fix: make timestamps strictly increasing

On Linux, this is almost always the case. Windows, however, doesn't have
nanosecond accuracy.

We make the timestamp sequence numbers strictly increasing by returning
the last timestamp + 1 where necessary.

peer/record.go

@@ -2,6 +2,7 @@ package peer
 
 import (
 	"fmt"
+	"sync/atomic"
 	"time"
 
 	pb "github.com/libp2p/go-libp2p-core/peer/pb"
@@ -125,9 +126,21 @@ func PeerRecordFromProtobuf(msg *pb.PeerRecord) (*PeerRecord, error) {
 	return record, nil
 }
 
+var lastTimestamp uint64
+
 // TimestampSeq is a helper to generate a timestamp-based sequence number for a PeerRecord.
 func TimestampSeq() uint64 {
-	return uint64(time.Now().UnixNano())
+	now := uint64(time.Now().UnixNano())
+	previous := atomic.LoadUint64(&lastTimestamp)
+	// If the new time is not greater than the last tiemstamp, or if someone else beats us to
+	// updateing the timestamp, just use last+1.
+	//
+	// Technically, last+1 could be before "now". But it's still strictly increasing and close
+	// enough.
+	if now <= previous || !atomic.CompareAndSwapUint64(&lastTimestamp, previous, now) {
+		now = atomic.AddUint64(&lastTimestamp, 1)
+	}
+	return now
 }
 
 // Domain is used when signing and validating PeerRecords contained in Envelopes.