math/rand/v2: remove Rand.Seed

Removing Rand.Seed lets us remove lockedSource as well,
along with the ambiguity in globalRand about which source
to use.

For #61716.

Change-Id: Ibe150520dd1e7dd87165eacaebe9f0c2daeaedfd
Reviewed-on: https://go-review.googlesource.com/c/go/+/502498
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Rob Pike <r@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>

5 files changed, 25 insertions, 315 deletions

diff --git a/api/next/61716.txt b/api/next/61716.txt
index 341c2e45c4..353ad379d0 100644
--- a/api/next/61716.txt
+++ b/api/next/61716.txt
@@ -12,8 +12,6 @@ pkg math/rand/v2, func NewSource(int64) Source #61716
 pkg math/rand/v2, func NewZipf(*Rand, float64, float64, uint64) *Zipf #61716
 pkg math/rand/v2, func NormFloat64() float64 #61716
 pkg math/rand/v2, func Perm(int) []int #61716
-pkg math/rand/v2, func Seed //deprecated #61716
-pkg math/rand/v2, func Seed(int64) #61716
 pkg math/rand/v2, func Shuffle(int, func(int, int)) #61716
 pkg math/rand/v2, func Uint32() uint32 #61716
 pkg math/rand/v2, func Uint64() uint64 #61716
@@ -28,17 +26,14 @@ pkg math/rand/v2, method (*Rand) Int64N(int64) int64 #61716
 pkg math/rand/v2, method (*Rand) IntN(int) int #61716
 pkg math/rand/v2, method (*Rand) NormFloat64() float64 #61716
 pkg math/rand/v2, method (*Rand) Perm(int) []int #61716
-pkg math/rand/v2, method (*Rand) Seed(int64) #61716
 pkg math/rand/v2, method (*Rand) Shuffle(int, func(int, int)) #61716
 pkg math/rand/v2, method (*Rand) Uint32() uint32 #61716
 pkg math/rand/v2, method (*Rand) Uint64() uint64 #61716
 pkg math/rand/v2, method (*Zipf) Uint64() uint64 #61716
 pkg math/rand/v2, type Rand struct #61716
-pkg math/rand/v2, type Source interface { Int64, Seed } #61716
+pkg math/rand/v2, type Source interface { Int64 } #61716
 pkg math/rand/v2, type Source interface, Int64() int64 #61716
-pkg math/rand/v2, type Source interface, Seed(int64) #61716
-pkg math/rand/v2, type Source64 interface { Int64, Seed, Uint64 } #61716
+pkg math/rand/v2, type Source64 interface { Int64, Uint64 } #61716
 pkg math/rand/v2, type Source64 interface, Int64() int64 #61716
-pkg math/rand/v2, type Source64 interface, Seed(int64) #61716
 pkg math/rand/v2, type Source64 interface, Uint64() uint64 #61716
 pkg math/rand/v2, type Zipf struct #61716
diff --git a/src/math/rand/v2/auto_test.go b/src/math/rand/v2/auto_test.go
index ad6cd8bdae..8b1f7547d7 100644
--- a/src/math/rand/v2/auto_test.go
+++ b/src/math/rand/v2/auto_test.go
@@ -16,20 +16,20 @@ import (
 func TestAuto(t *testing.T) {
 	// Pull out 10 int64s from the global source
 	// and then check that they don't appear in that
-	// order in the deterministic Seed(1) result.
+	// order in the deterministic seeded result.
 	var out []int64
 	for i := 0; i < 10; i++ {
 		out = append(out, Int64())
 	}
 
