Cross-program invocation with unauthorized signer or writable account when using PDA with seeds

Question

I've been able to write a simple program to create some accounts using a generated keypair. But, when I've tried to modify things in an attempt to make the user the authority of the account and use a PDA with seeds to create the account, I am coming across this error Error: failed to send transaction: Transaction simulation failed: Error processing Instruction 0: Cross-program invocation with unauthorized signer or writable account. I figure something is wrong with my seeds or how I'm signing, but I haven't been able to find what it is... I also tried without adding a signer (as I thought it was implicit), but the error is the same. I'll attach my validator and my TS test below.

I'm also quite new to writing Anchor/Rust, so if anyone sees anything that could be better, or any best practices that should be adhered to, I would love to know!

Anchor validator:

#[derive(Accounts)]
#[instruction(username: String, profile_pic_url: String, background_pic_url: String)]
pub struct CreateInstructor<'info> {

    // Create account of type Instructor and assign instructor's pubkey as the payer
    // Seeded with instructorWalletPubKey + "instructor"
    #[account(
        init, 
        seeds = [authority.key().as_ref(), "instructor".as_bytes().as_ref()], 
        constraint = instructor.to_account_info().owner == program_id,
        bump,
        payer = authority, 
        space = Instructor::LEN
    )]
    pub instructor: Account<'info, Instructor>,

    // Define user as mutable - money in their account, description
    #[account(mut)]
    pub authority: Signer<'info>,


    // Ensure System Program is the official one from Solana and handle errors
    pub system_program: Program<'info, System>,
}

Test:

    it("creates an instructor account", async () => {
      const provider = anchor.AnchorProvider.env();
      anchor.setProvider(provider);

      const instructorSeeds = [
        provider.wallet.publicKey.toBuffer(),
        Buffer.from(anchor.utils.bytes.utf8.encode("instructor")),
      ];

      const [instructorPubKey, _] = await anchor.web3.PublicKey.findProgramAddress(
        instructorSeeds,
        program.programId
      );

      await program.methods
        .createInstructor("username", "profile pic url", "background pic url")
        .accounts({
          instructor: instructorPubKey,
          authority: provider.wallet.publicKey,
          systemProgram: anchor.web3.SystemProgram.programId,
        })
        .signers([(provider.wallet as anchor.Wallet).payer])
        .rpc();

      // Assertions
      const instructorAccount = await program.account.instructor.fetch(
        instructorPubKey
      );
      assert.equal(instructorAccount.username, "username");
      assert.equal(instructorAccount.profilePicUrl, "profile pic url");
      assert.equal(instructorAccount.backgroundPicUrl, "background pic url");
    });

Answer 1

As far as I know, this is how PDAs are specified in the Rust program and derived in the client.

Rust:

..
seeds = [authority.key().as_ref(), "instructor".as_bytes().as_ref()]
..

Js/Ts:

const instructorSeeds = [
   provider.wallet.publicKey.toBuffer(),
   Buffer.from(anchor.utils.bytes.utf8.encode("instructor")),
];

b"instructor" and "instructor".as_bytes() mean the same thing and are interchangeable.

then:

const [instructorPubKey, _] = await anchor.web3.PublicKey.findProgramAddress(
  instructorSeeds,
  program.programId
);

The anchor.web3.PublicKey.findProgramAddress function returns a tuple, not a constant. const [instructorPubkey, _] is the proper way to get your derived pubkey value.

Answer 2

I solved the issue, but I actually don't know exactly how. I think trying to solve it led me to more issues and I'm unsure what actually wound up fixing it. What I did notice was that in trying to fix it so many times (and being a noob), I think I redeployed my program. After noticing this, copying the new program ID in my app/ directory, Anchor.toml, and lib.rs, and retrying the RPC call, it worked. I assume at the start my seeds were also messed up. But, unfortunately, I couldn't with confidence find the root cause.

Answer 3

This issue happens because either

account does not exist

OR missing #[account(mut)] to token accounts, also the mint accounts

OR seeds are incorrect