发现了Go并发下载的Bug - 《GO 开发知识笔记》

01 排查过程
02 curl 和 Go 代码行为异同

前几天我写了一篇文章：Go项目实战：一步步构建一个并发文件下载器，有小伙伴评论问，请求 [https://studygolang.com/dl/golang/go1.16.5.src.tar.gz](https://studygolang.com/dl/golang/go1.16.5.src.tar.gz) 为什么没有返回 Accept-Ranges。在写那篇文章时，我也试了，确实没有返回，因此我以为它不支持。
但有一个小伙伴很认真，他改用 GET 方法请求这个地址，结果却有 Accept-Ranges，于是就很困惑，问我什么原因。经过一顿操作猛如虎，终于知道原因了。记录下排查过程，供大家参考！（小伙伴的留言可以查看那篇文章）

01 排查过程

通过 curl 命令，分别用 GET 和 HEAD 方法请求这个地址，结果如下：

$ curl -X GET --head https://studygolang.com/dl/golang/go1.16.5.src.tar.gz
HTTP/1.1 303 See Other
Server: nginx
Date: Wed, 07 Jul 2021 09:09:35 GMT
Content-Length: 0
Connection: keep-alive
Location: https://golang.google.cn/dl/go1.16.5.src.tar.gz
X-Request-Id: 83ee595c-6270-4fb0-a2f1-98fdc4d315be
$ curl --head https://studygolang.com/dl/golang/go1.16.5.src.tar.gz
HTTP/1.1 200 OK
Server: nginx
Date: Wed, 07 Jul 2021 09:09:44 GMT
Connection: keep-alive
X-Request-Id: f2ba473d-5bee-44c3-a591-02c358551235

虽然都没有 Accept-Ranges，但有一个奇怪现象：一个状态码是 303，一个是 200。很显然，303 是正确的，HEAD 为什么会是 200？
我以为是 Nginx 对 HEAD 请求做了特殊处理，于是直接访问 Go 服务的方式（不经过 Nginx 代理），结果一样。
于是，我用 Go 实现一个简单的 Web 服务，Handler 里面也重定向。

func main() {
 http.HandleFunc("/dl", func(w http.ResponseWriter, r *http.Request) {
  http.Redirect(w, r, "/", http.StatusSeeOther)
 })
 http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
  fmt.Fprintf(w, "Hello World")
 })
 http.ListenAndServe(":2022", nil)
}

用 curl 请求 http://localhost:2022/dl，GET 和 HEAD 都返回 303。于是我怀疑是不是 Echo 框架哪里的问题（studygolang 使用 Echo 框架构建的）。
所以，我用 Echo 框架写个 Web 服务测试：

func main() {
 e := echo.New()
 e.GET("/dl", func(ctx echo.Context) error {
    return ctx.Redirect(http.StatusSeeOther, "/")
  })
  e.GET("/", func(ctx echo.Context) error {
    return ctx.String(http.StatusOK, "Hello World!")
  })
 e.Logger.Fatal(e.Start(":2022"))
}

同样用 curl 请求 http://localhost:2022/dl，GET 返回 303，而 HEAD 报 405 Method Not Allowed，这符合预期。我们的路由设置只允许 GET 请求。但为什么 studygolang 没有返回 405，因为它也限制只能 GET 请求。
于是我对随便一个地址发起 HEAD 请求，发现都返回 200，可见 HTTP 错误被“吞掉”了。查找 studygolang 的中间件，发现了这个：

