Fees buyback: Credit users for ui part of serum fees (#481)

At least for markets with USDC quote currency.

programs/mango-v4/src/instructions/serum3_liq_force_cancel_orders.rs

@@ -143,7 +143,9 @@ pub fn serum3_liq_force_cancel_orders(
     let mut account = ctx.accounts.account.load_full_mut()?;
     let mut base_bank = ctx.accounts.base_bank.load_mut()?;
     let mut quote_bank = ctx.accounts.quote_bank.load_mut()?;
+    let group = ctx.accounts.group.load()?;
     apply_settle_changes(
+        &group,
         ctx.accounts.account.key(),
         &mut account.borrow_mut(),
         &mut base_bank,

programs/mango-v4/src/instructions/serum3_place_order.rs

@@ -411,6 +411,7 @@ fn apply_vault_difference(
 /// Uses the changes in OpenOrders and vaults to adjust the user token position,
 /// collect fees and optionally adjusts the HealthCache.
 pub fn apply_settle_changes(
+    group: &Group,
     account_pk: Pubkey,
     account: &mut MangoAccountRefMut,
     base_bank: &mut Bank,
@@ -430,6 +431,17 @@ pub fn apply_settle_changes(
         .saturating_sub(after_oo.native_rebates());
     quote_bank.collected_fees_native += I80F48::from(received_fees);
 
+    // Ideally we could credit buyback_fees at the current value of the received fees,
+    // but the settle_funds instruction currently doesn't receive the oracle account
+    // that would be needed for it.
+    if quote_bank.token_index == QUOTE_TOKEN_INDEX {
+        let now_ts = Clock::get()?.unix_timestamp.try_into().unwrap();
+        account
+            .fixed
+            .expire_buyback_fees(now_ts, group.buyback_fees_expiry_interval);
+        account.fixed.accrue_buyback_fees(received_fees);
+    }
+
     // Don't count the referrer rebate fees as part of the vault change that should be
     // credited to the user.
     let after_quote_vault_adjusted = after_quote_vault - received_fees;

programs/mango-v4/src/instructions/serum3_settle_funds.rs

@@ -106,7 +106,9 @@ pub fn serum3_settle_funds(ctx: Context<Serum3SettleFunds>) -> Result<()> {
     let mut account = ctx.accounts.account.load_full_mut()?;
     let mut base_bank = ctx.accounts.base_bank.load_mut()?;
     let mut quote_bank = ctx.accounts.quote_bank.load_mut()?;
+    let group = ctx.accounts.group.load()?;
     apply_settle_changes(
+        &group,
         ctx.accounts.account.key(),
         &mut account.borrow_mut(),
         &mut base_bank,

programs/mango-v4/tests/cases/test_serum.rs

@@ -334,9 +334,9 @@ async fn test_serum_loan_origination_fees() -> Result<(), TransportError> {
     }
     .create(solana)
     .await;
-    let base_token = &tokens[0];
+    let base_token = &tokens[1];
     let base_bank = base_token.bank;
-    let quote_token = &tokens[1];
+    let quote_token = &tokens[0];
     let quote_bank = quote_token.bank;
 
     //
@@ -597,6 +597,12 @@ async fn test_serum_loan_origination_fees() -> Result<(), TransportError> {
             0.1
         ));
 
+        let account_data = solana.get_account::<MangoAccount>(account).await;
+        assert_eq!(
+            account_data.buyback_fees_accrued_current,
+            serum_fee(fill_amount) as u64
+        );
+
         assert_eq!(
             account_position(solana, account, quote_bank).await,
             deposit_amount + without_serum_taker_fee(fill_amount)

programs/mango-v4/tests/program_test/mod.rs

@@ -142,8 +142,8 @@ impl TestContextBuilder {
                 index: i,
                 decimals: 6,
                 unit: 10u64.pow(6) as f64,
-                base_lot: 0 as f64,
-                quote_lot: 0 as f64,
+                base_lot: 100 as f64,
+                quote_lot: 10 as f64,
                 pubkey: Pubkey::default(),
                 authority: TestKeypair::new(),
             });