Add persistence exception handling section

[psibi]

Let me know if this is not idiomatic. I did a related post regarding this on the mailing list: https://groups.google.com/forum/#!topic/yesodweb/MKEqVZdfam8

[snoyberg]

Perhaps "throws an exception"?

[snoyberg]

There's actually a problem with this. Once an exception is thrown within a transaction, most drivers will now consider the transaction in an invalid state, and therefore further changes will fail (I think). For example, I think the code you have here will fail for PostgreSQL (though unfortunately I can't easily test that at the moment).

[gregwebs]

Yes, generally speaking a DB exception must be caught outside the transaction because the DB will not necessarily throw an exception until commit in some cases (in this case imagine another thread doing the same insertion). In this particular example one could use the PersistUnique functionality. In general sometimes (and in this case) you can break up the transaction and catch errors outside it.

[psibi]

Yes, infact the code doesn't work with the PostgreSQL backend. I have been little occupied in my day job. Will update the PR by this week.

[psibi]

I have updated the code to handle it in a database like PostgreSQL. Have also added a helper function mapLiftSqlPersistMPool which is used for executing each transaction. Probably, it can be added to persistent if this is the proper way to do it.

[gregwebs]

Looks better!

[snoyberg]

LGTM too.

[psibi]

Thanks for the feedback, merging it.