1 files changed, 148 insertions, 0 deletions
diff --git a/src/math/rand/default_test.go b/src/math/rand/default_test.go
new file mode 100644
index 0000000..19fd75d
--- /dev/null
+++ b/src/math/rand/default_test.go
@@ -0,0 +1,148 @@
+// 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"
+	"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
+	}
+}