How do I use PDA account to pay for new PDA account

Question

I am struggling to successfully create a PDA account (pdaUSER) paid for by another PDA account.

I am doing it like this, and it does not work:

        invoke_signed(
        &system_instruction::create_account(
            &pdaGLOBAL.key,
            &pdaUSER.key,
            rentUSER,
            SIZE_USER.into(),
            &program_id,
        ),
        &[
            pdaGLOBAL.clone(),
            pdaUSER.clone(),
        ],
        &[&[&seedUSER, &[bumpUSER]]]
        )?;

The balance of pdaGLOBAL is 100 SOL.

On the other hand, when I pay for pdaUSER creation with the address that signed the transaction, just a regular solana account. It works fine, like this:

        invoke_signed(
        &system_instruction::create_account(
            &owner.key,
            &pdaUSER.key,
            rentUSER,
            SIZE_USER.into(),
            &program_id,
        ),
        &[
            owner.clone(),
            pdaUSER.clone(),
        ],
        &[&[&seedUSER, &[bumpUSER]]]
        )?;

Do I need to provide

&[&[&seedGLOBAL, &[bumpGLOBAL]]],
&[&[&seedUSER, &[bumpUSER]]],

instead? Or something entirely different altogether?

Thanks for your help.

Answer 1

It's relatively likely that you need to include the signatures of both accounts, the PDA payer and the PDA account that will be created.

You don't need to explicitly add the signature when there's an external signature because the Solana run time automatically includes any existing signatures in the CPI. But for PDAs which use seed based signing, you will need to include every seed for every PDA that needs to sign.

For signing with multiple PDAs though they all need to go into the same array. Hence it should the seeds should look like this:

&[&[&seedGLOBAL, &[bumpGLOBAL]], &[&seedUSER, &[bumpUSER]]]

Answer 2

The originally accepted answer above turned out to be incorrect (I was confounding different results with the incorrect solution's implementation I made. (sorry for the confusion)

According to sources, it is actually impossible to invoke system_instruction to modify balance/state on a pda, because the pda is owned by the pda program, not the system program.

The only way modify pda balance is by borrowing after any system_instruction calls have been made. (I do not understand why this is, but I verified it.)

The fix was to use a regular signing account ('owner') to fund create_account, then afterwards reimburse the regular account by borrowing from pdaGLOBAL. Working version below:

        &system_instruction::create_account(
            &owner.key,
            &pdaUSER.key,
            rentUSER,
            SIZE_USER.into(),
            &program_id,
        ),
        &[
            owner.clone(),
            pdaUSER.clone(),
        ],
        &[&[&seedUSER, &[bumpUSER]]],
        )?;
        msg!("Successfully created pdaUSER");
        
        // cover rent costs by transferring lamp to owner
        **pdaGLOBAL.try_borrow_mut_lamports()? -= rentUSER;
        **owner.try_borrow_mut_lamports()? += rentUSER;

To reiterate, multiple seed signers like &[&[&seedGLOBAL, &[bumpGLOBAL]], &[&seedUSER, &[bumpUSER]]] did NOT work for me.