From d2b6bdb0350e9b2b8f704b214ce5936b72de043d Mon Sep 17 00:00:00 2001
From: Paschalis T <paschalis.tsilias@gmail.com>
Date: Fri, 16 Aug 2024 01:29:18 +0300
Subject: [PATCH] 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 | 7 ++++
doc/next/6-stdlib/99-minor/math/rand/67273.md | 3 ++
src/internal/godebugs/table.go | 1 +
src/math/rand/rand.go | 11 ++++++
src/math/rand/rand_test.go | 36 +++++++++++++++++++
src/runtime/metrics/doc.go | 4 +++
6 files changed, 62 insertions(+)
create mode 100644 doc/next/6-stdlib/99-minor/math/rand/67273.md
diff --git a/doc/godebug.md b/doc/godebug.md
index b3a43664c42..d19de2374aa 100644
--- a/doc/godebug.md
+++ b/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
diff --git a/doc/next/6-stdlib/99-minor/math/rand/67273.md b/doc/next/6-stdlib/99-minor/math/rand/67273.md
new file mode 100644
index 00000000000..9882a3a332d
--- /dev/null
+++ b/doc/next/6-stdlib/99-minor/math/rand/67273.md
@@ -0,0 +1,3 @@
+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).
diff --git a/src/internal/godebugs/table.go b/src/internal/godebugs/table.go
index a802ac9c370..f8d30db5a3b 100644
--- a/src/internal/godebugs/table.go
+++ b/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"},
diff --git a/src/math/rand/rand.go b/src/math/rand/rand.go
index 61ff5c1b387..4be1ca208a8 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 7906f296743..1e1fad79cec 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) {
diff --git a/src/runtime/metrics/doc.go b/src/runtime/metrics/doc.go
index da3d956d480..906abb4102e 100644
--- a/src/runtime/metrics/doc.go
+++ 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=...
--
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