withdrawals: builtin fee return

src/common/sdpay.eas

@@ -0,0 +1,4 @@
+;;; payment via selfdestruct
+
+   calldataload
+   selfdestruct

src/withdrawals/main.eas

@@ -93,13 +93,31 @@
   push MIN_FEE          ;; [min_fee, excess, update_fraction]
   #include "../common/fake_expo.eas"
 
-  ;; Determine if the fee provided is enough to cover the withdrawal request fee.
-  callvalue             ;; [callvalue, req_fee]
+  ;; Revert if too little fee is given.
+  dup1                  ;; [req_fee, req_fee]
+  callvalue             ;; [callvalue, req_fee, req_fee]
   lt                    ;; [callvalue < req_fee]
+  jumpi @revert         ;; [req_fee]
+
+  ;; If the fee is correct, insert.
+  dup1                  ;; [req_fee, req_fee]
+  callvalue             ;; [callvalue, req_fee, req_fee]
+  eq                    ;; [callvalue==req_fee, req_fee]
+  jumpi @insert_request ;; [req_fee]
+
+  ;; Otherwise return the extra amount back to the caller before continuing.
+  callvalue             ;; [callvalue, req_fee]
+  sub                   ;; [extra_fee]
+  push @.sdpay_start    ;; [offset, extra_fee]
+  push sdpay_size       ;; [size, offset, extra_fee]
+  swap2                 ;; [extra_fee, offset, size]
+  create                ;; [addr]
+  iszero                ;; [ok]
   jumpi @revert         ;; []
 
+insert_request:
   ;; The request can pay, increment withdrawal request count.
-  push SLOT_COUNT
+  push SLOT_COUNT       ;; [slot]
   sload                 ;; [req_count]
   push 1                ;; [1, req_count]
   add                   ;; [req_count+1]
@@ -435,10 +453,17 @@ revert:
   push 0
   revert
 
+.sdpay_start:
+#assemble "../common/sdpay.eas"
+.sdpay_end:
+
 ;; ----------------------------------------------------------------------------
 ;; MACROS ---------------------------------------------------------------------
 ;; ----------------------------------------------------------------------------
 
+;; Size of the sdpay contract code.
+#define sdpay_size @.sdpay_end - @.sdpay_start
+
 ;; This defines a mask for accessing the top 16 bytes of a number.
 #define pk2_mask 0xffffffffffffffffffffffffffffffff00000000000000000000000000000000