tstest: add a helper for reproducable random seeds

We recently ran across a case of flaky tests that were seeding the
math/rand package and using that to generate data. The tests were
flaking on specific random values that were difficult to reproduce. If
the test had been using a helper like this, the flaky cases could have
been reliably reproduced.

Signed-off-by: James Tucker <james@tailscale.com>

tstest/tstest.go

@@ -6,6 +6,10 @@ package tstest
 
 import (
 	"context"
+	"math/rand"
+	"os"
+	"strconv"
+	"sync"
 	"testing"
 	"time"
 
@@ -46,3 +50,38 @@ func WaitFor(maxWait time.Duration, try func() error) error {
 	}
 	return err
 }
+
+var (
+	seed     int64
+	seedOnce sync.Once
+)
+
+// GetSeed gets the current global random test seed, by default this is based on
+// the current time, but can be fixed to a particular value using the
+// TS_TEST_SEED environment variable.
+func GetSeed(t testing.TB) int64 {
+	t.Helper()
+
+	seedOnce.Do(func() {
+		if s := os.Getenv("TS_TEST_SEED"); s != "" {
+			var err error
+			seed, err = strconv.ParseInt(s, 10, 64)
+			if err != nil {
+				t.Fatalf("invalid TS_TEST_SEED: %v", err)
+			}
+		} else {
+			seed = time.Now().UnixNano()
+		}
+	})
+	return seed
+}
+
+// SeedRand seeds the standard library global rand with the current test seed.
+func SeedRand(t testing.TB) {
+	t.Helper()
+
+	// Seed is called every time, as other tests may execute code that reseeds
+	// the global rand.
+	rand.Seed(GetSeed(t))
+	t.Logf("TS_TEST_SEED=%d", seed)
+}

tstest/tstest_test.go

@@ -3,7 +3,9 @@
 
 package tstest
 
-import "testing"
+import (
+	"testing"
+)
 
 func TestReplace(t *testing.T) {
 	before := "before"
@@ -22,3 +24,10 @@ func TestReplace(t *testing.T) {
 		t.Errorf("before = %q; want %q", before, "before")
 	}
 }
+
+func TestGetSeed(t *testing.T) {
+	t.Setenv("TS_TEST_SEED", "1234")
+	if got, want := GetSeed(t), int64(1234); got != want {
+		t.Errorf("GetSeed = %v; want %v", got, want)
+	}
+}