OP#792 initial commit after fork

internal/once/init.go

@@ -0,0 +1,77 @@
+// Copyright (c) 2015-2023 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package once
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+)
+
+// Inspired from Golang sync.Once but it is only marked
+// initialized when the provided function returns nil.
+
+// Init represents the structure.
+type Init struct {
+	done uint32
+	m    sync.Mutex
+}
+
+// Do is similar to sync.Once.Do - makes one successful
+// call to the function. ie, it invokes the function
+// if it is not successful yet.
+func (l *Init) Do(f func() error) error {
+	if atomic.LoadUint32(&l.done) == 0 {
+		return l.do(f)
+	}
+	return nil
+}
+
+func (l *Init) do(f func() error) error {
+	l.m.Lock()
+	defer l.m.Unlock()
+	if atomic.LoadUint32(&l.done) == 0 {
+		if err := f(); err != nil {
+			return err
+		}
+		// Mark as done only when f() is successful
+		atomic.StoreUint32(&l.done, 1)
+	}
+	return nil
+}
+
+// DoWithContext is similar to Do except that it accepts a context as an argument to be passed.
+func (l *Init) DoWithContext(ctx context.Context, f func(context.Context) error) error {
+	if atomic.LoadUint32(&l.done) == 0 {
+		return l.doWithContext(ctx, f)
+	}
+	return nil
+}
+
+func (l *Init) doWithContext(ctx context.Context, f func(context.Context) error) error {
+	l.m.Lock()
+	defer l.m.Unlock()
+	if atomic.LoadUint32(&l.done) == 0 {
+		if err := f(ctx); err != nil {
+			return err
+		}
+		// Mark as done only when f() is successful
+		atomic.StoreUint32(&l.done, 1)
+	}
+	return nil
+}

internal/once/singleton.go

@@ -0,0 +1,46 @@
+package once
+
+// Singleton contains a pointer to T that must be set once.
+// Until the value is set all Get() calls will block.
+type Singleton[T any] struct {
+	v   *T
+	set chan struct{}
+}
+
+// NewSingleton creates a new unset singleton.
+func NewSingleton[T any]() *Singleton[T] {
+	return &Singleton[T]{set: make(chan struct{}), v: nil}
+}
+
+// Get will return the singleton value.
+func (s *Singleton[T]) Get() *T {
+	<-s.set
+	return s.v
+}
+
+// GetNonBlocking will return the singleton value or nil if not set yet.
+func (s *Singleton[T]) GetNonBlocking() *T {
+	select {
+	case <-s.set:
+		return s.v
+	default:
+		return nil
+	}
+}
+
+// IsSet will return whether the singleton has been set.
+func (s *Singleton[T]) IsSet() bool {
+	select {
+	case <-s.set:
+		return true
+	default:
+		return false
+	}
+}
+
+// Set the value and unblock all Get requests.
+// This may only be called once, a second call will panic.
+func (s *Singleton[T]) Set(v *T) {
+	s.v = v
+	close(s.set)
+}