Re: [libmicrohttpd] multipart/form-data Partial Write in Iterator

[Evgeny Grin]

Hi Gregory,

Unfortunately currently MHD has no API to attach arbitrary FD to 
internal event loop.

However, there are several ways how to handle it correctly.
The easiest way is thread-per-connection mode (flag 
MHD_USE_THREAD_PER_CONNECTION). In this mode individual threads are 
created for every connection. As every thread is responsible for a 
single connection only, you can use blocking writes, and this will not 
affect other requests/connections.

The proper way of handling it in other modes is more complicated. You 
can report partial processing of the upload data by setting 
`*upload_data_size` to non-zero value. However, this cause MHD to 
iterate over all connections wait for new network event and call "access 
handler" callback again, but only when something happens on the 
monitored sockets. The next MHD version will not wait for new data in 
such case, but call callback after iterating all connections. To handle 
partial write situation correctly, when incomplete data was processed by 
write you need to setup some external watcher for the file FD and call 
MHD_suspend_connection() before return from access handler. Then watcher 
should call MHD_resume_connection() when more data could be written to 
the file.

If you are not expecting very high traffic and you file storage device 
is not very slow, you can use blocking writes even without 
thread-per-connection. It will delay processing other requests' data but 
significantly simplify your code logic. When using thread pool, it will 
delay data processing only for connection in the same thread.

--
Evgeny

On 08.08.2023 14:46, Gregory Oakes via libmicrohttpd wrote:
> Hello everyone,
>
> I'm currently writing an application using libmicrohttpd, and I've
> encountered a question that I can't find an answer to in the manual. I
> am handling a file upload on a POST request by writing the contents to
> a temporary file. When I went to actually write the data, it struck me
> that the write would block the event loop.
>
> Am I able to attach my file write to libmicrohttpd's event loop
> somehow? Otherwise, am I able to report a partial write in the
> iterator?
>
> Respectfully,
> Gregory Oakes

OpenPGP_0x460A317C3326D2AE.asc
Description: OpenPGP public key

OpenPGP_signature
Description: OpenPGP digital signature