func HTTPError() echo.MiddlewareFunc {
 return func(next echo.HandlerFunc) echo.HandlerFunc {
  return func(ctx echo.Context) error {
   if err := next(ctx); err != nil {
    if !ctx.Response().Committed {
     if he, ok := err.(*echo.HTTPError); ok {
      switch he.Code {
      case http.StatusNotFound:
       if util.IsAjax(ctx) {
        return ctx.String(http.StatusOK, `{"ok":0,"error":"接口不存在"}`)
       }
       return Render(ctx, "404.html", nil)
      case http.StatusForbidden:
       if util.IsAjax(ctx) {
        return ctx.String(http.StatusOK, `{"ok":0,"error":"没有权限访问"}`)
       }
       return Render(ctx, "403.html", map[string]interface{}{"msg": he.Message})
      case http.StatusInternalServerError:
       if util.IsAjax(ctx) {
        return ctx.String(http.StatusOK, `{"ok":0,"error":"接口服务器错误"}`)
       }
       return Render(ctx, "500.html", nil)
     }
    }
   }
   return nil
  }
 }
}

这里对 404、403、500 错误都做了处理，但其他 HTTP 错误直接忽略了，导致最后返回了 200 OK。只需要在上面 switch 语句加一个 default 分支，同时把 err 原样 return，采用系统默认处理方式：

default:
 return err

这样 405 Method Not Allowed 会正常返回。
同时，为了解决 HEAD 能用来判断下载行为，针对下载路由，我加上了允许 HEAD 请求，这样就解决了小伙伴们的困惑。

02 curl 和 Go 代码行为异同

不知道大家发现没有，通过 curl 请求 [https://studygolang.com/dl/golang/go1.16.5.src.tar.gz](https://studygolang.com/dl/golang/go1.16.5.src.tar.gz) 和 Go 代码请求，结果是不一样的：

$ curl -X GET --head https://studygolang.com/dl/golang/go1.16.5.src.tar.gz
HTTP/1.1 303 See Other
Server: nginx
Date: Thu, 08 Jul 2021 02:05:10 GMT
Content-Length: 0
Connection: keep-alive
Location: https://golang.google.cn/dl/go1.16.5.src.tar.gz
X-Request-Id: 14d741ca-65c1-4b05-90b8-bef5c8b5a0a3

返回的是 303 重定向，自然没有 Accept-Ranges 头。
但改用如下 Go 代码：

resp, err := http.Get("https://studygolang.com/dl/golang/go1.16.5.src.tar.gz")
if err != nil {
  fmt.Println("get err", err)
  return
}
fmt.Println(resp)
fmt.Println("ranges", resp.Header.Get("Accept-Ranges"))

返回的是 200，且有 Accept-Ranges 头。可以猜测，应该是 Go 根据重定向递归请求重定向后的地址。可以查看源码确认下。
通过这个可以看到：https://docs.studygolang.com/src/net/http/client.go?s=20406:20458#L574，核心代码如下（比较容易看懂）：

