tai64n: make the test deterministic

In the presence of preemption, the current test may fail transiently.
This uses static test data instead to ensure consistent behavior.

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

1 files changed, 25 insertions, 15 deletions

diff --git a/tai64n/tai64n_test.go b/tai64n/tai64n_test.go
index 05a9d8f..6df7367 100644
--- a/tai64n/tai64n_test.go
+++ b/tai64n/tai64n_test.go
@@ -10,21 +10,31 @@ import (
 	"time"
 )
 
-/* Testing the essential property of the timestamp
- * as used by WireGuard.
- */
+// Test that timestamps are monotonic as required by Wireguard and that
+// nanosecond-level information is whitened to prevent side channel attacks.
 func TestMonotonic(t *testing.T) {
-	old := Now()
-	for i := 0; i < 50; i++ {
-		next := Now()
-		if next.After(old) {
-			t.Error("Whitening insufficient")
-		}
-		time.Sleep(time.Duration(whitenerMask)/time.Nanosecond + 1)
-		next = Now()
-		if !next.After(old) {
-			t.Error("Not monotonically increasing on whitened nano-second scale")
-		}
-		old = next
+	startTime := time.Unix(0, 123456789) // a nontrivial bit pattern
+	// Whitening should reduce timestamp granularity
+	// to more than 10 but fewer than 20 milliseconds.
+	tests := []struct {
+		name      string
+		t1, t2    time.Time
+		wantAfter bool
+	}{
+		{"after_10_ns", startTime, startTime.Add(10 * time.Nanosecond), false},
+		{"after_10_us", startTime, startTime.Add(10 * time.Microsecond), false},
+		{"after_1_ms", startTime, startTime.Add(time.Millisecond), false},
+		{"after_10_ms", startTime, startTime.Add(10 * time.Millisecond), false},
+		{"after_20_ms", startTime, startTime.Add(20 * time.Millisecond), true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ts1, ts2 := stamp(tt.t1), stamp(tt.t2)
+			got := ts2.After(ts1)
+			if got != tt.wantAfter {
+				t.Errorf("after = %v; want %v", got, tt.wantAfter)
+			}
+		})
 	}
 }