Selfdestruct – How to Destroy a Contract Using Suicide Accounts in Solidity

selfdestruct

I learnt from the yellowpaper that at the end of a contract execution, there could be a list of suicide accounts that will be removed.

my question is, why and how would one smart contract remove other accounts (smart contracts).

for example, I understand the need of self destruction. when one smart contract is finished, it destructs itself. but why do you need to destroy others and how?

Can my smart contract destroy a contract owned by another person?

also, engineering wise, how this destruction is implemented? for example, do we actually modify previous blocks' contents related to destructed accounts? or we just update the change in the tree structure?

Best Answer

To start my answer, I have to say that once you put something on the blockchain it is "immutable", and by that I mean that it can not be modified or deleted.

(there is a special type of "upgradable" contracts but this is not the answer for that)

You can find here a lot of information about self-destruction.

Why would you need to destroy others? For example, to make a contract not accessible, you can not delete it from the chain but you can disable it.

Can my smart contract destroy a contract owned by another person? This requires a long explanation and has multiple factors at play. But, in summary, no, you can't do it.

How is this destruction implemented? You can use selfdestruct/destroy (The old mean suicide/kill). You can find some information here and a good but old tutorial here - the latter explains how this implementation works. Also, here's yet another link with information on the topic.

Do we actually modify previous blocks' contents related to destructed accounts? or we just update the change in the tree structure? We never can modify other blocks, we can only put a new block in the blockchain with more information. Blocks are permanent and they contain the information of the transactions and the contracts. Maybe this can help you with the block concepts.

Related Solutions

Blockchain – What is Removed During a Contract Suicide and Its Impact

EDIT: I tried but realized I don't have the answer to "If the contract's data is removed to free up space, then surely the block in which it resides changes, and therefore that block's hash also changes. Wouldn't this therefore mean changing all subsequent blocks that rely on this hash?"

Leaving this here in case it helps others explain.

Each block has a "state root" which contains accounts. With a SELFDESTRUCT (original name SUICIDE) opcode, the account is zeroed out.

An account has a nonce, balance, storageRoot, codeHash. The storageRoot is a pointer to a tree structure of the contract's storage: when it is set to zero, it's setting the pointer to null (in languages like C) and this removes all the children of the tree. You're correct that this leads to a new stateRoot, but it doesn't mean an entire copy of the state since everything is pointers.

Picture is worth 1000 words, for example: Ethereum block architecture (all credit to https://ethereum.stackexchange.com/users/124/zanzu for efforts on the diagram)

Solidity – How to Destroy a Smart Contract and Collect Its Funds?

All you need to do is have the selfdestruct(address payable recipient) function. selfdestruct takes a single parameter that sends all ETH in the contract to that address. In your case, you can do:

function finalize() public creatorOnly biddingClosedOnly {
    selfdestruct(_creator);
}

From the docs:

selfdestruct(address payable recipient): destroy the current contract, sending its funds to the given address

The reason you can still call the function after the contract has been selfdestructed is because technically the address is still valid. However, no contract (data) lives there any more.

Because of this, you can still send ETH to the address and you can still send transactions with data to the address, but the EVM will not execute the function as it would with a non-selfdestructed address.

Edit: You can use the get_code RPC method to verify that the contract was, in fact, destroyed.

Using ethers.js, the following output will be given:

// Deploy contract

ethersProvider.getCode('0xDCcd6331401b62ebcE7F3a22e966b26ACe27559d').then(console.log)
> 0x608060405260043610603f576000357c0100000000000000000000000000000000000000000000000000000000900463ffffffff1680634bb278f3146044575b600080fd5b348015604f57600080fd5b5060566058565b005b3373ffffffffffffffffffffffffffffffffffffffff16ff00a165627a7a7230582077ca7684f4d93293e360c5c695d0e416f54dde89713426cc4d6fddb9f9963faa0029

// Self destruct contract

ethersProvider.getCode('0xDCcd6331401b62ebcE7F3a22e966b26ACe27559d').then(console.log)
> 0x

Best Answer

Related Solutions

Blockchain – What is Removed During a Contract Suicide and Its Impact

Solidity – How to Destroy a Smart Contract and Collect Its Funds?

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