Close Mint Extension

Summary

• The original Token Program only allowed closing token accounts, but not mint accounts.
• Token Extensions Program includes the close mint extension which allows mint accounts to be closed.
• To close a mint with the close mint extension, the supply of said mint needs to be 0.
• The mintCloseAuthority can be updated by calling setAuthority

Overview

The original Token Program only allows owners to close token accounts, not mint accounts. So if you create a mint, you'll never be able to close the account. This has resulted in a lot of wasted space on the blockchain. To remedy this, the Token Extensions Program introduced the close mint extension. This simply allows a mint account to be closed and the lamports refunded. The only caveat, is the supply of said mint needs to be 0.

This extension is a nice improvement for developers, who may have thousands of mint accounts that could be cleaned up and be refunded for. Additionally it's great for NFT holders who wish to burn their NFT. They will now be able to recuperate all of the costs, ie closing the mint, metadata and token accounts. Whereas before, if someone burned an NFT would only recuperate the metadata and token account's rents. Note, the burner would also have to be the mintCloseAuthority.

The close mint extension, adds an additional field mintCloseAuthority to the mint account. This is the address of the authority to actually close the account.

Again, for a mint to be closed with this extension, the supply has to be 0. So if any of this token is minted, it will have to be burned first.

Create Mint with Close Authority

Initializing the mint with the close authority extension involves three instructions:

• SystemProgram.createAccount
• createInitializeMintCloseAuthorityInstruction
• createInitializeMintInstruction

The first instruction SystemProgram.createAccount allocates space on the blockchain for the mint account. However like all Token Extensions Program mints, we need to calculate the size and cost of the mint. This can be accomplished by using getMintLen and getMinimumBalanceForRentExemption. In this case, we'll call getMintLen with only the ExtensionType.MintCloseAuthority.

To get the mint length and create account instruction, do the following:

const extensions = [ExtensionType.MintCloseAuthority];
const mintLength = getMintLen(extensions);

const mintLamports =
  await connection.getMinimumBalanceForRentExemption(mintLength);

const createAccountInstruction = SystemProgram.createAccount({
  fromPubkey: payer,
  newAccountPubkey: mint,
  space: mintLength,
  lamports: mintLamports,
  programId: TOKEN_2022_PROGRAM_ID,
});

The second instruction createInitializeMintCloseAuthorityInstruction initializes the close authority extension. The only notable parameter is the mintCloseAuthority in the second position. This is the address that can close the mint.

const initializeMintCloseAuthorityInstruction =
  createInitializeMintCloseAuthorityInstruction(
    mint,
    authority,
    TOKEN_2022_PROGRAM_ID,
  );

The last instruction createInitializeMintInstruction initializes the mint.

const initializeMintInstruction = createInitializeMintInstruction(
  mint,
  decimals,
  payer.publicKey,
  null,
  TOKEN_2022_PROGRAM_ID,
);

Finally we add the instructions to the transaction and submit it to the Solana network.

const mintTransaction = new Transaction().add(
  createAccountInstruction,
  initializeMintCloseAuthorityInstruction,
  initializeMintInstruction,
);

const signature = await sendAndConfirmTransaction(
  connection,
  mintTransaction,
  [payer, mintKeypair],
  { commitment: "finalized" },
);

When the transaction is sent, a new mint account is created with the specified close authority.

Close Mint with Close Authority

To close a mint with the close mint extension, all that is needed is to call the closeAccount function.

Remember, that to close the mint account, the total supply has to be 0. So if any tokens exist, they have to be burned first. You can do this with the burn function.

The closeAccount function works for mints and token accounts alike.

// burn tokens to 0
const burnSignature = await burn(
  connection, // connection - Connection to use
  payer, // payer -Payer of the transaction fees
  sourceAccount, // account - Account to burn tokens from
  mintKeypair.publicKey, // mint - Mint for the account
  sourceKeypair, // owner - Account owner
  sourceAccountInfo.amount, // amount -  Amount to burn
  [], // multiSigners - Signing accounts if `owner` is a multisig
  { commitment: "finalized" }, // confirmOptions - Options for confirming the transaction
  TOKEN_2022_PROGRAM_ID, // programId - SPL Token program account
);

