[Taler] transaction state/history; fulfillment

[Florian Dold]

Hi all,

I'm currently implementing the transaction history in the WX wallet, so
that it is actually possible to view the transactions you've made.

We should talk about when a transaction is considered completed.  I
think that basically a transaction should count as completed once the
merchant deposited the coins.  But there is currently no good way to
certainly know this, since

* the merchant does not give us a signature that they "accepted" our coins
* AFAIK it is not possible to request the state of a coin, e.g. the
transactions a coin was involved in, except when double spending is
attempted.

So we should either require the merchant to sign that they successfully
deposited our coin, or have an explicit API for the coin state.

Since we need the ability to request infos about coins anyway (when we
have multiple synchronized wallets with shared coins), we might also use
this for probing the state of a transaction.

But should the merchant also give us a signature after payment?



Also the current demo merchant fulfillment URL is a bit strange, since
the URL does not contain the necessary information to actually view your
"purchase"; that information is in the PHP session cookie and not known
to the wallet.  Should we require that the fulfillment URL contains all
necessary information (maybe modulo authentication)?  Do we need more
than a URL?

In theory we can always recover the state that's not included in the
fulfillment URL by submitting the same contract with the same deposit
permission again.  But do we want to rely on that?

Any opinions?

- Florian