Update Not So Cosmos for Miss Erroe Handler

Author: slendermaan

not-so-smart-contracts/cosmos/README.md

@@ -24,7 +24,7 @@ Each _Not So Smart Cosmos_ includes a standard set of information:
 | [Broken bookkeeping](broken_bookkeeping) | Exploit mismatch between different modules' views on balances |
 | [Rounding errors](rounding_errors) | Bugs related to imprecision of finite precision arithmetic |
 | [Unregistered message handler](unregistered_msg_handler) | Broken functionality because of unregistered msg handler |
-
+| [Missing error handler](missing_error_handler) | Missing error handling leads to successful execution of a transaction that should have failed |
 ## Credits
 
 These examples are developed and maintained by [Trail of Bits](https://www.trailofbits.com/).

not-so-smart-contracts/cosmos/missing_error_handler/README.md

@@ -0,0 +1,46 @@
+# Missing error handler
+
+The idiomatic way of handling errors in `Go` is to compare the returned error to nil. This way of checking for errors gives the programmer a lot of control. However, when error handling is ignored it can also lead to numerous problems. The impact of this is most obvious in method calls in the `bankKeeper` module, which even causes some accounts with insufficient balances to perform `SendCoin` operations normally without triggering a transaction failure.
+
+
+## Example 
+In the following code, `k.bankKeeper.SendCoins(ctx, lender, borrower, amount)` does not have any return values being used, including `err`. This results in `SendCoin` not being able to prevent the transaction from executing even if there is an `error` due to insufficient balance in `SendCoin`.
+```golang
+func (k msgServer) ApproveLoan(goCtx context.Context, msg *types.MsgApproveLoan) (*types.MsgApproveLoanResponse, error) {
+	ctx := sdk.UnwrapSDKContext(goCtx)
+
+	loan, found := k.GetLoans(ctx, msg.Id)
+	if !found {
+		return nil, sdkerrors.Wrapf(sdkerrors.ErrKeyNotFound, "key %d doesn't exist", msg.Id)
+	}
+
+	// TODO: for some reason the error doesn't get printed to the terminal
+	if loan.State != "requested" {
+		return nil, sdkerrors.Wrapf(types.ErrWrongLoanState, "%v", loan.State)
+	}
+
+	lender, _ := sdk.AccAddressFromBech32(msg.Creator)
+	borrower, _ := sdk.AccAddressFromBech32(loan.Borrower)
+	amount, err := sdk.ParseCoinsNormalized(loan.Amount)
+	if err != nil {
+		return nil, sdkerrors.Wrap(types.ErrWrongLoanState, "Cannot parse coins in loan amount")
+	}
+
+	k.bankKeeper.SendCoins(ctx, lender, borrower, amount)
+
+	loan.Lender = msg.Creator
+	loan.State = "approved"
+
+	k.SetLoans(ctx, loan)
+
+	return &types.MsgApproveLoanResponse{}, nil
+}
+```
+## Mitigations
+
+- Implement the error handling process instead of missing it
+
+## External examples
+- [ignite's tutorials](https://github.com/ignite/cli/issues/2828). 
+- [JackalLabs](https://github.com/JackalLabs/canine-chain/issues/8).
+- [OllO](https://github.com/OllO-Station/ollo/issues/20)