remove Lsn::sub in favor of sub_checked

There is only one place doing subtraction, and it had a manually
implemented check.

pageserver/src/page_cache.rs

@@ -831,8 +831,9 @@ impl PageCache {
         loop {
             thread::sleep(conf.gc_period);
             let last_lsn = self.get_last_valid_lsn();
-            if last_lsn.0 > conf.gc_horizon {
-                let horizon = last_lsn - conf.gc_horizon;
+
+            // checked_sub() returns None on overflow.
+            if let Some(horizon) = last_lsn.checked_sub(conf.gc_horizon) {
                 let mut maxkey = CacheKey {
                     tag: BufferTag {
                         rel: RelTag {

zenith_utils/src/lsn.rs

@@ -1,7 +1,7 @@
 #![warn(missing_docs)]
 
 use std::fmt;
-use std::ops::{Add, AddAssign, Sub};
+use std::ops::{Add, AddAssign};
 use std::path::Path;
 use std::str::FromStr;
 
@@ -18,6 +18,12 @@ impl Lsn {
     /// Maximum possible value for an LSN
     pub const MAX: Lsn = Lsn(u64::MAX);
 
+    /// Subtract a number, returning None on overflow.
+    pub fn checked_sub<T: Into<u64>>(self, other: T) -> Option<Lsn> {
+        let other: u64 = other.into();
+        self.0.checked_sub(other).map(Lsn)
+    }
+
     /// Parse an LSN from a filename in the form `0000000000000000`
     pub fn from_filename<F>(filename: F) -> Result<Self, LsnParseError>
     where
@@ -86,15 +92,6 @@ impl fmt::Display for Lsn {
     }
 }
 
-impl Sub<u64> for Lsn {
-    type Output = Lsn;
-
-    fn sub(self, other: u64) -> Self::Output {
-        // panic if the subtraction overflows/underflows.
-        Lsn(self.0.checked_sub(other).unwrap())
-    }
-}
-
 impl Add<u64> for Lsn {
     type Output = Lsn;