Cross Program Invocation

Modify Update Instruction

First, we'll implement a simple "pay-to-update" mechanism by modifying the Update struct and update function.

Begin by updating the lib.rs file to bring into scope items from the system_program module.

use anchor_lang::system_program::{transfer, Transfer};

Next, update the Update struct to include an additional account called vault_account. This account, controlled by our program, will receive SOL from a user when they update their message account.

#[account(
    mut,
    seeds = [b"vault", user.key().as_ref()],
    bump,
)]
pub vault_account: SystemAccount<'info>,

Next, implement the CPI logic in the update instruction to transfer 0.001 SOL from the user's account to the vault account.

let transfer_accounts = Transfer {
    from: ctx.accounts.user.to_account_info(),
    to: ctx.accounts.vault_account.to_account_info(),
};
let cpi_context = CpiContext::new(
    ctx.accounts.system_program.to_account_info(),
    transfer_accounts,
);
transfer(cpi_context, 1_000_000)?;

Rebuild the program.

build

Modify Delete Instruction

We'll now implement a "refund on delete" mechanism by modifying the Delete struct and delete function.

First, update the Delete struct to include the vault_account. This allows us to transfer any SOL in the vault back to the user when they close their message account.

#[account(
    mut,
    seeds = [b"vault", user.key().as_ref()],
    bump,
)]
pub vault_account: SystemAccount<'info>,

Also add the system_program as the CPI for the transfer requires invoking the System Program.

pub system_program: Program<'info, System>,

Next, implement the CPI logic in the delete instruction to transfer SOL from the vault account back to the user's account.

let user_key = ctx.accounts.user.key();
let signer_seeds: &[&[&[u8]]] =
    &[&[b"vault", user_key.as_ref(), &[ctx.bumps.vault_account]]];
 
let transfer_accounts = Transfer {
    from: ctx.accounts.vault_account.to_account_info(),
    to: ctx.accounts.user.to_account_info(),
};
let cpi_context = CpiContext::new(
    ctx.accounts.system_program.to_account_info(),
    transfer_accounts,
).with_signer(signer_seeds);
transfer(cpi_context, ctx.accounts.vault_account.lamports())?;

Note that we updated _ctx: Context<Delete> to ctx: Context<Delete> as we'll be using the context in the body of the function.

Rebuild the program.

build

Redeploy Program

After making these changes, we need to redeploy our updated program. This ensures that our modified program is available for testing. On Solana, updating a program simply requires deploying the program at the same program ID.

deploy

Update Test File

Next, we'll update our anchor.test.ts file to include the new vault account in our instructions. This requires deriving the vault PDA and including it in our update and delete instruction calls.

Derive Vault PDA

First, add the vault PDA derivation:

const [vaultPda, vaultBump] = PublicKey.findProgramAddressSync(
  [Buffer.from("vault"), wallet.publicKey.toBuffer()],
  program.programId,
);

Modify Update Test

Then, update the update instruction to include the vaultAccount.

const transactionSignature = await program.methods
  .update(message)
  .accounts({
    messageAccount: messagePda,
    vaultAccount: vaultPda,
  })
  .rpc({ commitment: "confirmed" });

Modify Delete Test

Then, update the delete instruction to include the vaultAccount.

const transactionSignature = await program.methods
  .delete()
  .accounts({
    messageAccount: messagePda,
    vaultAccount: vaultPda,
  })
  .rpc({ commitment: "confirmed" });

Rerun Test

After making these changes, run the tests to ensure everything is working as expected:

test

You can then inspect the SolanaFM links to view the transaction details, where you'll find the CPIs for the transfer instructions within the update and delete instructions.

Update CPI

Delete CPI

If you encounter any errors, you can reference the final code.