The current recoup API is broken. I guess this is another example where "trivial" API changes turn out to have (multiple!) unexpected consequences.

The current "/recoup" API does not have clear idempotency semantics, as we've discussed on the phone.  This is already bad by itself, as it makes it hard to write down what the API does other than "whatever the implementation does".

However, it actually breaks correctness in this (admittedly kinda contrived, but not impossible) case:

Say that we have a coin A obtained via withdrawal and a coin B obtained via refreshing coin A. Now the denominations of A gets revoked..

The wallet does a recoup of A for EUR:1.

Now the denomination of B also gets revoked.  The wallet recoups B (incidentally also for EUR:1) and now A can be recouped again for EUR:1.  But now the exchange is in a state where it will refuse a legitimate recoup request for A because the detection for an idempotent request kicks in.

This is IMHO bad API design, and the exchange should simply always recoup the maximum amount.

Furthermore, we usually follow the principle of "API calls that take up DB space are paid".  With the current recoup API, I can do many tiny recoup requests which the exchange then has to store, right?

I guess it would not be a big change to remove the "amount" value from the recoup/recoup-refresh request bodies, right?

- Florian

1 files changed, 1 insertions, 5 deletions

diff --git a/src/include/taler_signatures.h b/src/include/taler_signatures.h
index 947c7e831..3ad1121ca 100644
--- a/src/include/taler_signatures.h
+++ b/src/include/taler_signatures.h
@@ -1507,6 +1507,7 @@ struct TALER_RecoupRequestPS
 {
   /**
    * Purpose is #TALER_SIGNATURE_WALLET_COIN_RECOUP
+   * or #TALER_SIGNATURE_WALLET_COIN_RECOUP_REFRESH.
    */
   struct GNUNET_CRYPTO_EccSignaturePurpose purpose;
 
@@ -1520,11 +1521,6 @@ struct TALER_RecoupRequestPS
    */
   union TALER_DenominationBlindingKeyP coin_blind;
 
-  /**
-   * How much of the coin's value will be recouped?
-   */
-  struct TALER_AmountNBO recoup_amount;
-
 };