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Re: [libmicrohttpd] upgrading and life cycle of sockets, issue when used

From: Christian Grothoff
Subject: Re: [libmicrohttpd] upgrading and life cycle of sockets, issue when used with epoll
Date: Sun, 3 Jan 2021 14:22:25 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0

Hi Jose,

I've figured out the issue.  The bug is actually in your code.

The problem is that you are using an _external_ event loop, and if you
do so, you are responsible for re-triggering MHD_run() if you do
anything that might change the state of an MHD connection.
This (mostly) applies to MHD_resume_connection() -- but also to closing
an upgraded connection.

Without this modification, MHD does close the connection on the _next_
request, so if you would connect with another client, MHD will then
close the connection.

This is the only way to fix this, as when MHD is not 'run', it cannot
act. And if you are controlling the external event loop, you are
responsible for triggering MHD 'when necessary'.

I've attached a modified version of your code that adds the necessary
signalling logic to your poll()-loop. (Using eventfd(); non-portable.
Use a pipe() to do this in a more portable way.)

Happy hacking!


On 12/10/20 4:08 PM, José Bollo wrote:
> Hello,
> My code uses LMHD embedded with its EPOLL mechanism. Part of that code
> deals with upgrading to websocket. It then call somewhere:
>   response = MHD_create_response_for_upgrade(
>                upgrade_to_websocket, memo);
> and the callback function upgrade_to_websocket looks as here below:
>   void upgrade_to_websocket(
>              void *cls,
>              struct MHD_Connection *connection,
>              void *con_cls,
>              const char *extra_in,
>              size_t extra_in_size,
>              MHD_socket sock,
>              struct MHD_UpgradeResponseHandle *urh
>   ) {
>       struct memo *memo = cls;
>       struct ws *ws = ws_create(sock, memo, close_websocket, urh);
>       if (ws == NULL) close_websocket(urh);
>   }
>   void close_websocket(struct MHD_UpgradeResponseHandle *urh) {
>       MHD_upgrade_action (urh, MHD_UPGRADE_ACTION_CLOSE);
>   }
> Thank you for your attention until here. So far, so good.
> The issue now: when the functiuon ws_create returns NULL, the program
> returns to some polling and wait for an events BUT DOES NOT CLOSE THE
> SOCKET, leading to starvation of the client.
> I guess that calling some function after calling MHD_upgrade_action
> (urh, MHD_UPGRADE_ACTION_CLOSE) could unlock the situation by
> performing correct close. Though the called function should not be
> MHD_run because it dispatch events, what is not expected here.
> I join a sample demo. When I connect on websocket on it, the client
> starves. I recorded and joined the output of strace.
> Best regards
> José Bollo

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