Add a test for concurrent use of global loggers

In working on UberFX, I realized that zap's global loggers are actually safe to
replace at runtime; because they're pointers, they're a single machine word.
Since loggers are also never mutated, we don't need any locks to make this code
race-free.

This commit removes the incorrect (and scary) documentation about races and adds
a test that attempts to trigger a race.

global.go

@@ -29,9 +29,6 @@ import (
 var (
 	// L is a global Logger. It defaults to a no-op implementation but can be
 	// replaced using ReplaceGlobals.
-	//
-	// Both L and S are unsynchronized, so replacing them while they're in
-	// use isn't safe.
 	L = New(nil)
 	// S is a global SugaredLogger, similar to L. It also defaults to a no-op
 	// implementation.
@@ -40,11 +37,9 @@ var (
 
 // ReplaceGlobals replaces the global Logger L and the global SugaredLogger S,
 // and returns a function to restore the original values.
-//
-// Note that replacing the global loggers isn't safe while they're being used;
-// in practice, this means that only the owner of the application's main
-// function should use this method.
 func ReplaceGlobals(logger *Logger) func() {
+	// This doesn't require synchronization to be safe, since pointers are a
+	// single word.
 	prev := *L
 	L = logger
 	S = logger.Sugar()

global_test.go

@@ -22,11 +22,15 @@ package zap
 
 import (
 	"log"
+	"sync"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
+	"go.uber.org/atomic"
 
 	"go.uber.org/zap/internal/observer"
+	"go.uber.org/zap/testutils"
 	"go.uber.org/zap/zapcore"
 )
 
@@ -59,6 +63,34 @@ func TestReplaceGlobals(t *testing.T) {
 	assert.Equal(t, initialS, *S, "Expected func returned from ReplaceGlobals to restore initial S.")
 }
 
+func TestGlobalsConcurrentUse(t *testing.T) {
+	var (
+		stop atomic.Bool
+		wg   sync.WaitGroup
+	)
+
+	for i := 0; i < 100; i++ {
+		wg.Add(2)
+		go func() {
+			for stop.Load() {
+				ReplaceGlobals(New(nil))
+			}
+			wg.Done()
+		}()
+		go func() {
+			for stop.Load() {
+				L.Info("")
+				S.Info("")
+			}
+			wg.Done()
+		}()
+	}
+
+	testutils.Sleep(100 * time.Millisecond)
+	stop.Toggle()
+	wg.Wait()
+}
+
 func TestRedirectStdLog(t *testing.T) {
 	initialFlags := log.Flags()
 	initialPrefix := log.Prefix()