ioutil.ReadAll goroutine 14509 [sync.Cond.Wait]：

Question

我正在编写一个Web爬网程序，使用30个goroutine并行爬网程序URL，该程序运行一段时间后会出现紧急情况，这是示例代码和紧急情况详细信息。 没有ioutil.ReadAll会读取httpResponse正文。

示例代码

import (
  "io/ioutil"
  "log"
  "net/http"
  "golang.org/x/sync/semaphore"
)

func main(){
  urls:=make([]string,100)
  crawl(urls)   
}

func crawl(urls []string){
    sem := semaphore.NewWeighte(30)
    for _,u:=range urls {
       c, _ := context.WithTimeout(context.Background(), 10*time.Second)
       sem.Acquire(c, 1)
       go func(u string){
          defer sem.Release(1)
          b,e:=down(u)
          if e==nil{
             log.Println(string(b))
          }else{
             log.Println("Error ",e)
          }
       }(u)
    }
}

func down(u string) ([]byte,error){
    resp,err:=http.Get(u)
    if err!=nil {
        return nil,err
    }
    defer resp.Body.Close()
    return ioutil.ReadAll(resp.Body)
}
panic：

goroutine 14509 [sync.Cond.Wait]:
runtime.goparkunlock(...)
         runtime/proc.go:307
sync.runtime_notifyListWait(0xc0000c2400, 0xc000000068)
         runtime/sema.go:510 +0xf9
sync.(*Cond).Wait(0xc0000c23f0)
         sync/cond.go:56 +0x9e
net/http.(*http2pipe).Read(0xc0000c23e8, 0xc011471bef, 0x90211, 0x90211, 0x0, 0x0, 0x0)
         net/http/h2_bundle.go:3511 +0x8a
net/http.http2transportResponseBody.Read(0xc0000c23c0, 0xc011471bef, 0x90211, 0x90211, 0x0, 0x0, 0x0)
         net/http/h2_bundle.go:8375 +0x95
bytes.(*Buffer).ReadFrom(0xc00046ee60, 0x7ff1d838ff40, 0xc0000c23c0, 0xc000136000, 0xc0008b02d0, 0xc004c05e88)
         bytes/buffer.go:207 +0xbd
io/ioutil.readAll(0x7ff1d838ff40, 0xc0000c23c0, 0x200, 0x0, 0x0, 0x0, 0x0, 0x0)
         io/ioutil/ioutil.go:36 +0xe5
io/ioutil.ReadAll(...)
         io/ioenter code hereutil/ioutil.go:45