// 循环处理所有需要处理的 url（包括重定向后的）
for {
  // For all but the first request, create the next
  // request hop and replace req.
  if len(reqs) > 0 {
      // 如果是重定向，请求重定向地址
   loc := resp.Header.Get("Location")
   if loc == "" {
    resp.closeBody()
    return nil, uerr(fmt.Errorf("%d response missing Location header", resp.StatusCode))
   }
   u, err := req.URL.Parse(loc)
   if err != nil {
    resp.closeBody()
    return nil, uerr(fmt.Errorf("failed to parse Location header %q: %v", loc, err))
   }
   host := ""
   if req.Host != "" && req.Host != req.URL.Host {
    // If the caller specified a custom Host header and the
    // redirect location is relative, preserve the Host header
    // through the redirect. See issue #22233.
    if u, _ := url.Parse(loc); u != nil && !u.IsAbs() {
     host = req.Host
    }
   }
   ireq := reqs[0]
   req = &Request{
    Method:   redirectMethod,
    Response: resp,
    URL:      u,
    Header:   make(Header),
    Host:     host,
    Cancel:   ireq.Cancel,
    ctx:      ireq.ctx,
   }
   if includeBody && ireq.GetBody != nil {
    req.Body, err = ireq.GetBody()
    if err != nil {
     resp.closeBody()
     return nil, uerr(err)
    }
    req.ContentLength = ireq.ContentLength
   }
   // Copy original headers before setting the Referer,
   // in case the user set Referer on their first request.
   // If they really want to override, they can do it in
   // their CheckRedirect func.
   copyHeaders(req)
   // Add the Referer header from the most recent
   // request URL to the new one, if it's not https->http:
   if ref := refererForURL(reqs[len(reqs)-1].URL, req.URL); ref != "" {
    req.Header.Set("Referer", ref)
   }
   err = c.checkRedirect(req, reqs)
   // Sentinel error to let users select the
   // previous response, without closing its
   // body. See Issue 10069.
   if err == ErrUseLastResponse {
    return resp, nil
   }
   // Close the previous response's body. But
   // read at least some of the body so if it's
   // small the underlying TCP connection will be
   // re-used. No need to check for errors: if it
   // fails, the Transport won't reuse it anyway.
   const maxBodySlurpSize = 2 << 10
   if resp.ContentLength == -1 || resp.ContentLength <= maxBodySlurpSize {
    io.CopyN(io.Discard, resp.Body, maxBodySlurpSize)
   }
   resp.Body.Close()
   if err != nil {
    // Special case for Go 1 compatibility: return both the response
    // and an error if the CheckRedirect function failed.
    // See https://golang.org/issue/3795
    // The resp.Body has already been closed.
    ue := uerr(err)
    ue.(*url.Error).URL = loc
    return resp, ue
   }
  }
  reqs = append(reqs, req)
  var err error
  var didTimeout func() bool
  if resp, didTimeout, err = c.send(req, deadline); err != nil {
   // c.send() always closes req.Body
   reqBodyClosed = true
   if !deadline.IsZero() && didTimeout() {
    err = &httpError{
     // TODO: early in cycle: s/Client.Timeout exceeded/timeout or context cancellation/
     err:     err.Error() + " (Client.Timeout exceeded while awaiting headers)",
     timeout: true,
    }
   }
   return nil, uerr(err)
  }
   // 确认重定向行为
  var shouldRedirect bool
  redirectMethod, shouldRedirect, includeBody = redirectBehavior(req.Method, resp, reqs[0])
  if !shouldRedirect {
   return resp, nil
  }
  req.closeBody()
 }

可以进一步看 redirectBehavior 函数 https://docs.studygolang.com/src/net/http/client.go?s=20406:20458#L497：

func redirectBehavior(reqMethod string, resp *Response, ireq *Request) (redirectMethod string, shouldRedirect, includeBody bool) {
 switch resp.StatusCode {
 case 301, 302, 303:
  redirectMethod = reqMethod
  shouldRedirect = true
  includeBody = false
  // RFC 2616 allowed automatic redirection only with GET and
  // HEAD requests. RFC 7231 lifts this restriction, but we still
  // restrict other methods to GET to maintain compatibility.
  // See Issue 18570.
  if reqMethod != "GET" && reqMethod != "HEAD" {
   redirectMethod = "GET"
  }
 case 307, 308:
  redirectMethod = reqMethod
  shouldRedirect = true
  includeBody = true
  // Treat 307 and 308 specially, since they're new in
  // Go 1.8, and they also require re-sending the request body.
  if resp.Header.Get("Location") == "" {
   // 308s have been observed in the wild being served
   // without Location headers. Since Go 1.7 and earlier
   // didn't follow these codes, just stop here instead
   // of returning an error.
   // See Issue 17773.
   shouldRedirect = false
   break
  }
  if ireq.GetBody == nil && ireq.outgoingLength() != 0 {
   // We had a request body, and 307/308 require
   // re-sending it, but GetBody is not defined. So just
   // return this response to the user instead of an
   // error, like we did in Go 1.7 and earlier.
   shouldRedirect = false
  }
 }
 return redirectMethod, shouldRedirect, includeBody
}

很清晰了吧。