fix: resolve concurrent get may lead to empty result in ImmutableResource (#5065)

Co-authored-by: hsun <hsun@apac.freewheel.com>
Co-authored-by: Kevin Wan <wanjunfeng@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

core/syncx/immutableresource_test.go

@@ -2,6 +2,8 @@ package syncx
 
 import (
 	"errors"
+	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -56,6 +58,46 @@ func TestImmutableResourceError(t *testing.T) {
 	assert.Equal(t, 2, count)
 }
 
+func TestImmutableResourceConcurrent(t *testing.T) {
+	var count int32
+	ready := make(chan struct{})
+	r := NewImmutableResource(func() (any, error) {
+		atomic.AddInt32(&count, 1)
+		close(ready)                      // signal that fetch started
+		time.Sleep(10 * time.Millisecond) // simulate slow fetch
+		return "hello", nil
+	})
+
+	const goroutines = 100
+	var wg sync.WaitGroup
+	results := make([]any, goroutines)
+	errors := make([]error, goroutines)
+
+	wg.Add(goroutines)
+	for i := 0; i < goroutines; i++ {
+		go func(idx int) {
+			defer wg.Done()
+			res, err := r.Get()
+			results[idx] = res
+			errors[idx] = err
+		}(i)
+	}
+
+	// wait for fetch to start
+	<-ready
+
+	wg.Wait()
+
+	// fetch should only be called once despite concurrent access
+	assert.Equal(t, int32(1), atomic.LoadInt32(&count))
+
+	// all goroutines should eventually get the same result
+	for i := 0; i < goroutines; i++ {
+		assert.Nil(t, errors[i])
+		assert.Equal(t, "hello", results[i])
+	}
+}
+
 func TestImmutableResourceErrorRefreshAlways(t *testing.T) {
 	var count int
 	r := NewImmutableResource(func() (any, error) {