tevent: fd events do not work correctly for UDP socket?

Stefan Metzmacher metze at samba.org
Wed Aug 19 19:24:19 UTC 2020

Hi Pavel,

>> Do strace ./client to see why this is:
>> The key is here:
>> write(1, "File descriptor is readable!\n", 29File descriptor is readable!
>> ) = 29
>> read(4, 0x7fffeee02750, 254)            = -1 ECONNREFUSED (Connection refused)
>> Your connect call succeeds, as it's setting up the local
>> binding to the remote address, but as it hasn't sent any
>> data yet the client hasn't noticed there's no one listening.
>> Once you do the:
>>       const char *msg = "I AM CONNECTED\n";
>>       write(fd, msg, strlen(msg));
>> call then the kernel tries to send the data, notices
>> there's nothing listening and so the read fd becomes
>> ready via EPOLL - it needs to return the error
>> So when you call the read() in the tevent handler,
>> that's when you'd pick up the errno = ECONNREFUSED
>> error.
> I see. If I understand it correctly epoll returns EPOLLERR and the code hits this [1] line?
>> I don't think this is tevent specific behavior.
> If the above is true then tevent should provide way for the handler to check for errors or don't call a read handler on an error so read does not get called.
> My use case is that I'm trying to implement a CLDAP ping over UDP in SSSD and when a Domain Controller is unreachable the read handler is fired, then ldap tries to receive
> a reply and blocks until network timeout is reached which is undesirable.
> [1] https://github.com/samba-team/samba/blob/master/lib/tevent/tevent_epoll.c#L707

You need to mark the socket non-blocking in order to avoid any blocking.

I have some patches to add TEVENT_FD_ERROR, but they are not upstream as there wasn't a strict need for.
In the beginning select was the main backend of tevent, there was no POLLERR,POLLHUP.
So errors are reported with TEVENT_FD_READ, as a read/recv is typically required to get the error.


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