deposit limit: always allow obv1-selling deposits (#869)

Previously serum3_place_order would fail when deposit limits were
exhausted on the payer token side. Now the failure only happens when
payer tokens need to be borrowed.

(cherry picked from commit a4cddf3)

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

@@ -316,18 +316,13 @@ pub fn serum3_place_order(
         )?
     };
 
-    // Deposit limit check: Placing an order can increase deposit limit use on both
-    // the payer and receiver bank. Imagine placing a bid for 500 base @ 0.5: it would
-    // use up 1000 quote and 500 base because either could be deposit on cancel/fill.
-    // This is why this must happen after update_bank_potential_tokens() and any withdraws.
+    // Deposit limit check, receiver side:
+    // Placing an order can always increase the receiver bank deposits on fill.
     {
         let receiver_bank = receiver_bank_ai.load::<Bank>()?;
         receiver_bank
             .check_deposit_and_oo_limit()
             .with_context(|| std::format!("on {}", receiver_bank.name()))?;
-        payer_bank
-            .check_deposit_and_oo_limit()
-            .with_context(|| std::format!("on {}", payer_bank.name()))?;
     }
 
     // Payer bank safety checks like reduce-only, net borrows, vault-to-deposits ratio
@@ -342,6 +337,18 @@ pub fn serum3_place_order(
         );
         payer_bank.enforce_max_utilization_on_borrow()?;
         payer_bank.check_net_borrows(payer_bank_oracle)?;
+
+        // Deposit limit check, payer side:
+        // The payer bank deposits could increase when cancelling the order later:
+        // Imagine the account borrowing payer tokens to place the order, repaying the borrows
+        // and then cancelling the order to create a deposit.
+        //
+        // However, if the account only decreases its deposits to place an order it can't
+        // worsen the situation and should always go through, even if payer deposit limits are
+        // already exceeded.
+        payer_bank
+            .check_deposit_and_oo_limit()
+            .with_context(|| std::format!("on {}", payer_bank.name()))?;
     } else {
         payer_bank.enforce_borrows_lte_deposits()?;
     }

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

@@ -1563,6 +1563,7 @@ async fn test_serum_deposit_limits() -> Result<(), TransportError> {
     //
     let deposit_amount = 5000; // for 10k tokens over both order_placers
     let CommonSetup {
+        serum_market_cookie,
         group_with_tokens,
         mut order_placer,
         quote_token,
@@ -1677,11 +1678,15 @@ async fn test_serum_deposit_limits() -> Result<(), TransportError> {
     let remaining_quote = {
         || async {
             let b: Bank = solana2.get_account(quote_bank).await;
-            b.remaining_deposits_until_limit().round().to_num::<u64>()
+            b.remaining_deposits_until_limit().round().to_num::<i64>()
         }
     };
 
     order_placer.cancel_all().await;
+    context
+        .serum
+        .consume_spot_events(&serum_market_cookie, &[order_placer.open_orders])
+        .await;
 
     //
     // TEST: even when placing all quote tokens into a bid, they still count
@@ -1705,6 +1710,43 @@ async fn test_serum_deposit_limits() -> Result<(), TransportError> {
     assert_mango_error(&r, MangoError::BankDepositLimit.into(), "dep limit".into());
     order_placer.try_ask(5.0, 399).await.unwrap(); // not 400 due to rounding
 
+    // reset
+    order_placer.cancel_all().await;
+    context
+        .serum
+        .consume_spot_events(&serum_market_cookie, &[order_placer.open_orders])
+        .await;
+    order_placer.settle().await;
+
+    //
+    // TEST: can place a bid even if quote deposit limit is exhausted
+    //
+    send_tx(
+        solana,
+        TokenEdit {
+            group: group_with_tokens.group,
+            admin: group_with_tokens.admin,
+            mint: quote_token.mint.pubkey,
+            fallback_oracle: Pubkey::default(),
+            options: mango_v4::instruction::TokenEdit {
+                deposit_limit_opt: Some(1),
+                ..token_edit_instruction_default()
+            },
+        },
+    )
+    .await
+    .unwrap();
+    assert!(remaining_quote().await < 0);
+    assert_eq!(
+        account_position(solana, order_placer.account, quote_token.bank).await,
+        5000
+    );
+    // borrowing might lead to a deposit increase later
+    let r = order_placer.try_bid(1.0, 5001, false).await;
+    assert_mango_error(&r, MangoError::BankDepositLimit.into(), "dep limit".into());
+    // but just selling deposits is fine
+    order_placer.try_bid(1.0, 4999, false).await.unwrap();
+
     Ok(())
 }