math/rand: make calls to Seed no-op

Makes calls to the global Seed a no-op. The GODEBUG=randseednop=0
setting can be used to revert this behavior.

Fixes #67273

Change-Id: I79c1b2b23f3bc472fbd6190cb916a9d7583250f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/606055
Auto-Submit: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 47 insertions, 0 deletions

diff --git a/src/math/rand/rand.go b/src/math/rand/rand.go
index 61ff5c1b38..4be1ca208a 100644
--- a/src/math/rand/rand.go
+++ b/src/math/rand/rand.go
@@ -313,6 +313,9 @@ var globalRandGenerator atomic.Pointer[Rand]
 
 var randautoseed = godebug.New("randautoseed")
 
+// randseednop controls whether the global Seed is a no-op.
+var randseednop = godebug.New("randseednop")
+
 // globalRand returns the generator to use for the top-level convenience
 // functions.
 func globalRand() *Rand {
@@ -391,7 +394,15 @@ func (fs *runtimeSource) read(p []byte, readVal *int64, readPos *int8) (n int, e
 // a random value. Programs that call Seed with a known value to get
 // a specific sequence of results should use New(NewSource(seed)) to
 // obtain a local random generator.
+//
+// As of Go 1.24 [Seed] is a no-op. To restore the previous behavior set
+// GODEBUG=randseednop=0.
 func Seed(seed int64) {
+	if randseednop.Value() != "0" {
+		return
+	}
+	randseednop.IncNonDefault()
+
 	orig := globalRandGenerator.Load()
 
 	// If we are already using a lockedSource, we can just re-seed it.
diff --git a/src/math/rand/rand_test.go b/src/math/rand/rand_test.go
index 7906f29674..1e1fad79ce 100644
--- a/src/math/rand/rand_test.go
+++ b/src/math/rand/rand_test.go
@@ -556,6 +556,42 @@ func TestUniformFactorial(t *testing.T) {
 	}
 }
 
+func TestSeedNop(t *testing.T) {
+	// If the global Seed takes effect, then resetting it to a certain value
+	// should provide predictable output to functions using it.
+	t.Run("randseednop=0", func(t *testing.T) {
+		t.Setenv("GODEBUG", "randseednop=0")
+		Seed(1)
+		before := Int63()
+		Seed(1)
+		after := Int63()
+		if before != after {
+			t.Fatal("global Seed should take effect")
+		}
+	})
+	// If calls to the global Seed are no-op then functions using it should
+	// provide different output, even if it was reset to the same value.
+	t.Run("randseednop=1", func(t *testing.T) {
+		t.Setenv("GODEBUG", "randseednop=1")
+		Seed(1)
+		before := Int63()
+		Seed(1)
+		after := Int63()
+		if before == after {
+			t.Fatal("global Seed should be a no-op")
+		}
+	})
+	t.Run("GODEBUG unset", func(t *testing.T) {
+		Seed(1)
+		before := Int63()
+		Seed(1)
+		after := Int63()
+		if before == after {
+			t.Fatal("global Seed should default to being a no-op")
+		}
+	})
+}
+
 // Benchmarks
 
 func BenchmarkInt63Threadsafe(b *testing.B) {