fix(gossipsub): avoid direct casting from u128 to u64

Replaces the lossy cast with a safer/checked version when initializing sequence numbers from the unix timestamp in nanoseconds.

Pull-Request: #6211.

Author: onur-ozkan

protocols/gossipsub/CHANGELOG.md

@@ -32,6 +32,9 @@
 - Refactor gossipsub with in-place negative-score peer removal.
   See [PR 6209](https://github.com/libp2p/rust-libp2p/pull/6209).
 
+- Avoid direct casting from u128 to u64.
+  See [PR 6211](https://github.com/libp2p/rust-libp2p/pull/6211).
+
 ## 0.49.2
 
 - Relax `Behaviour::with_metrics` requirements, do not require DataTransform and TopicSubscriptionFilter to also impl Default

protocols/gossipsub/src/behaviour.rs

@@ -183,7 +183,7 @@ enum PublishConfig {
 
 /// A strictly linearly increasing sequence number.
 ///
-/// We start from the current time as unix timestamp in milliseconds.
+/// We start from the current time as unix timestamp in nanoseconds.
 #[derive(Debug)]
 struct SequenceNumber(u64);
 
@@ -194,7 +194,10 @@ impl SequenceNumber {
             .expect("time to be linear")
             .as_nanos();
 
-        Self(unix_timestamp as u64)
+        Self(
+            u64::try_from(unix_timestamp)
+                .expect("timestamp in nanos since UNIX_EPOCH should fit in u64"),
+        )
     }
 
     fn next(&mut self) -> u64 {