
private txn.Op analogue where Assert, Insert and Update are bson.D instead
of interface{}?

- Menno
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