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>

doc/godebug.md

@@ -150,6 +150,13 @@ for example,
 see the [runtime documentation](/pkg/runtime#hdr-Environment_Variables)
 and the [go command documentation](/cmd/go#hdr-Build_and_test_caching).
 
+### Go 1.24
+
+Go 1.24 changed the global [`math/rand.Seed`](/pkg/math/rand/#Seed) to be a
+no-op. This setting is controlled by the `randseednop` setting.
+For Go 1.24 it defaults to `randseednop=1`.
+Using `randseednop=0` reverts to the pre-Go 1.24 behavior.
+
 ### Go 1.23
 
 Go 1.23 changed the channels created by package time to be unbuffered

doc/next/6-stdlib/99-minor/math/rand/67273.md

+Calls to the deprecated top-level [Seed] function no longer have any effect. To
+restore the old behavior set `GODEBUG=randseednop=0`. For more background see
+the proposal [#67273](/issue/67273).

src/internal/godebugs/table.go

@@ -47,6 +47,7 @@ var All = []Info{
 	{Name: "netedns0", Package: "net", Changed: 19, Old: "0"},
 	{Name: "panicnil", Package: "runtime", Changed: 21, Old: "1"},
 	{Name: "randautoseed", Package: "math/rand"},
+	{Name: "randseednop", Package: "math/rand", Changed: 24, Old: "0"},
 	{Name: "tarinsecurepath", Package: "archive/tar"},
 	{Name: "tls10server", Package: "crypto/tls", Changed: 22, Old: "1"},
 	{Name: "tls3des", Package: "crypto/tls", Changed: 23, Old: "1"},

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.

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) {

src/runtime/metrics/doc.go

@@ -306,6 +306,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the math/rand
 		package due to a non-default GODEBUG=randautoseed=... setting.
 
+	/godebug/non-default-behavior/randseednop:events
+		The number of non-default behaviors executed by the math/rand
+		package due to a non-default GODEBUG=randseednop=... setting.
+
 	/godebug/non-default-behavior/tarinsecurepath:events
 		The number of non-default behaviors executed by the archive/tar
 		package due to a non-default GODEBUG=tarinsecurepath=...