Why can any wallet close a bid?

Question

I setup a simple start, bid, close in Plutus Playground where the following occurs:

• Wallet 1 starts the bidding
• Wallet 2 makes a bid above the min amount
• Wallet 3 calls close

In the end Wallet 2 gets the token and Wallet 1 gets the amount in the script. But why is wallet 3 allowed to end the bidding when they weren't the one to start the bidding?

Answer 1

Any party can end the bidding according to the code in this contract. The constraints for what needs met is defined at this part in the contract: https://github.com/input-output-hk/plutus-pioneer-program/blob/ecafd204b56defceed9fd3a69ebede09256c12c0/code/week01/src/Week01/EnglishAuction.hs#L320. There is no incentive though for wallet 3 or any other wallet other than 1 or 2 to close bidding as those are the only two wallets that would benefit from the auction being closed after the deadline is reached.

Answer 2

Closing the auction makes you pay a fee. Thus, only interested parties are likely to actually perform the action. Moreover, it is not possible to close the auction before its deadline, even if you are the one who started the auction - valid bids after close but before the deadline will still be taken into account.