// account can be closed as total supply is now 0
await closeAccount(
  connection, // connection - Connection to use
  payer, // payer - Payer of the transaction fees
  mintKeypair.publicKey, // account - Account to close
  payer.publicKey, // destination - Account to receive the remaining balance of the closed account
  payer, // authority - Authority which is allowed to close the account
  [], // multiSigners - Signing accounts if `authority` is a multisig
  { commitment: "finalized" }, // confirmOptions - Options for confirming the transaction
  TOKEN_2022_PROGRAM_ID, // programIdSPL Token program account
);

Update Close Mint Authority

To change the closeMintAuthority you can call the setAuthority function and pass in the right accounts, as well as the authorityType, which in this case is AuthorityType.CloseMint

/**
 * Assign a new authority to the account
 *
 * @param connection       Connection to use
 * @param payer            Payer of the transaction fees
 * @param account          Address of the account
 * @param currentAuthority Current authority of the specified type
 * @param authorityType    Type of authority to set
 * @param newAuthority     New authority of the account
 * @param multiSigners     Signing accounts if `currentAuthority` is a multisig
 * @param confirmOptions   Options for confirming the transaction
 * @param programId        SPL Token program account
 *
 * @return Signature of the confirmed transaction
 */

await setAuthority(
  connection,
  payer,
  mint,
  currentAuthority,
  AuthorityType.CloseMint,
  newAuthority,
  [],
  undefined,
  TOKEN_2022_PROGRAM_ID,
);

Lab

In this lab, we'll create a mint with the close mint extension. We will then mint some of the tokens and see what happens when we try to close it with a non-zero supply (hint, the close transaction will fail). Lastly, we will burn the supply and close the account.

1. Getting Started

To get started, create an empty directory named close-mint and navigate to it. We'll be initializing a brand new project. Run npm init and follow through the prompts.

Next, we'll need to add our dependencies. Run the following to install the required packages:

npm i @solana-developers/helpers @solana/spl-token @solana/web3.js esrun dotenv typescript

Create a directory named src. In this directory, create a file named index.ts. This is where we will run checks against the rules of this extension. Paste the following code in index.ts:

import { Connection, Keypair, LAMPORTS_PER_SOL } from "@solana/web3.js";
import { initializeKeypair } from "@solana-developers/helpers";
// import { createClosableMint } from './create-mint' // - uncomment this in a later step
import {
  TOKEN_2022_PROGRAM_ID,
  burn,
  closeAccount,
  createAccount,
  getAccount,
  getMint,
  mintTo,
} from "@solana/spl-token";
import dotenv from "dotenv";
dotenv.config();

/**
 * Create a connection and initialize a keypair if one doesn't already exists.
 * If a keypair exists, airdrop a SOL if needed.
 */
const connection = new Connection("http://127.0.0.1:8899");
const payer = await initializeKeypair(connection);

console.log(`public key: ${payer.publicKey.toBase58()}`);

const mintKeypair = Keypair.generate();
const mint = mintKeypair.publicKey;
console.log("\nmint public key: " + mintKeypair.publicKey.toBase58() + "\n\n");

// CREATE A MINT WITH CLOSE AUTHORITY

// MINT TOKEN

// VERIFY SUPPLY

// TRY CLOSING WITH NON ZERO SUPPLY

// BURN SUPPLY

// CLOSE MINT

index.ts creates a connection to the specified validator node and calls initializeKeypair. This is where we'll end up calling the rest of our script once we've written it.

Go ahead and run the script. You should see the payer and mint public key logged to your terminal.

esrun src/index.ts

If you run into an error in initializeKeypair with airdropping, follow the next step.

2. Run validator node

For the sake of this guide, we'll be running our own validator node.

In a separate terminal, run the following command: solana-test-validator. This will run the node and also log out some keys and values. The value we need to retrieve and use in our connection is the JSON RPC URL, which in this case is http://127.0.0.1:8899. We then use that in the connection to specify to use the local RPC URL.

const connection = new Connection("http://127.0.0.1:8899", "confirmed");

Alternatively, if you’d like to use testnet or devnet, import the clusterApiUrl from @solana/web3.js and pass it to the connection as such:

const connection = new Connection(clusterApiUrl("devnet"), "confirmed");

If you decide to use devnet, and have issues with airdropping SOL, feel free to add the keypairPath parameter to initializeKeypair. You can get this from running solana config get in your terminal. And then go to faucet.solana.com and airdrop some SOL to your address. You can get your address from running solana address in your terminal.

