Re: [libmicrohttpd] MHD_create_response_from_callback

[Christian Grothoff]

Hi Regis,

I've written a new testcase based on your specifications, and I'm now even more 
convinced that there is no bug.  The testcase will shortly be in SVN HEAD.  
The test starts a MHD server (using most available threading modes) and forks 
a 'curl' process for the download. MHD then generates an "infiinte" webpage of 
"d" characters and the test driver eventually (after ~1s) kills curl 
(SIGTERM).  The test driver then checks that the cleanup function is indeed 
called.

Now, there is one *unusual* case which might have tripped you up (certainly 
tripped me up when writing the testcase at first) in case of the multithreaded 
(one-thread-per-connection) MHD.  Here, the "cleanup" function is only called 
after MHD accepts the *next* inbound connection -- or upon MHD shutdown.

The reason for this is simple: the master MHD thread sleeps until it is awoken 
from listen and only then cleans up 'left-over' connections. This, potentially 
'untimely' cleanup is usually not an issue as it still avoids unbounded (or 
even large) resource consumption.  However, if you test with only a single 
connection and without MHD_daemon_stop, you would think that MHD failed to 
call your cleanup function.  Just remember this: the API guarantees that your 
cleanup function will be called, but it makes no claims as to when exactly 
this will happen exactly.


I hope this helps!

Happy hacking!

Christian

On Thursday, September 08, 2011 08:41:46 PM Regis Louge wrote:
> Here is my callback function :
> 
> static ssize_t
> push_callback (void *cls, uint64_t pos, char *buf, size_t max)
> {
>     PushElement *push = cls;
>     if(push->pushReceived == 1){
>         ...
>         Store push data in buf
>         ...
>         return strlen(buf);
>     }
>     else return 0;
> }
> 
> In my answer_to_connection for the URL /push I have :
> 
> response = MHD_create_response_from_callback (MHD_SIZE_UNKNOWN,
>                                                          32 * 1024,
>                                                          &push_callback,
>                                                          push,
> 
> &push_free_callback);
>            if(response == NULL)
>            {
>           return MHD_NO;
>            }
>            MHD_add_response_header (response, "Content-Type", "text/html;
> charset=utf-8");
>            ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
>            MHD_destroy_response (response);
>            return ret;
> 
> For a test case, when my client accesses to /push using curl -N -GET
> ".../push" he receives all the push notifications like it is suppose to,
> when I cancel it (^C), nothing happens on my server, request_completed is
> not called and neither is push_free_callback (which is the cleanup
> function)
> 
> On Thu, Sep 8, 2011 at 8:19 PM, Christian Grothoff
> 
> <address@hidden>wrote:
> > Hi!
> > 
> > Can you provide some kind of testcase that shows that MHD doesn't
> > properly call the cleanup function?
> > 
> > Happy hacking!
> > 
> > Christian
> > 
> > On Thursday, September 08, 2011 12:56:13 PM Regis Louge wrote:
> > > Hi,
> > > 
> > > I am currently trying to implement push notifications using
> > > libmicrohttpd and MHD_create_response_from_callback returning 0 until
> > > an event triggers the push. This seems to work pretty fine except for
> > > when the client interrupts the communication, then the cleanup method
> > > is not called and thus creates a lot of problems and impossibility to
> > > "subscribe" again to the push stream.
> > > 
> > > --
> > > Regis