-	// Look for out in Seed(1)'s output.
+	// Look for out in seeded output.
 	// Strictly speaking, we should look for them in order,
 	// but this is good enough and not significantly more
 	// likely to have a false positive.
-	Seed(1)
+	r := New(NewSource(1))
 	found := 0
 	for i := 0; i < 1000; i++ {
-		x := Int64()
+		x := r.Int64()
 		if x == out[found] {
 			found++
 			if found == len(out) {
diff --git a/src/math/rand/v2/default_test.go b/src/math/rand/v2/default_test.go
deleted file mode 100644
index 6b42e2e19a..0000000000
--- a/src/math/rand/v2/default_test.go
+++ /dev/null
@@ -1,148 +0,0 @@
-// Copyright 2023 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package rand_test
-
-import (
-	"fmt"
-	"internal/race"
-	"internal/testenv"
-	. "math/rand/v2"
-	"os"
-	"runtime"
-	"strconv"
-	"sync"
-	"testing"
-)
-
-// Test that racy access to the default functions behaves reasonably.
-func TestDefaultRace(t *testing.T) {
-	// Skip the test in short mode, but even in short mode run
-	// the test if we are using the race detector, because part
-	// of this is to see whether the race detector reports any problems.
-	if testing.Short() && !race.Enabled {
-		t.Skip("skipping starting another executable in short mode")
-	}
-
-	const env = "GO_RAND_TEST_HELPER_CODE"
-	if v := os.Getenv(env); v != "" {
-		doDefaultTest(t, v)
-		return
-	}
-
-	t.Parallel()
-
-	for i := 0; i < 6; i++ {
-		i := i
-		t.Run(strconv.Itoa(i), func(t *testing.T) {
-			t.Parallel()
-			exe, err := os.Executable()
-			if err != nil {
-				exe = os.Args[0]
-			}
-			cmd := testenv.Command(t, exe, "-test.run=TestDefaultRace")
-			cmd = testenv.CleanCmdEnv(cmd)
-			cmd.Env = append(cmd.Env, fmt.Sprintf("GO_RAND_TEST_HELPER_CODE=%d", i/2))
-			if i%2 != 0 {
-				cmd.Env = append(cmd.Env, "GODEBUG=randautoseed=0")
-			}
-			out, err := cmd.CombinedOutput()
-			if len(out) > 0 {
-				t.Logf("%s", out)
-			}
-			if err != nil {
-				t.Error(err)
-			}
-		})
-	}
-}
-
-// doDefaultTest should be run before there have been any calls to the
-// top-level math/rand functions. Make sure that we can make concurrent
-// calls to top-level functions and to Seed without any duplicate values.
-// This will also give the race detector a change to report any problems.
-func doDefaultTest(t *testing.T, v string) {
-	code, err := strconv.Atoi(v)
-	if err != nil {
-		t.Fatalf("internal error: unrecognized code %q", v)
-	}
-
-	goroutines := runtime.GOMAXPROCS(0)
-	if goroutines < 4 {
-		goroutines = 4
-	}
-
-	ch := make(chan uint64, goroutines*3)
