Merge #139104

139104: metamorphic: use globalSeed from `randutil.NewTestRand` r=stevendanna,RaduBerinde,herkolategan a=srosenberg

A minor quality of life improvement to ensure
metamorphic constants are derived from the
globalSeed. This makes it possible to reproduce
both the metamoprhic constants and the individual
test's rng values, from `COCKROACH_RANDOM_SEED`.

We also log the seed, when metamoprhic constants
are enabled. As of this change, we get a message
of this form, logged to `stderr`,
```
metamorphic: use COCKROACH_RANDOM_SEED=9076506593373491438 for reproduction
```

Epic: none
Release note: None

Co-authored-by: Stan Rosenberg <[email protected]>

pkg/util/metamorphic/constants.go

@@ -229,6 +229,8 @@ var (
 		r *rand.Rand
 		syncutil.Mutex
 	}
+	// Exposed for testing.
+	rngSeed int64
 )
 
 // Returns true iff the current process is eligible to enable metamorphic
@@ -267,8 +269,13 @@ func valueFromOverride[T any](name string, parser func(string) (T, error)) (T, b
 func init() {
 	if metamorphicEligible() {
 		if !disableMetamorphicTesting {
-			rng.r, _ = randutil.NewTestRand()
+			rng.r, rngSeed = randutil.NewPseudoRandWithGlobalSeed()
 			metamorphicutil.IsMetamorphicBuild = rng.r.Float64() < IsMetamorphicBuildProbability
+			if metamorphicutil.IsMetamorphicBuild {
+				logf("metamorphic: use COCKROACH_RANDOM_SEED=%d for reproduction\n", rngSeed)
+			} else {
+				logf("IsMetamorphicBuild==false\n")
+			}
 		}
 
 		if overrideList, ok := envutil.EnvString(MetamorphicOverridesEnvVar, 0); ok {

pkg/util/metamorphic/constants_test.go

@@ -6,8 +6,10 @@
 package metamorphic
 
 import (
+	"math/rand"
 	"testing"
 
+	"github.com/cockroachdb/cockroach/pkg/util/randutil"
 	"github.com/stretchr/testify/require"
 )
 
@@ -27,3 +29,25 @@ func TestMetamorphicFromOverride(t *testing.T) {
 	)
 	require.Equal(t, 7, v)
 }
+
+// TestMetamorphicRngSeed checks that `constants.rngSeed` corresponds to `randutil.globalSeed`
+func TestMetamorphicRngSeed(t *testing.T) {
+	// Make sure metamorphic testing is enabled.
+	require.True(t, metamorphicEligible())
+	require.False(t, disableMetamorphicTesting)
+	// N.B. We can't access `randutil.globalSeed` directly.
+	// Instead, the call below forces NewTestRand to reseed its iternal RNG with the globalSeed.
+	// The returned seed is _not_ the globalSeed, but one (application of) `Int63` away.
+	testRng, testSeed := randutil.NewTestRand()
+	// Thus, if rngSeed == globalSeed, it implies r(rngSeed).Int63() == testSeed.
+	// N.B. The converse doesn't necessarily hold, as two different seeds could yield the same PRNG sequence. However,
+	// it's extremely improbable. Hence, this assertion should suffice.
+	expectedSeed := rand.New(rand.NewSource(rngSeed)).Int63()
+	require.Equal(t, expectedSeed, testSeed)
+	// On the off-chance there is a collision, let's do 100 iterations. The probability of two different seeds yielding
+	// the same sequence of length 101 is infinitesimally small.
+	r := rand.New(rand.NewSource(expectedSeed))
+	for i := 0; i < 100; i++ {
+		require.Equal(t, r.Int63(), testRng.Int63())
+	}
+}

pkg/util/randutil/rand.go

@@ -99,6 +99,16 @@ func NewLockedPseudoRand() (*rand.Rand, int64) {
 	return rand.New(NewLockedSource(seed)), seed
 }
 
+// NewPseudoRandWithGlobalSeed returns an instance of math/rand.Rand, which is
+// seeded with the global seed.
+// It's _not_ intended to be called directly from a test; use NewTestRand for that.
+// Instead, this function is useful for seeding other random number generators, on which the tests
+// may depend; e.g., metamorphic constants.
+// N.B. unlike NewTestRand, this function _never_ reseeds rng.
+func NewPseudoRandWithGlobalSeed() (*rand.Rand, int64) {
+	return rand.New(rand.NewSource(globalSeed)), globalSeed
+}
+
 // NewTestRand returns an instance of math/rand.Rand seeded from rng, which is
 // seeded with the global seed. If the caller is a test with a different
 // path-qualified name than the previous caller, rng is reseeded from the global