From 26c14bbc9d13299626acd9f14549fbdd0d0be9d5 Mon Sep 17 00:00:00 2001
From: asahi <mikiyashiki@outlook.com>
Date: Sat, 11 Jan 2025 16:17:27 +0800
Subject: [PATCH] =?UTF-8?q?=E9=98=85=E8=AF=BBgolang=20sync.Once=E6=96=87?=
 =?UTF-8?q?=E6=A1=A3?=
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---
 Golang/Golang Document.md | 101 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 99 insertions(+), 2 deletions(-)

diff --git a/Golang/Golang Document.md b/Golang/Golang Document.md
index 14b9a99..10ed0a7 100644
--- a/Golang/Golang Document.md	
+++ b/Golang/Golang Document.md	
@@ -1,3 +1,46 @@
+- [Golang](#golang)
+  - [Get Started](#get-started)
+    - [Enable dependency tracking](#enable-dependency-tracking)
+      - [go mod init](#go-mod-init)
+      - [go mod tidy](#go-mod-tidy)
+  - [multi-module workspace](#multi-module-workspace)
+    - [go work init](#go-work-init)
+    - [go work use](#go-work-use)
+  - [Gin框架构建restful api](#gin框架构建restful-api)
+    - [向response中写入返回数据](#向response中写入返回数据)
+    - [解析request中的数据](#解析request中的数据)
+    - [将请求的endpoint注册到server中](#将请求的endpoint注册到server中)
+  - [golang generic](#golang-generic)
+    - [不使用泛型的代码编写](#不使用泛型的代码编写)
+    - [使用泛型的代码编写](#使用泛型的代码编写)
+      - [comparable](#comparable)
+    - [泛型方法调用](#泛型方法调用)
+    - [type constraint](#type-constraint)
+  - [Fuzzing](#fuzzing)
+    - [unit test编写](#unit-test编写)
+    - [fuzz test编写](#fuzz-test编写)
+      - [执行fuzz test](#执行fuzz-test)
+  - [sync.Pool](#syncpool)
+    - [sync.Pool使用示例](#syncpool使用示例)
+    - [Pool和垃圾回收](#pool和垃圾回收)
+      - [poolCleanup](#poolcleanup)
+      - [allPools \& oldPools](#allpools--oldpools)
+    - [Proc Pining](#proc-pining)
+      - [per-P](#per-p)
+      - [local \& localSize](#local--localsize)
+      - [PinSlow](#pinslow)
+    - [Pool Local](#pool-local)
+    - [pool的Put/Get](#pool的putget)
+      - [Put](#put)
+      - [Get](#get)
+        - [slow path](#slow-path)
+  - [Sync.once](#synconce)
+    - [use case](#use-case)
+  - [syntax](#syntax)
+    - [iota](#iota)
+
+
+
 # Golang 
 ## Get Started
 ### Enable dependency tracking
@@ -310,8 +353,7 @@ ok      git.kazusa.red/asahi/fuzz-demo  10.360s
 > #### new interesting
 > `new interesting`指会扩充code coverage的用例输入，在fuzz test刚开始时，new interesting数量通常会因发现新的代码路径快速增加，然后，会随着时间的推移逐渐减少
 
-## Go Sync
-### sync.Pool
+## sync.Pool
 `sync.Pool`为golang标准库中的实现，用于降低allocation和减少垃圾回收。
 
 ### sync.Pool使用示例
@@ -708,6 +750,61 @@ func (p *Pool) getSlow(pid int) any {
   - 如果所有victim中的poolLocal对象都返回为空，那么会将victim中`p.victimSize`标识为空，后续再次执行slow path时，如果感知到victimSize为空，那么便不会再次查找victim
 
 
+## Sync.once
+sync.Once是golang中提供的工具包，确保指定的代码块在并发环境下只会被执行一次。
+
+sync.Once为struct，其中只包含一个方法`Do(f func())`。sync.Once保证指定方法只会被执行一次，即使在多routine环境下被调用了多次。`并且，sync.Once方法是线程安全的`。
+
+### use case
+sync.Once的用例如下：
+- 对共享的资源进行初始化
+- 设置单例
+- 执行只需要运行一次的高开销计算
+- 导入配置文件
+
+sync.Once的使用示例如下：
+```go
+var instance *singleton
+var once sync.Once
+
+func getInstance() *singleton {
+    once.Do(func() {
+        instance = &singleton{}
+    })
+    return instance
+}
+```
+
+sync.Once实现逻辑如下：
+```go
+type Once struct {
+	// done indicates whether the action has been performed.
+	// It is first in the struct because it is used in the hot path.
+	// The hot path is inlined at every call site.
+	// Placing done first allows more compact instructions on some architectures (amd64/386),
+	// and fewer instructions (to calculate offset) on other architectures.
+	done atomic.Uint32
+	m    Mutex
+}
+
+func (o *Once) Do(f func()) {
+
+	if o.done.Load() == 0 {
+		// Outlined slow-path to allow inlining of the fast-path.
+		o.doSlow(f)
+	}
+}
+
+func (o *Once) doSlow(f func()) {
+	o.m.Lock()
+	defer o.m.Unlock()
+	if o.done.Load() == 0 {
+		defer o.done.Store(1)
+		f()
+	}
+}
+```
+
 ## syntax
 ### iota
 `iota`关键字代表连续的整数变量，`0, 1, 2`，每当`const`关键字出现时，其重置为0