util-time: Add panic for overflow/underflow

Cause the timestamp functions with addition and subtraction to panic if
they would overflow. This helps to prevent logic issues. However, this
is potentially dangerous if the timestamp can be forced to overflow and
cause a panic; so, added TODOs as a note for future refactoring.

src/util/time.rs

@@ -251,13 +251,29 @@ impl Timestamp {
     }
 
     /// Calculates elapsed time of a `Timestamp`.
+    /// TODO: Rework this function to return the result of checked_sub and make calling code
+    /// check whether it is Some/None
     pub fn elapsed(&self) -> u64 {
-        UNIX_EPOCH.elapsed().unwrap().as_secs() - self.0
+        let now = UNIX_EPOCH.elapsed().unwrap().as_secs();
+        if let Some(elapsed) = now.checked_sub(self.0) {
+            elapsed
+        } else {
+            panic!(
+                "Cannot subtract Timestamp value {} from current time {}. (Integer underflow)",
+                self.0, now
+            );
+        }
     }
 
     /// Increment a 'Timestamp'.
+    /// TODO: Rework this function to return the result of checked_add and make calling code
+    /// check whether it is Some/None
     pub fn add(&mut self, inc: u64) {
-        self.0 += inc;
+        if let Some(sum) = self.0.checked_add(inc) {
+            self.0 = sum
+        } else {
+            panic!("Cannot add {} to Timestamp {}. (Integer overflow)", self.0, inc);
+        }
     }
 }
 
@@ -439,4 +455,21 @@ mod tests {
             TimeKeeperSafe { timekeeper: TimeKeeper::new(Timestamp::current_time(), 0, 0, 0) };
         tk_unsafe.slot_epoch(0);
     }
+
+    #[test]
+    #[should_panic]
+    fn panic_on_add_overflow() {
+        // Panic when the Timestamp func add() overflows u64.
+        let mut ts = Timestamp::current_time();
+        ts.add(u64::MAX);
+    }
+
+    #[test]
+    #[should_panic]
+    fn panic_on_elapsed_underflow() {
+        // Panic when the Timestamp function elapsed() underflows u64.
+        let mut ts = Timestamp::current_time();
+        ts.add(10_000);
+        ts.elapsed();
+    }
 }