testutils: log random seed to test output

Include the used seed in the test log so that test2json contains
the full information needed to recreate a test failure. This is
useful on CI which parses the output and provides a nice UI
to see failures.

Also allow overriding the used seed via an environment variable.

Signed-off-by: Lorenz Bauer <[email protected]>

btf/core_test.go

@@ -649,10 +649,11 @@ func TestCORECopyWithoutQualifiers(t *testing.T) {
 	}
 
 	t.Run("long chain", func(t *testing.T) {
+		rng := testutils.Rand(t)
 		root := &Int{Name: "abc"}
 		v := Type(root)
 		for i := 0; i < maxTypeDepth; i++ {
-			q := qualifiers[testutils.Rand().Intn(len(qualifiers))]
+			q := qualifiers[rng.Intn(len(qualifiers))]
 			v = q.fn(v)
 			t.Log(q.name)
 		}

btf/marshal_test.go

@@ -75,7 +75,7 @@ func TestRoundtripVMlinux(t *testing.T) {
 
 	// Randomize the order to force different permutations of walking the type
 	// graph. Keep Void at index 0.
-	testutils.Rand().Shuffle(len(types[1:]), func(i, j int) {
+	testutils.Rand(t).Shuffle(len(types[1:]), func(i, j int) {
 		types[i+1], types[j+1] = types[j+1], types[i+1]
 	})
 

internal/testutils/seed.go

@@ -1,9 +1,11 @@
 package testutils
 
 import (
-	"fmt"
 	"math/rand"
+	"os"
+	"strconv"
 	"sync"
+	"testing"
 	"time"
 )
 
@@ -12,10 +14,20 @@ var randSeed struct {
 	once  sync.Once
 }
 
-func Rand() *rand.Rand {
+func Rand(tb testing.TB) *rand.Rand {
 	randSeed.once.Do(func() {
 		randSeed.value = time.Now().UnixMicro()
-		fmt.Printf("Random seed is %d\n", randSeed.value)
 	})
-	return rand.New(rand.NewSource(randSeed.value))
+
+	seed := randSeed.value
+	if seedStr, ok := os.LookupEnv("TEST_SEED"); ok {
+		var err error
+		seed, err = strconv.ParseInt(seedStr, 0, 64)
+		if err != nil {
+			tb.Fatal("Parse TEST_SEED environment variable:", err)
+		}
+	}
+
+	tb.Logf("TEST_SEED=%d\n", seed)
+	return rand.New(rand.NewSource(seed))
 }