feat: prevent possible division by zero in difficulty calculation (#5828

)

Description
---
Prevented possible division by zero panic in `fn
u256_scalar_to_difficulty(scalar: U256) -> Result<Difficulty,
DifficultyError>`

Motivation and Context
---
Code should not be able to panic.

How Has This Been Tested?
---
Unit test added

What process can a PR reviewer use to test or verify this change?
---
Code walkthrough

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Breaking Changes
---

- [x] None
- [ ] Requires data directory on base node to be deleted
- [ ] Requires hard fork
- [ ] Other - Please specify

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base_layer/core/src/proof_of_work/difficulty.rs

@@ -76,6 +76,9 @@ impl Difficulty {
     }
 
     fn u256_scalar_to_difficulty(scalar: U256) -> Result<Difficulty, DifficultyError> {
+        if scalar == U256::zero() {
+            return Err(DifficultyError::DivideByZero);
+        }
         let result = U256::MAX / scalar;
         let result = result.min(u64::MAX.into());
         Difficulty::from_u64(result.low_u64())
@@ -285,4 +288,14 @@ mod test {
             Difficulty::from_u64(expected).unwrap()
         );
     }
+
+    #[test]
+    fn u256_scalar_to_difficulty_division_by_zero() {
+        let bytes = [];
+        assert!(Difficulty::little_endian_difficulty(&bytes).is_err());
+        assert!(Difficulty::big_endian_difficulty(&bytes).is_err());
+        let bytes = [0u8; 32];
+        assert!(Difficulty::little_endian_difficulty(&bytes).is_err());
+        assert!(Difficulty::big_endian_difficulty(&bytes).is_err());
+    }
 }

base_layer/core/src/proof_of_work/error.rs

@@ -60,4 +60,6 @@ pub enum DifficultyError {
     InvalidDifficulty,
     #[error("Maximum block time overflowed u64")]
     MaxBlockTimeOverflow,
+    #[error("Divide by zero")]
+    DivideByZero,
 }