system: Add create account helper (#282)

* Add create account helper

* More comments

* Use signer seeds on transfer

* Cosmetics

* Add signed variant

programs/system/src/lib.rs

@@ -1,5 +1,86 @@
 #![no_std]
 
+use pinocchio::{
+    account_info::AccountInfo,
+    instruction::Signer,
+    pubkey::Pubkey,
+    sysvars::{rent::Rent, Sysvar},
+    ProgramResult,
+};
+
+use crate::instructions::{Assign, CreateAccount, Transfer};
+
 pub mod instructions;
 
 pinocchio_pubkey::declare_id!("11111111111111111111111111111111");
+
+/// Create an account with a minimum balance to be rent-exempt.
+#[inline(always)]
+pub fn create_account_with_minimum_balance(
+    account: &AccountInfo,
+    space: usize,
+    owner: &Pubkey,
+    payer: &AccountInfo,
+    rent_sysvar: Option<&AccountInfo>,
+) -> ProgramResult {
+    create_account_with_minimum_balance_signed(account, space, owner, payer, rent_sysvar, &[])
+}
+
+/// Create an account with a minimum balance to be rent-exempt.
+///
+/// When creating a PDA `account`, the PDA signer seeds must be provided
+/// via the `signers`.
+///
+/// The account will be funded by the `payer` if its current lamports
+/// are insufficient for rent-exemption. The payer can be a PDA signer
+/// owned by the system program and its signer seeds can be provided
+/// via the `signers`.
+#[inline(always)]
+pub fn create_account_with_minimum_balance_signed(
+    account: &AccountInfo,
+    space: usize,
+    owner: &Pubkey,
+    payer: &AccountInfo,
+    rent_sysvar: Option<&AccountInfo>,
+    signers: &[Signer],
+) -> ProgramResult {
+    let lamports = if let Some(rent_sysvar) = rent_sysvar {
+        let rent = Rent::from_account_info(rent_sysvar)?;
+        rent.minimum_balance(space)
+    } else {
+        Rent::get()?.minimum_balance(space)
+    };
+
+    if account.lamports() == 0 {
+        // Create the account if it does not exist.
+        CreateAccount {
+            from: payer,
+            to: account,
+            lamports,
+            space: space as u64,
+            owner,
+        }
+        .invoke_signed(signers)
+    } else {
+        let required_lamports = lamports.saturating_sub(account.lamports());
+
+        // Transfer lamports from `payer` to `account` if needed.
+        if required_lamports > 0 {
+            Transfer {
+                from: payer,
+                to: account,
+                lamports: required_lamports,
+            }
+            .invoke_signed(signers)?;
+        }
+
+        // Assign the account to the specified owner.
+        Assign { account, owner }.invoke_signed(signers)?;
+
+        // Allocate the required space for the account.
+        //
+        // SAFETY: There are no active borrows of the `account`.
+        // This was checked by the `Assign` CPI above.
+        unsafe { account.resize_unchecked(space) }
+    }
+}