fmt: incorrect usage of sync.Pool

[dsnet]

Consider this snippet:

type CustomStringer int

func (c CustomStringer) String() string {
	time.Sleep(500 * time.Millisecond) // simulate processing time
	return string(make([]byte, int(c)))
}

func main() {
	processRequest := func(size int) {
		fmt.Sprintf("<%v>", CustomStringer(size))
		time.Sleep(1 * time.Millisecond) // simulate idle time
	}

	// Simulate a steady stream of infrequent large requests.
	go func() {
		for {
			processRequest(1 << 28) // 256MiB
		}
	}()

	// Simulate a storm of small requests.
	for i := 0; i < 1000; i++ {
		go func() {
			for {
				processRequest(1 << 10) // 1KiB
			}
		}()
	}

	// Continually run a GC and track the allocated bytes.
	var stats runtime.MemStats
	for i := 0; ; i++ {
		runtime.ReadMemStats(&stats)
		fmt.Printf("Cycle %d: %dB\n", i, stats.Alloc)
		time.Sleep(time.Second)
		runtime.GC()
	}
}

This is a variation of #23199 (comment). See that comment for why this memory leak occurs.

After the 50th GC cycle, this pins 7GiB of heap, when I expect it to use no more than 256MiB.

[gopherbot]

Change https://golang.org/cl/136116 mentions this issue: fmt: fix usage of sync.Pool