Creating/Learning about services

[Aki Tuomi]

> 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