solana: Add more err handling and documentation

Author: johnsaigle

solana/programs/ntt-quoter/src/processor/request_relay.rs

@@ -92,6 +92,7 @@ const GWEI: u64 = u64::pow(10, 9);
 //     might be more efficient to use f64 or ruint crate
 // SECURITY: Integer division is OK here. The calling code is responsible for understanding that
 // this function returns the quotient of the operation and that the remainder will be lost.
+// The divisor is explicitly checked in this function.
 #[allow(clippy::integer_division)]
 fn mul_div(scalar: u64, numerator: u64, denominator: u64) -> StdResult<u64, NttQuoterError> {
     if denominator == 0 {

solana/programs/ntt-quoter/src/processor/update_prices.rs

@@ -16,11 +16,15 @@ pub struct UpdateSolPrice<'info> {
     )]
     pub authority: Signer<'info>,
 
-    #[account(mut)]
     pub instance: Account<'info, Instance>,
 }
 
 pub fn update_sol_price(ctx: Context<UpdateSolPrice>, args: UpdateSolPriceArgs) -> Result<()> {
+    // `sol_price` is used as a divisor in arithmetic operations so it is invalid for its value to
+    // be zero.
+    if args.sol_price == 0 {
+        return Err(NttQuoterError::PriceCannotBeZero.into());
+    }
     ctx.accounts.instance.sol_price = args.sol_price;
     Ok(())
 }