-	var wg sync.WaitGroup
-
-	// The various tests below should not cause race detector reports
-	// and should not produce duplicate results.
-	//
-	// Note: these tests can theoretically fail when using fastrand64
-	// in that it is possible to coincidentally get the same random
-	// number twice. That could happen something like 1 / 2**64 times,
-	// which is rare enough that it may never happen. We don't worry
-	// about that case.
-
-	switch code {
-	case 0:
-		// Call Seed and Uint64 concurrently.
-		wg.Add(goroutines)
-		for i := 0; i < goroutines; i++ {
-			go func(s int64) {
-				defer wg.Done()
-				Seed(s)
-			}(int64(i) + 100)
-		}
-		wg.Add(goroutines)
-		for i := 0; i < goroutines; i++ {
-			go func() {
-				defer wg.Done()
-				ch <- Uint64()
-			}()
-		}
-	case 1:
-		// Call Uint64 concurrently with no Seed.
-		wg.Add(goroutines)
-		for i := 0; i < goroutines; i++ {
-			go func() {
-				defer wg.Done()
-				ch <- Uint64()
-			}()
-		}
-	case 2:
-		// Start with Uint64 to pick the fast source, then call
-		// Seed and Uint64 concurrently.
-		ch <- Uint64()
-		wg.Add(goroutines)
-		for i := 0; i < goroutines; i++ {
-			go func(s int64) {
-				defer wg.Done()
-				Seed(s)
-			}(int64(i) + 100)
-		}
-		wg.Add(goroutines)
-		for i := 0; i < goroutines; i++ {
-			go func() {
-				defer wg.Done()
-				ch <- Uint64()
-			}()
-		}
-	default:
-		t.Fatalf("internal error: unrecognized code %d", code)
-	}
-
-	go func() {
-		wg.Wait()
-		close(ch)
-	}()
-
-	m := make(map[uint64]bool)
-	for i := range ch {
-		if m[i] {
-			t.Errorf("saw %d twice", i)
-		}
-		m[i] = true
-	}
-}
diff --git a/src/math/rand/v2/race_test.go b/src/math/rand/v2/race_test.go
index 963b37e91d..5ab7a21fa5 100644
--- a/src/math/rand/v2/race_test.go
+++ b/src/math/rand/v2/race_test.go
@@ -23,9 +23,8 @@ func TestConcurrent(t *testing.T) {
 	for i := 0; i < numRoutines; i++ {
 		go func(i int) {
 			defer wg.Done()
-			buf := make([]byte, 997)
+			var seed int64
 			for j := 0; j < numCycles; j++ {
-				var seed int64
 				seed += int64(ExpFloat64())
 				seed += int64(Float32())
 				seed += int64(Float64())
@@ -38,11 +37,8 @@ func TestConcurrent(t *testing.T) {
 				for _, p := range Perm(10) {
 					seed += int64(p)
 				}
-				for _, b := range buf {
-					seed += int64(b)
-				}
-				Seed(int64(i*j) * seed)
 			}
+			_ = seed
 		}(i)
 	}
 }
diff --git a/src/math/rand/v2/rand.go b/src/math/rand/v2/rand.go
index dd2213ff83..c9d1ec11df 100644
--- a/src/math/rand/v2/rand.go
+++ b/src/math/rand/v2/rand.go
@@ -18,9 +18,6 @@
 package rand
 