3. Create a mint with close authority

Let's create a closable mint by creating the function createClosableMint in a new file src/create-mint.ts.

To create a closable mint, we need several instructions:

• getMintLen: Gets the space needed for the mint account
• SystemProgram.getMinimumBalanceForRentExemption: Tells us the cost of the rent for the mint account
• SystemProgram.createAccount: Creates the instruction to allocates space on Solana for the mint account
• createInitializeMintCloseAuthorityInstruction: Creates the instruction to initialize the close mint extension - this takes the closeMintAuthority as a parameter.
• createInitializeMintInstruction: Creates the instruction to initialize the mint
• sendAndConfirmTransaction: Sends the transaction to the blockchain

We'll call all of these functions in turn. But before that, let's define the inputs to our createClosableMint function:

• connection: Connection : The connection object
• payer: Keypair : Payer for the transaction
• mintKeypair: Keypair : Keypair for new mint
• decimals: number : Mint decimals

Putting it all together we get:

import {
  sendAndConfirmTransaction,
  Connection,
  Keypair,
  SystemProgram,
  Transaction,
  TransactionSignature,
} from "@solana/web3.js";

import {
  ExtensionType,
  createInitializeMintInstruction,
  getMintLen,
  TOKEN_2022_PROGRAM_ID,
  createInitializeMintCloseAuthorityInstruction,
} from "@solana/spl-token";

export async function createClosableMint(
  connection: Connection,
  payer: Keypair,
  mintKeypair: Keypair,
  decimals: number,
): Promise<TransactionSignature> {
  const extensions = [ExtensionType.MintCloseAuthority];
  const mintLength = getMintLen(extensions);

  const mintLamports =
    await connection.getMinimumBalanceForRentExemption(mintLength);

  console.log("Creating a transaction with close mint instruction...");
  const mintTransaction = new Transaction().add(
    SystemProgram.createAccount({
      fromPubkey: payer.publicKey,
      newAccountPubkey: mintKeypair.publicKey,
      space: mintLength,
      lamports: mintLamports,
      programId: TOKEN_2022_PROGRAM_ID,
    }),
    createInitializeMintCloseAuthorityInstruction(
      mintKeypair.publicKey,
      payer.publicKey,
      TOKEN_2022_PROGRAM_ID,
    ),
    createInitializeMintInstruction(
      mintKeypair.publicKey,
      decimals,
      payer.publicKey,
      null,
      TOKEN_2022_PROGRAM_ID,
    ),
  );

  console.log("Sending transaction...");
  const signature = await sendAndConfirmTransaction(
    connection,
    mintTransaction,
    [payer, mintKeypair],
    { commitment: "finalized" },
  );

  return signature;
}

Now let's call this function in src/index.ts. First you'll need to import our new function. Then paste the following under the right comment section:

// CREATE A MINT WITH CLOSE AUTHORITY
const decimals = 9;

await createClosableMint(connection, payer, mintKeypair, decimals);

This will create a transaction with close mint instruction.

Feel free to run this and check that everything is working:

esrun src/index.ts

4. Closing the mint

We're going to close the mint, but first, lets explore what happens when we have a supply when trying to close (hint, it'll fail).

To do this, we are going to mint some tokens, try to close, then burn the tokens and then actually close.

4.1 Mint a token

In src/index.ts, create an account and mint 1 token to that account.

We can accomplish this by calling 2 functions: createAccount and mintTo:

// MINT TOKEN
/**
 * Creating an account and mint 1 token to that account
 */
console.log("Creating an account...");
const sourceKeypair = Keypair.generate();
const sourceAccount = await createAccount(
  connection,
  payer,
  mint,
  sourceKeypair.publicKey,
  undefined,
  { commitment: "finalized" },
  TOKEN_2022_PROGRAM_ID,
);

console.log("Minting 1 token...\n\n");
const amount = 1 * LAMPORTS_PER_SOL;
await mintTo(
  connection,
  payer,
  mint,
  sourceAccount,
  payer,
  amount,
  [payer],
  { commitment: "finalized" },
  TOKEN_2022_PROGRAM_ID,
);

