Solitaire: Use sysvar for rent instead of default (#1505)

* Solitaire: Use sysvar for rent instead of default

Using default has some risks such as:
- Network having a different values (such as Pythnet)
- Network changes the value

It avoids using default as fallback to have Error more explicitly
avoid exposing above risks under rare conditions
that the sysvar might return Err.

* Update comments

* Format the code

solana/solitaire/program/src/lib.rs

@@ -13,6 +13,7 @@ use solana_program::{
     account_info::AccountInfo,
     pubkey::Pubkey,
     rent::Rent,
+    sysvar::Sysvar as SolanaSysvar,
 };
 
 use std::slice::Iter;
@@ -72,10 +73,15 @@ pub enum CreationLamports {
 
 impl CreationLamports {
     /// Amount of lamports to be paid in account creation
-    pub fn amount(self, size: usize) -> u64 {
+    pub fn amount(self, size: usize) -> Result<u64> {
         match self {
-            CreationLamports::Exempt => Rent::default().minimum_balance(size),
-            CreationLamports::Amount(v) => v,
+            // Rent::get() is used over Rent::default() here in case on-chain Rent has diverged from
+            // the Solana defaults. Unlike `default()`, the `get()` call only works on-chain and in
+            // Solana's test framework, it will fail in unit tests. In those environments one can use
+            // the following:
+            // `CreationLamports::Amount(Rent::default().minimum_balance(size))`
+            CreationLamports::Exempt => Ok(Rent::get()?.minimum_balance(size)),
+            CreationLamports::Amount(v) => Ok(v),
         }
     }
 }

solana/solitaire/program/src/types/accounts.rs

@@ -188,7 +188,7 @@ pub fn create_account(
     owner: &Pubkey,
     seeds: IsSigned,
 ) -> Result<()> {
-    let target_rent = lamports.amount(size);
+    let target_rent = lamports.amount(size)?;
     // top up account to target rent
     if account.lamports() < target_rent {
         let transfer_ix =