dovecot - Aug 2021 - Creating/Learning about services

Hello,

I'm currently researching how services work within Dovecot and what 
scope these services are capable of. For example, I created a service as 
such:

service myservice {
 ? executable = /home/me/executable
 ? type????????? = worker
 ? unix_listener myservice {
 ??? mode = 0666
 ? }
}


When data is provided to the myservice unix socket, Dovecot appears to 
create a new process by executing "/home/me/executable". I found out 
about "status updates" through examining the logs. At first I received
this:

master: Error: service(myservice): Initial status notification not 
received in 30 seconds, killing the process

What is this status notification?

I found out that writing to file descriptor 5 from the executable does 
seem to be valid. When I write out 12 bytes I don't get an error, but 
when I write out 13 bytes I get the following:

master: Error: service(dmon): service sent partial status update (13 bytes)

What I've also noticed is that exiting 0 will cause Dovecot to spawn the 
process again, in what appears to be infinitely.

Are there any resources/documentation that I could reference to learn 
more about how services work on a lower level besides just looking at 
the source code?

Thanks.

> On 25/08/2021 20:22 Dan Conway <darkc0de at archnix6.net> wrote:
> 
>  
> Hello,
> 
> I'm currently researching how services work within Dovecot and what 
> scope these services are capable of. For example, I created a service as 
> such:
> 
> service myservice {
>  ? executable = /home/me/executable
>  ? type????????? = worker
>  ? unix_listener myservice {
>  ??? mode = 0666
>  ? }
> }
> 
> 
> When data is provided to the myservice unix socket, Dovecot appears to 
> create a new process by executing "/home/me/executable". I found
out
> about "status updates" through examining the logs. At first I
received this:
> 
> master: Error: service(myservice): Initial status notification not 
> received in 30 seconds, killing the process
> 
> What is this status notification?
> 
> I found out that writing to file descriptor 5 from the executable does 
> seem to be valid. When I write out 12 bytes I don't get an error, but 
> when I write out 13 bytes I get the following:
> 
> master: Error: service(dmon): service sent partial status update (13 bytes)
> 
> What I've also noticed is that exiting 0 will cause Dovecot to spawn
the
> process again, in what appears to be infinitely.
> 
> Are there any resources/documentation that I could reference to learn 
> more about how services work on a lower level besides just looking at 
> the source code?
> 
> Thanks.
Making native services requires practically C program using lib-master. We
provide a simple helpers called `script` and `login-script` which wrap your
program as a Dovecot service and provide you parameters from stdin and, when
possible, lets you write into stdout your reply.

See https://github.com/dovecot/core/blob/master/src/util/script.c#L83 for
provided input.

You can use it like

service foobar {
  executable = script /path/to/your/stuff
  unix_listener myservice {
    mode = 0666
  }
}

btw, there is no `type = worker` setting, see
https://doc.dovecot.org/configuration_manual/service_configuration/

Aki