update

utils/singleton/singleton.go

@@ -0,0 +1,69 @@
+package singleton
+
+import (
+	"fmt"
+	"reflect"
+	"sync"
+
+	"github.com/navidrome/navidrome/log"
+)
+
+var (
+	instances = map[string]interface{}{}
+	pending   = map[string]chan struct{}{}
+	lock      sync.RWMutex
+)
+
+func GetInstance[T any](constructor func() T) T {
+	var v T
+	name := reflect.TypeOf(v).String()
+
+	// First check with read lock
+	lock.RLock()
+	if instance, ok := instances[name]; ok {
+		defer lock.RUnlock()
+		return instance.(T)
+	}
+	lock.RUnlock()
+
+	// Now check if someone is already creating this type
+	lock.Lock()
+
+	// Check again with the write lock - someone might have created it
+	if instance, ok := instances[name]; ok {
+		lock.Unlock()
+		return instance.(T)
+	}
+
+	// Check if creation is pending
+	wait, isPending := pending[name]
+	if !isPending {
+		// We'll be the one creating it
+		pending[name] = make(chan struct{})
+		wait = pending[name]
+	}
+	lock.Unlock()
+
+	// If someone else is creating it, wait for them
+	if isPending {
+		<-wait // Wait for creation to complete
+
+		// Now it should be in the instances map
+		lock.RLock()
+		defer lock.RUnlock()
+		return instances[name].(T)
+	}
+
+	// We're responsible for creating the instance
+	newInstance := constructor()
+
+	// Store it and signal other goroutines
+	lock.Lock()
+	instances[name] = newInstance
+	close(wait)           // Signal that creation is complete
+	delete(pending, name) // Clean up
+	log.Trace("Created new singleton", "type", name, "instance", fmt.Sprintf("%+v", newInstance))
+	lock.Unlock()
+
+	return newInstance
+}