RFD: Rework/extending functionality of mdev

[Michael Conrad]

On 3/13/2015 9:48 AM, Harald Becker wrote:
> On 13.03.2015 12:29, Michael Conrad wrote:
>> On 3/13/2015 3:25 AM, Harald Becker wrote:
>>>>    1 - kernel-spawned hotplug helpers is the traditional way,
>>>>    2 - netlink socket daemon is the "right way to solve the forkbomb
>>>> problem"
>>>
>>> ACK, but #2 blocks usage for those who like / need to stay at #1 / #0
>>>
>>>> In that case, I would offer this idea:
>>>
>>> All you do, is throwing in complex code sharing and the need to chose
>>> a mechanism ahead at build time, to allow for switching to some newer
>>> stuff ... but what about pre-generated binary versions, which
>>> mechanism shall be used in the default options, which mechanism shall
>>> be offered?
>>
>> Please review it again.  My solution solves both #1 and #2 in the same
>> binary, with no code duplication.
>
> At first complex code reusage, and then: How will you do it without 
> suffering from hotplug handler problem as current mdev? I'm don't 
> seeing, that you try to handle this problem. My solution is to enable 
> kernel hotplug handler mechanism to also benefit and avoid that 
> parallel parsing for each event.
>
> ... beside that, this close / open_netlink look suspicious, looks like 
> possible race condition. 

I thought pseudocode would be clearer than English text, but I suppose 
my pseudocode is still really just English...  Maybe some comments will 
help.

The new code would not be run like a hotplug helper, it would be run as 
a daemon, probably from a supervisor.  But the old code is still there 
and can still be run as a hotplug helper.

   mdev_main() {
     read_options();
     load_config();
     // if user requests "--netlink" mode, we act like a daemon
     if (option_netlink) {
       // If "--netlink-on-stdin" then netlink is open for us already
       // if not, then we need to create our netlink socket
       if (!option_netlink_on_stdin) {
         close(0); // the new socket will now be file descriptor 0
         open_netlink_socket();
       }
       // use 'select()' to see if a new netlink message is ready.
       // if the user gave us a "--timeout" then we exit if no new
       //  netlink message in a certain amount of time
       while (select([0], timeout)) {
         if (recv(0, message)) {
           // Netlink message is a list of variables.  We call 'setenv' 
for each.
           apply_env_from_message(message);
           // Now we have all the hotplug variables.  So call the old code.
           process_request();
         }
         // keep running in a loop until timeout (or forever if no timeout)
       }
     }
     else
     #endif
       process_request();
   }