go如何优雅关闭Graceful Shutdown服务

func (srv *Server) Shutdown(ctx context.Context) error

func (srv *Server) RegisterOnShutdown(f func())

type Server struct {
    ...
    onShutdown []func()
}

func (srv *Server) RegisterOnShutdown(f func()) {
    srv.mu.Lock()
    srv.onShutdown = append(srv.onShutdown, f)
    srv.mu.Unlock()
}

func (srv *Server) Shutdown(ctx context.Context) error {
    ...

    srv.mu.Lock()
    ...
    for _, f := range srv.onShutdown {
        go f()
    }
    srv.mu.Unlock()

    ...
}

package main
import (
    "context"
    "log"
    "net/http"
    "os"
    "os/signal"
    "time"
)
func main() {
    var srv http.Server
    srv.RegisterOnShutdown(func() {
        time.Sleep(3 * time.Second)
        log.Println("Exit func")
    })
    idleConnsClosed := make(chan struct{})
    go func() {
        sigint := make(chan os.Signal, 1)
        signal.Notify(sigint, os.Interrupt)
        <-sigint
        // We received an interrupt signal, shut down.
        if err := srv.Shutdown(context.Background()); err != nil {
            // Error from closing listeners, or context timeout:
            log.Printf("HTTP server Shutdown: %v", err)
        }
        close(idleConnsClosed)
    }()
    if err := srv.ListenAndServe(); err != http.ErrServerClosed {
        // Error starting or closing listener:
        log.Fatalf("HTTP server ListenAndServe: %v", err)
    }
    <-idleConnsClosed
}

package main
import (
    "context"
    "log"
    "net/http"
    "os"
    "os/signal"
    "sync"
    "time"
)
func main() {
    var srv http.Server
    var wg sync.WaitGroup
    register := func(f func()) {
        wg.Add(1)
        log.Println("Exit func register")
        srv.RegisterOnShutdown(func() {
            defer wg.Done()
            f()
            log.Println("Exit func done")
        })
    }
    register(func() {
        time.Sleep(3 * time.Second)
        log.Println("Called on Shutdown")
    })
    idleConnsClosed := make(chan struct{})
    go func() {
        sigint := make(chan os.Signal, 1)
        signal.Notify(sigint, os.Interrupt)
        <-sigint
        // We received an interrupt signal, shut down.
        log.Println("Shutdown start")
        if err := srv.Shutdown(context.Background()); err != nil {
            // Error from closing listeners, or context timeout:
            log.Printf("HTTP server Shutdown: %v", err)
        }
        close(idleConnsClosed)
    }()
    if err := srv.ListenAndServe(); err != http.ErrServerClosed {
        // Error starting or closing listener:
        log.Fatalf("HTTP server ListenAndServe: %v", err)
    }
    <-idleConnsClosed
    log.Println("Shutdown finish")
    wg.Wait()
}

wg.Add(1)
go func() {
    defer wg.Done()
    ...
}