lib: enhance client reader resume + rewind

- update client reader documentation
- client reader, add rewind capabilities
    - tell creader to rewind on next start
    - Curl_client_reset() will keep reader for future rewind if requested
    - add Curl_client_cleanup() for freeing all resources independent of
      rewinds
    - add Curl_client_start() to trigger rewinds
    - move rewind code from multi.c to sendf.c and make part of
      "cr-in"'s implementation
- http, move the "resume_from" handling into the client readers
    - the setup of a HTTP request is reshuffled to follow:
      * determine method, target, auth negotiation
      * install the client reader(s) for the request, including crlf
        conversions and "chunked" encoding
      * apply ranges to client reader
      * concat request headers, upgrades, cookies, etc.
      * complete request by determining Content-Length of installed
        readers in combination with method
      * send
    - add methods for client readers to
      * return the overall length they will generate (or -1 when unknown)
      * return the amount of data on the CLIENT level, so that
        expect-100 can decide if it want to apply itself
      * set a "resume_from" offset or fail if unsupported
    - struct HTTP has become largely empty now
- rename `Client_reader_*` to `Curl_creader_*`

Closes #13026

lib/multi.c

@@ -1816,93 +1816,6 @@ static CURLcode protocol_connect(struct Curl_easy *data,
   return result; /* pass back status */
 }
 
-/*
- * readrewind() rewinds the read stream. This is typically used for HTTP
- * POST/PUT with multi-pass authentication when a sending was denied and a
- * resend is necessary.
- */
-static CURLcode readrewind(struct Curl_easy *data)
-{
-#if !defined(CURL_DISABLE_MIME) || !defined(CURL_DISABLE_FORM_API)
-  curl_mimepart *mimepart = &data->set.mimepost;
-#endif
-  DEBUGASSERT(data->conn);
-
-  data->state.rewindbeforesend = FALSE; /* we rewind now */
-
-  /* explicitly switch off sending data on this connection now since we are
-     about to restart a new transfer and thus we want to avoid inadvertently
-     sending more data on the existing connection until the next transfer
-     starts */
-  data->req.keepon &= ~KEEP_SEND;
-
-  /* We have sent away data. If not using CURLOPT_POSTFIELDS or
-     CURLOPT_HTTPPOST, call app to rewind
-  */
-#if !defined(CURL_DISABLE_HTTP) && !defined(CURL_DISABLE_MIME)
-  if(data->conn->handler->protocol & PROTO_FAMILY_HTTP) {
-    if(data->state.mimepost)
-      mimepart = data->state.mimepost;
-  }
-#endif
-  if(data->set.postfields ||
-     (data->state.httpreq == HTTPREQ_GET) ||
-     (data->state.httpreq == HTTPREQ_HEAD))
-    ; /* no need to rewind */
-#if !defined(CURL_DISABLE_MIME) || !defined(CURL_DISABLE_FORM_API)
-  else if(data->state.httpreq == HTTPREQ_POST_MIME ||
-          data->state.httpreq == HTTPREQ_POST_FORM) {
-    CURLcode result = Curl_mime_rewind(mimepart);
-    if(result) {
-      failf(data, "Cannot rewind mime/post data");
-      return result;
-    }
-  }
-#endif
-  else {
-    if(data->set.seek_func) {
-      int err;
-
-      Curl_set_in_callback(data, true);
-      err = (data->set.seek_func)(data->set.seek_client, 0, SEEK_SET);
-      Curl_set_in_callback(data, false);
-      if(err) {
-        failf(data, "seek callback returned error %d", (int)err);
-        return CURLE_SEND_FAIL_REWIND;
-      }
-    }
-    else if(data->set.ioctl_func) {
-      curlioerr err;
-
-      Curl_set_in_callback(data, true);
-      err = (data->set.ioctl_func)(data, CURLIOCMD_RESTARTREAD,
-                                   data->set.ioctl_client);
-      Curl_set_in_callback(data, false);
-      infof(data, "the ioctl callback returned %d", (int)err);
-
-      if(err) {
-        failf(data, "ioctl callback returned error %d", (int)err);
-        return CURLE_SEND_FAIL_REWIND;
-      }
-    }
-    else {
-      /* If no CURLOPT_READFUNCTION is used, we know that we operate on a
-         given FILE * stream and we can actually attempt to rewind that
-         ourselves with fseek() */
-      if(data->state.fread_func == (curl_read_callback)fread) {
-        if(-1 != fseek(data->state.in, 0, SEEK_SET))
-          /* successful rewind */
-          return CURLE_OK;
-      }
-
-      /* no callback set or failure above, makes us fail at once */
-      failf(data, "necessary data rewind wasn't possible");
-      return CURLE_SEND_FAIL_REWIND;
-    }
-  }
-  return CURLE_OK;
-}
-
 /*
  * Curl_preconnect() is called immediately before a connect starts. When a
  * redirect is followed, this is then called multiple times during a single
@@ -2169,9 +2082,6 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
       break;
 
     case MSTATE_PROTOCONNECT:
-      if(data->state.rewindbeforesend)
-        result = readrewind(data);
-
       if(!result && data->conn->bits.reuse) {
         /* ftp seems to hang when protoconnect on reused connection
          * since we handle PROTOCONNECT in general inside the filers, it