A reproduction of a leak in go due to a blocked finalizer

leak.go

package main

import (
	"fmt"
	"math/rand/v2"
	"net/http"
	_ "net/http/pprof"
	"runtime"
	"sync"
	"time"
)

type Leaky struct {
	data []string
	mu   sync.Mutex
}

func (r *Leaky) Close() error {
	fmt.Println("[Finalizer Locked]")
	r.mu.Lock()
	fmt.Println("You should not see this message")
	return nil
}

//go:noinline
func newLeaky(size int) *Leaky {
	z := &Leaky{
		data: make([]string, 0, size),
		mu:   sync.Mutex{},
	}
	runtime.SetFinalizer(z, (*Leaky).Close)
	return z
}

//go:noinline
func objThatWillBlockFinalizer() {
	z := newLeaky(0)
	runtime.KeepAlive(z)
	z.mu.Lock() // block the finalizer
}

//go:noinline
func objWithNormalFinalizer() {
	z := newLeaky(1024 * 1024)
	runtime.KeepAlive(z)
}

func main() {
	go func() {
		addr := fmt.Sprintf("0.0.0.0:%d", 6060)
		fmt.Printf("Starting PPROF: http://%s/debug/pprof/\n", addr)
		fmt.Printf("PPROF stopped: %s\n", http.ListenAndServe(addr, nil).Error())
	}()

	fmt.Println("Blocking finalizer forever...")
	go objThatWillBlockFinalizer()
	time.Sleep(1 * time.Second)

	// Apparently, this usage causes a leak...
	for {
		go objWithNormalFinalizer()
		runtime.GC()
		time.Sleep(10 * time.Millisecond)

		if rand.IntN(10) == 0 {
			runtime.GC()
			runtime.GC()
			fmt.Println("Give GC some space to run...")
			time.Sleep(5 * time.Second)
		}
	}
}