 import (
-	"internal/godebug"
-	"sync"
-	"sync/atomic"
 	_ "unsafe" // for go:linkname
 )
 
@@ -30,7 +27,6 @@ import (
 // A Source is not safe for concurrent use by multiple goroutines.
 type Source interface {
 	Int64() int64
-	Seed(seed int64)
 }
 
 // A Source64 is a Source that can also generate
@@ -71,16 +67,6 @@ func New(src Source) *Rand {
 	return &Rand{src: src, s64: s64}
 }
 
-// Seed uses the provided seed value to initialize the generator to a deterministic state.
-// Seed should not be called concurrently with any other Rand method.
-func (r *Rand) Seed(seed int64) {
-	if lk, ok := r.src.(*lockedSource); ok {
-		lk.Seed(seed)
-		return
-	}
-	r.src.Seed(seed)
-}
-
 // Int64 returns a non-negative pseudo-random 63-bit integer as an int64.
 func (r *Rand) Int64() int64 { return r.src.Int64() }
 
@@ -259,46 +245,9 @@ func (r *Rand) Shuffle(n int, swap func(i, j int)) {
  * Top-level convenience functions
  */
 
-// globalRandGenerator is the source of random numbers for the top-level
-// convenience functions. When possible it uses the runtime fastrand64
-// function to avoid locking. This is not possible if the user called Seed,
-// either explicitly or implicitly via GODEBUG=randautoseed=0.
-var globalRandGenerator atomic.Pointer[Rand]
-
-var randautoseed = godebug.New("randautoseed")
-
-// globalRand returns the generator to use for the top-level convenience
-// functions.
-func globalRand() *Rand {
-	if r := globalRandGenerator.Load(); r != nil {
-		return r
-	}
-
-	// This is the first call. Initialize based on GODEBUG.
-	var r *Rand
-	if randautoseed.Value() == "0" {
-		randautoseed.IncNonDefault()
-		r = New(new(lockedSource))
-		r.Seed(1)
-	} else {
-		r = &Rand{
-			src: &fastSource{},
-			s64: &fastSource{},
-		}
-	}
-
-	if !globalRandGenerator.CompareAndSwap(nil, r) {
-		// Two different goroutines called some top-level
-		// function at the same time. While the results in
-		// that case are unpredictable, if we just use r here,
-		// and we are using a seed, we will most likely return
-		// the same value for both calls. That doesn't seem ideal.
-		// Just use the first one to get in.
-		return globalRandGenerator.Load()
-	}
-
-	return r
-}
+// globalRand is the source of random numbers for the top-level
+// convenience functions.
+var globalRand = &Rand{src: &fastSource{}}
 
 //go:linkname fastrand64
 func fastrand64() uint64
@@ -310,107 +259,60 @@ func (*fastSource) Int64() int64 {
 	return int64(fastrand64() & rngMask)
 }
 
-func (*fastSource) Seed(int64) {
-	panic("internal error: call to fastSource.Seed")
-}
-
 func (*fastSource) Uint64() uint64 {
 	return fastrand64()
 }
 
-// Seed uses the provided seed value to initialize the default Source to a
-// deterministic state. Seed values that have the same remainder when
-// divided by 2³¹-1 generate the same pseudo-random sequence.
-// Seed, unlike the Rand.Seed method, is safe for concurrent use.
-//
-// If Seed is not called, the generator is seeded randomly at program startup.
-//
-// Prior to Go 1.20, the generator was seeded like Seed(1) at program startup.
-// To force the old behavior, call Seed(1) at program startup.
-// Alternately, set GODEBUG=randautoseed=0 in the environment
-// before making any calls to functions in this package.
-//
-// Deprecated: As of Go 1.20 there is no reason to call Seed with
-// 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.
-func Seed(seed int64) {
-	orig := globalRandGenerator.Load()
-
-	// If we are already using a lockedSource, we can just re-seed it.
-	if orig != nil {
-		if _, ok := orig.src.(*lockedSource); ok {
-			orig.Seed(seed)
-			return
-		}
-	}
-
-	// Otherwise either
-	// 1) orig == nil, which is the normal case when Seed is the first
-	// top-level function to be called, or
-	// 2) orig is already a fastSource, in which case we need to change
-	// to a lockedSource.
-	// Either way we do the same thing.
-
-	r := New(new(lockedSource))
-	r.Seed(seed)
-
-	if !globalRandGenerator.CompareAndSwap(orig, r) {
-		// Something changed underfoot. Retry to be safe.
-		Seed(seed)
-	}
-}
-
 // Int64 returns a non-negative pseudo-random 63-bit integer as an int64
 // from the default Source.
-func Int64() int64 { return globalRand().Int64() }
+func Int64() int64 { return globalRand.Int64() }
 
 // Uint32 returns a pseudo-random 32-bit value as a uint32
 // from the default Source.
-func Uint32() uint32 { return globalRand().Uint32() }
+func Uint32() uint32 { return globalRand.Uint32() }
 
 // Uint64 returns a pseudo-random 64-bit value as a uint64
 // from the default Source.
-func Uint64() uint64 { return globalRand().Uint64() }
+func Uint64() uint64 { return globalRand.Uint64() }
 
 // Int32 returns a non-negative pseudo-random 31-bit integer as an int32
 // from the default Source.
-func Int32() int32 { return globalRand().Int32() }
+func Int32() int32 { return globalRand.Int32() }
 
 // Int returns a non-negative pseudo-random int from the default Source.
-func Int() int { return globalRand().Int() }
+func Int() int { return globalRand.Int() }
 
