Looking at the code where it is used in fasthttp's client.go func clientGetURLDeadlineFreeConn (line 672), you can see that if there is a timeout, dst's contents are copied to body at line 712. So based on what I read in the code (and debugged with Delve using your code), I saw that dst doesn't get updated in this usage. It seems that it can be used to provide default content to body in the event of a timeout - probably worth a direct question to fasthttp's author for more detail.
func clientGetURLDeadlineFreeConn(dst []byte, url string, deadline time.Time, c clientDoer) (statusCode int, body []byte, err error) {
timeout := -time.Since(deadline)
if timeout <= 0 {
return 0, dst, ErrTimeout
}
var ch chan clientURLResponse
chv := clientURLResponseChPool.Get()
if chv == nil {
chv = make(chan clientURLResponse, 1)
}
ch = chv.(chan clientURLResponse)
req := AcquireRequest()
// Note that the request continues execution on ErrTimeout until
// client-specific ReadTimeout exceeds. This helps limiting load
// on slow hosts by MaxConns* concurrent requests.
//
// Without this 'hack' the load on slow host could exceed MaxConns*
// concurrent requests, since timed out requests on client side
// usually continue execution on the host.
go func() {
statusCodeCopy, bodyCopy, errCopy := doRequestFollowRedirects(req, dst, url, c)
ch <- clientURLResponse{
statusCode: statusCodeCopy,
body: bodyCopy,
err: errCopy,
}
}()
tc := acquireTimer(timeout)
select {
case resp := <-ch:
ReleaseRequest(req)
clientURLResponseChPool.Put(chv)
statusCode = resp.statusCode
body = resp.body
err = resp.err
case <-tc.C:
body = dst
err = ErrTimeout
}
releaseTimer(tc)
return statusCode, body, err
}
In client.go func doRequestFollowRedirects (line 743) it is used at line 748: bodyBuf.B = dst
func doRequestFollowRedirects(req *Request, dst []byte, url string, c clientDoer) (statusCode int, body []byte, err error) {
resp := AcquireResponse()
bodyBuf := resp.bodyBuffer()
resp.keepBodyBuffer = true
oldBody := bodyBuf.B
bodyBuf.B = dst
redirectsCount := 0
for {
req.parsedURI = false
req.Header.host = req.Header.host[:0]
req.SetRequestURI(url)
if err = c.Do(req, resp); err != nil {
break
}
statusCode = resp.Header.StatusCode()
if statusCode != StatusMovedPermanently && statusCode != StatusFound && statusCode != StatusSeeOther {
break
}
redirectsCount++
if redirectsCount > maxRedirectsCount {
err = errTooManyRedirects
break
}
location := resp.Header.peek(strLocation)
if len(location) == 0 {
err = errMissingLocation
break
}
url = getRedirectURL(url, location)
}
body = bodyBuf.B
bodyBuf.B = oldBody
resp.keepBodyBuffer = false
ReleaseResponse(resp)
return statusCode, body, err
}