feat: (de/in)crease allowance (#56)

Author: AuHau

ethers/erc20.nim

@@ -54,6 +54,24 @@ method approve*(token: Erc20Token,
                 amount: UInt256): ?TransactionResponse {.base, contract.}
   ## Sets `amount` as the allowance of `spender` over the caller's tokens.
 
+method increaseAllowance*(token: Erc20Token,
+                spender: Address,
+                addedValue: UInt256): ?TransactionResponse {.base, contract.}
+  ## Atomically increases the allowance granted to spender by the caller.
+  ## This is an alternative to approve that can be used as a mitigation for problems described in IERC20.approve.
+  ## Emits an Approval event indicating the updated allowance.
+  ##
+  ## WARNING: THIS IS NON-STANDARD ERC-20 FUNCTION, DOUBLE CHECK THAT YOUR TOKEN HAS IT!
+
+method decreaseAllowance*(token: Erc20Token,
+                spender: Address,
+                addedValue: UInt256): ?TransactionResponse {.base, contract.}
+  ## Atomically decreases the allowance granted to spender by the caller.
+  ## This is an alternative to approve that can be used as a mitigation for problems described in IERC20.approve.
+  ## Emits an Approval event indicating the updated allowance.
+  ##
+  ## WARNING: THIS IS NON-STANDARD ERC-20 FUNCTION, DOUBLE CHECK THAT YOUR TOKEN HAS IT!
+
 method transferFrom*(token: Erc20Token,
                      spender: Address,
                      recipient: Address,

testmodule/testErc20.nim

@@ -69,6 +69,32 @@ for url in ["ws://localhost:8545", "http://localhost:8545"]:
       check (await token.balanceOf(accounts[0])) == 100.u256
       check (await token.balanceOf(accounts[1])) == 0.u256
 
+    test "increase/decrease allowance":
+      discard await testToken.mint(accounts[0], 100.u256)
+
+      check (await token.allowance(accounts[0], accounts[1])) == 0.u256
+      check (await token.balanceOf(accounts[0])) == 100.u256
+      check (await token.balanceOf(accounts[1])) == 0.u256
+
+      discard await token.increaseAllowance(accounts[1], 50.u256)
+
+      check (await token.allowance(accounts[0], accounts[1])) == 50.u256
+      check (await token.balanceOf(accounts[0])) == 100.u256
+      check (await token.balanceOf(accounts[1])) == 0.u256
+
+      discard await token.increaseAllowance(accounts[1], 50.u256)
+
+      check (await token.allowance(accounts[0], accounts[1])) == 100.u256
+      check (await token.balanceOf(accounts[0])) == 100.u256
+      check (await token.balanceOf(accounts[1])) == 0.u256
+
+      discard await token.decreaseAllowance(accounts[1], 50.u256)
+
+      check (await token.allowance(accounts[0], accounts[1])) == 50.u256
+      check (await token.balanceOf(accounts[0])) == 100.u256
+      check (await token.balanceOf(accounts[1])) == 0.u256
+
+
     test "transferFrom tokens":
       let senderAccount = accounts[0]
       let receiverAccount = accounts[1]