Now we can verify that the mint supply is non-zero by fetching the mint info. Underneath the minting functions, add the following code block:

// VERIFY SUPPLY
/**
 * Get mint information to verify supply
 */
const mintInfo = await getMint(
  connection,
  mintKeypair.publicKey,
  "finalized",
  TOKEN_2022_PROGRAM_ID,
);
console.log("Initial supply: ", mintInfo.supply);

Let's run the script and check the initial supply:

esrun src/index.ts

You should see the following in your terminal:

Initial supply:  1000000000n

4.2 Closing the mint with non zero supply

Now we'll attempt to close the mint when supply is non-zero. We know this is going to fail, since the close mint extension requires a non-zero supply. So to see the resulting error message, we'll wrap the closeAccount function in a try catch and log out the error:

// TRY CLOSING WITH NON ZERO SUPPLY
/**
 * Try closing the mint account when supply is not 0
 *
 * Should throw `SendTransactionError`
 */
try {
  await closeAccount(
    connection,
    payer,
    mintKeypair.publicKey,
    payer.publicKey,
    payer,
    [],
    { commitment: "finalized" },
    TOKEN_2022_PROGRAM_ID,
  );
} catch (e) {
  console.log(
    "Close account fails here because the supply is not zero. Check the program logs:",
    (e as any).logs,
    "\n\n",
  );
}

Give this a run:

esrun src/index.ts

We'll see that the program throws an error along with the program logs. You should see the following:

Close account fails here because the supply is not zero.

4.3 Burning the supply

Let's burn the whole supply so we can actually close the mint. We do this by calling burn:

// BURN SUPPLY
const sourceAccountInfo = await getAccount(
  connection,
  sourceAccount,
  "finalized",
  TOKEN_2022_PROGRAM_ID,
);

console.log("Burning the supply...");
const burnSignature = await burn(
  connection,
  payer,
  sourceAccount,
  mintKeypair.publicKey,
  sourceKeypair,
  sourceAccountInfo.amount,
  [],
  { commitment: "finalized" },
  TOKEN_2022_PROGRAM_ID,
);

4.4 Close the mint

With no tokens in circulation, we can now close the mint. At this point, we can simply call closeAccount, however, for the sake of visualizing how this works, we'll do the following:

- Retrieve Mint Information: Initially, we fetch and inspect the mint's details, particularly focusing on the supply, which should be zero at this stage. This shows that the mint is eligible to be closed.

- Verify Account Status: Next, we confirm the status of the account to ensure that it is still open and active.

- Close the Account: Once we've verified the account's open status, we proceed to close the mint account.

- Confirm Closure: Finally, after invoking the `closeAccount` function, we check the account status once more to confirm that it has indeed been closed successfully.

We can accomplish all of this with the following functions:

• getMint: Grabs the mint account and deserializes the information
• getAccountInfo: Grabs the mint account, so we can check it exists - we'll call this before and after the close.
• closeAccount: Closes the mint

Putting this all together we get:

// CLOSE MINT
const mintInfo = await getMint(
  connection,
  mintKeypair.publicKey,
  "finalized",
  TOKEN_2022_PROGRAM_ID,
);

console.log("After burn supply: ", mintInfo.supply);

const accountInfoBeforeClose = await connection.getAccountInfo(
  mintKeypair.publicKey,
  "finalized",
);

console.log("Account closed? ", accountInfoBeforeClose === null);

console.log("Closing account after burning the supply...");
const closeSignature = await closeAccount(
  connection,
  payer,
  mintKeypair.publicKey,
  payer.publicKey,
  payer,
  [],
  { commitment: "finalized" },
  TOKEN_2022_PROGRAM_ID,
);

const accountInfoAfterClose = await connection.getAccountInfo(
  mintKeypair.publicKey,
  "finalized",
);

console.log("Account closed? ", accountInfoAfterClose === null);

Run the script one last time.

esrun src/index.ts

You should see the whole process of creating a closable mint, minting a token, trying to close, burning the token, and finally closing the account.

That's it! We have successfully created a mint with close authority. If you get stuck at any point, you can find working code in the solution branch of this repository.

Challenge

For the challenge, try and create your own mint and mint to several token accounts, then create a script to burn all of those token accounts, then close the mint.