fix(buffer): Consistent cmp vs == for Priority (#4022)

Derive `Priority::Eq` from `Priority::PartialOrd`.

After INC-875, the custom `Ord` implementation for `Priority` was a
suspect for occurring panics
rust-lang/rust#129561.

It turned out the panic occurred somewhere else, but making `Eq` and
`PartialOrd` consistent cannot hurt.

relay-server/src/services/buffer/envelope_buffer/mod.rs

@@ -511,11 +511,10 @@ impl<K: PartialEq, V> PartialEq for QueueItem<K, V> {
 
 impl<K: PartialEq, V> Eq for QueueItem<K, V> {}
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 struct Priority {
     readiness: Readiness,
     received_at: Instant,
-    // FIXME(jjbayer): `last_peek` is currently never updated, see https://github.com/getsentry/relay/pull/3960.
     last_peek: Instant,
 }
 
@@ -529,22 +528,6 @@ impl Priority {
     }
 }
 
-impl PartialEq for Priority {
-    fn eq(&self, other: &Self) -> bool {
-        self.readiness.ready() == other.readiness.ready()
-            && self.received_at == other.received_at
-            && self.last_peek == other.last_peek
-    }
-}
-
-impl PartialOrd for Priority {
-    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
-impl Eq for Priority {}
-
 impl Ord for Priority {
     fn cmp(&self, other: &Self) -> Ordering {
         match (self.readiness.ready(), other.readiness.ready()) {
@@ -565,7 +548,21 @@ impl Ord for Priority {
     }
 }
 
-#[derive(Debug)]
+impl PartialOrd for Priority {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl PartialEq for Priority {
+    fn eq(&self, other: &Self) -> bool {
+        self.cmp(other).is_eq()
+    }
+}
+
+impl Eq for Priority {}
+
+#[derive(Debug, Clone, Copy)]
 struct Readiness {
     own_project_ready: bool,
     sampling_project_ready: bool,
@@ -948,6 +945,24 @@ mod tests {
         assert_eq!(buffer.priority_queue.len(), 2);
     }
 
+    #[test]
+    fn test_total_order() {
+        let p1 = Priority {
+            readiness: Readiness {
+                own_project_ready: true,
+                sampling_project_ready: true,
+            },
+            received_at: Instant::now(),
+            last_peek: Instant::now(),
+        };
+        let mut p2 = p1.clone();
+        p2.last_peek += Duration::from_millis(1);
+
+        // Last peek does not matter because project is ready:
+        assert_eq!(p1.cmp(&p2), Ordering::Equal);
+        assert_eq!(p1, p2);
+    }
+
     #[tokio::test]
     async fn test_last_peek_internal_order() {
         let mut buffer = EnvelopeBuffer::<MemoryStackProvider>::new(mock_memory_checker());