Content length is zero for pages created with AutoConnect

[faanskit]

I've noted the following warning for each request to AutoConnect portal pages created by me.

[ 79445][W][WebServer.cpp:436] send(): content length is zero
[ 82495][W][WebServer.cpp:436] send(): content length is zero
[ 85596][W][WebServer.cpp:436] send(): content length is zero
[354996][W][WebServer.cpp:436] send(): content length is zero
[358046][W][WebServer.cpp:436] send(): content length is zero

A simple example page:

// Building and creating the Configuration resul page
AutoConnectAux* MyPortal::configResultPage(){
        AutoConnectElement* myText = new AutoConnectText("area_results", "","","", AC_Tag_None);
        AutoConnectElement* myDone = new AutoConnectSubmit("done", "Klar", HOME_PAGE);
        return new AutoConnectAux(CONFIG_DONE_PAGE, CONFIG_PAGE_HEADER, false, {*myText, *myDone});;
}

I do use a callback:

AutoConnectPortal->on(CONFIG_DONE_PAGE, std::bind<String>(&MyPortal::onAreaResults, this, std::placeholders::_1, std::placeholders::_2));

And I suspect it may be related to the fact that I return an "empty" string.

String MyPortal::onAreaResults(AutoConnectAux& aux, PageArgument& args){
    ...
    return String();
}

This is not impacting the behavior, but I wanted to make a note that this occurs and also try to understand what the callback is expected to return. return String() looks fishy. But, it is a common design pattern in most example files.

/Marcus

[Hieromon]

@faanskit

return String() looks fishy. But, it is a common design pattern in most example files.

The send member function of the WebServer class logs "content length is zero" because your AutoConnectAux is something wrong.
I have no idea if you are being dubious based on your legacy C knowledge, but if you mean that there are cases where RVO does not apply, here is a way to verify it.
Please verify it yourself. If your MyPortal class code is properly coded, then return String() should be fine. It would be used rather aggressively.

And, Perhaps the answer to what you want to do is in src/AutoConnectOTA.cpp or src/AutoConnectUpdate.cpp. I recommend that you trace it thoroughly.

[faanskit]

When I access the page .../area from the sample code you provided in #554 (comment) I also get the warning:

It's one send() warning for each reload.

--- Terminal on COM4 | 115200 8-N-1
--- Available filters and text transformations: colorize, debug, default, direct, esp32_exception_decoder, hexlify, log2file, nocontrol, printable, send_on_enter, time
--- More details at https://bit.ly/pio-monitor-filters
--- Quit: Ctrl+C | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H

[ 14879][W][WebServer.cpp:436] send(): content length is zero
[ 15001][E][WebServer.cpp:649] _handleRequest(): request handler not found
[ 23168][W][WebServer.cpp:436] send(): content length is zero
[429298][W][WebServer.cpp:436] send(): content length is zero
[432367][W][WebServer.cpp:436] send(): content length is zero
[434208][W][WebServer.cpp:436] send(): content length is zero

Take note that I compile with ARDUHAL_LOG_LEVEL_INFO to see this warning:

[env:az-delivery-devkit-v4]
platform = espressif32
board = az-delivery-devkit-v4
framework = arduino
monitor_speed = 115200
upload_speed = 460800
lib_deps = hieromon/AutoConnect@^1.4.1
build_flags = 
	-DCORE_DEBUG_LEVEL=ARDUHAL_LOG_LEVEL_INFO

Even this basic page from your documentation throws the same warning:

#include <Arduino.h>
#include <WiFi.h>
#include <WebServer.h>
#include <AutoConnect.h>

static const char PAGE_HELLO[] = R"(
{
  "uri": "/",
  "title": "Hello",
  "menu": false,
  "element": [
    {
      "name": "caption",
      "type": "ACText",
      "value": "Hello, World"
    }
  ]
}
)";


WebServer Server;
AutoConnect Portal(Server);
void setup() {
  delay(1000);
  Serial.begin(115200);
  Serial.println();
  Portal.load(PAGE_HELLO);
  Portal.begin();
  Serial.println("Web Server started:" + WiFi.localIP().toString());
}

void loop() {
  Portal.handleClient();
}

Web Server started:192.168.1.193
[  5418][W][WebServer.cpp:436] send(): content length is zero

So, don't think it's my code that is causing this; even though it should be the first and usual suspect ;-)
And again, everything works as expected. Only this warning that indicates something is not right.

/Marcus

[Hieromon]

?? Hm ??
Isn't that simply the start sign of a chunk? Because PageBuilder will chunk the transfer encoding if the content is not determined. It is not a problem with AutoConnect, rather the opposite, PageBuilder auto-chunking seems to work fine.

[Hieromon]

?? Hm ??
Isn't that simply the start sign of a chunk? Because PageBuilder will chunk the transfer encoding if the content length is not predetermined. It is not a problem with AutoConnect, nor of course with return String(). Rather, it is the other way around, and PageBuilder's auto-chanking seems to work fine.

EDIT

I have confirmed on the client browser side that the chunked transfer is successful.

[faanskit]

Great, I can determine it's not an issue with my code.

I guess this is a false positive from WebServer.cpp, which should not warn in this case. RFC2616 is even quite specific about it:
"Messages must not include both a Content-Length header field and a non-identity transfer-coding. If the message does include a non-identity transfer-coding, the Content-Length must be ignored."

Thanks for the clarification!

[Hieromon]

This has a history, and the ESP8266 WebServer class suppresses this warning; the ESP32 WebServer class has been modified to zero-check content length specifications, regardless of whether they are CONTENT_LENGTH_UNKNOWN or not.

https://github.com/espressif/arduino-esp32/blob/a95d838aab436588afb23656ef8190c3ff2e7b94/libraries/WebServer/src/WebServer.cpp#L430-L442

On the other hand, chunk transfer when dealing with the ESP8266WebServer class is:
https://github.com/Hieromon/PageBuilder/blob/6193e09a2eaafc71ace0a5651f18209f0cce2e7b/src/PageBuilder.cpp#L692-L696

server.setContentLength(CONTENT_LENGTH_UNKNOWN);
server.send(code, "text/html", "");.

In fact, if the above blank transmission does not exist, the content transmission will not be complete.

However, sending empty content with Chunked Transfer Encoding essentially means a trailer. Let me quote from the RFC like you.

 Chunked-Body   = *chunk
                  last-chunk
                  trailer
                  CRLF

 chunk          = chunk-size [ chunk-extension ] CRLF
                  chunk-data CRLF
 chunk-size     = 1*HEX
 last-chunk     = 1*("0") [ chunk-extension ] CRLF

 chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
 chunk-ext-name = token
 chunk-ext-val  = token | quoted-string
 chunk-data     = chunk-size(OCTET)
 trailer        = *(entity-header CRLF)

I think there is a difference of opinion between ESP8266WebServer and ESP32WebServer as to which is the more appropriate implementation. (As for me, I don't care which one is better.) The low level library cannot determine if it is a redundant warning or not.