 // Int64N returns, as an int64, a non-negative pseudo-random number in the half-open interval [0,n)
 // from the default Source.
 // It panics if n <= 0.
-func Int64N(n int64) int64 { return globalRand().Int64N(n) }
+func Int64N(n int64) int64 { return globalRand.Int64N(n) }
 
 // Int32N returns, as an int32, a non-negative pseudo-random number in the half-open interval [0,n)
 // from the default Source.
 // It panics if n <= 0.
-func Int32N(n int32) int32 { return globalRand().Int32N(n) }
+func Int32N(n int32) int32 { return globalRand.Int32N(n) }
 
 // IntN returns, as an int, a non-negative pseudo-random number in the half-open interval [0,n)
 // from the default Source.
 // It panics if n <= 0.
-func IntN(n int) int { return globalRand().IntN(n) }
+func IntN(n int) int { return globalRand.IntN(n) }
 
 // Float64 returns, as a float64, a pseudo-random number in the half-open interval [0.0,1.0)
 // from the default Source.
-func Float64() float64 { return globalRand().Float64() }
+func Float64() float64 { return globalRand.Float64() }
 
 // Float32 returns, as a float32, a pseudo-random number in the half-open interval [0.0,1.0)
 // from the default Source.
-func Float32() float32 { return globalRand().Float32() }
+func Float32() float32 { return globalRand.Float32() }
 
 // Perm returns, as a slice of n ints, a pseudo-random permutation of the integers
 // in the half-open interval [0,n) from the default Source.
-func Perm(n int) []int { return globalRand().Perm(n) }
+func Perm(n int) []int { return globalRand.Perm(n) }
 
 // Shuffle pseudo-randomizes the order of elements using the default Source.
 // n is the number of elements. Shuffle panics if n < 0.
 // swap swaps the elements with indexes i and j.
-func Shuffle(n int, swap func(i, j int)) { globalRand().Shuffle(n, swap) }
+func Shuffle(n int, swap func(i, j int)) { globalRand.Shuffle(n, swap) }
 
 // NormFloat64 returns a normally distributed float64 in the range
 // [-math.MaxFloat64, +math.MaxFloat64] with
@@ -420,7 +322,7 @@ func Shuffle(n int, swap func(i, j int)) { globalRand().Shuffle(n, swap) }
 // adjust the output using:
 //
 //	sample = NormFloat64() * desiredStdDev + desiredMean
-func NormFloat64() float64 { return globalRand().NormFloat64() }
+func NormFloat64() float64 { return globalRand.NormFloat64() }
 
 // ExpFloat64 returns an exponentially distributed float64 in the range
 // (0, +math.MaxFloat64] with an exponential distribution whose rate parameter
@@ -429,39 +331,4 @@ func NormFloat64() float64 { return globalRand().NormFloat64() }
 // callers can adjust the output using:
 //
 //	sample = ExpFloat64() / desiredRateParameter
-func ExpFloat64() float64 { return globalRand().ExpFloat64() }
-
-type lockedSource struct {
-	lk sync.Mutex
-	s  *rngSource
-}
-
-func (r *lockedSource) Int64() (n int64) {
-	r.lk.Lock()
-	n = r.s.Int64()
-	r.lk.Unlock()
-	return
-}
-
-func (r *lockedSource) Uint64() (n uint64) {
-	r.lk.Lock()
-	n = r.s.Uint64()
-	r.lk.Unlock()
-	return
-}
-
-func (r *lockedSource) Seed(seed int64) {
-	r.lk.Lock()
-	r.seed(seed)
-	r.lk.Unlock()
-}
-
-// seed seeds the underlying source.
-// The caller must have locked r.lk.
-func (r *lockedSource) seed(seed int64) {
-	if r.s == nil {
-		r.s = newSource(seed)
-	} else {
-		r.s.Seed(seed)
-	}
-}
+func ExpFloat64() float64 { return globalRand.ExpFloat64() }