in a program, is it possible to tell if an instruction was invoked by a CPI call?

Question

Say the program is in the middle of executing Instruction 2b.

Instruction 1
Instruction 2
  Instruction 2a
  Instruction 2b
  Instruction 2c
Instruction 3

Is it possible to programmatically figure out that 2b was called by instruction 2?

Context: I want to forbid anyone from calling a specific instruction in my program via CPI.

Answer 1

Yes! Solana provides an Instructions sysvar. Here's an example.

use anchor_lang::solana_program::sysvar::instructions as tx_instructions;
...

#[account(address = tx_instructions::ID)]
pub instructions: UncheckedAccount<'info>,

...
// Forbid creating voter accounts from CPI.
{
    let ixns = ctx.accounts.instructions.to_account_info();
    let current_index = tx_instructions::load_current_index_checked(&ixns)? as usize;
    let current_ixn = tx_instructions::load_instruction_at_checked(current_index, &ixns)?;
    require_keys_eq!(
        current_ixn.program_id,
        *ctx.program_id,
        VsrError::ForbiddenCpi
    );
}