Re: [Nix-dev] How to override/replace part of a service definition?

[Nicolas Pierron]

Hi Michael,

On Sun, Feb 22, 2015 at 2:45 PM, Michael Alan Dorman mdor...@jaunder.io wrote:
 All the documentation I can find about overriding things seems focuses
 on packages, but what I'm interested in overriding is the systemd
 pre-start-script.  I tried what seemed obvious:

 { config, ... }:

 let
   cfg = config.services.couchdb;

 in {
   systemd.services.couchdb.preStart =
   ''
   […]
   '';
 }

 But that appears to have had an *additive* effect---that is, a copy of
 the modified text appears *after* the existing, non-working, definition.

First, you can double check the content of this option without
building a system, with

  $ nixos-option systemd.services.couchdb.preStart

The *additive* effect is a feature, which is used in many cases, such
as defining fileSystems, systemd.services, and many more.  This
gives the ability to have a module system where multiple modules can
register them-self concurrently in other modules.

You can override other modules definitions, almost as you expected, by
using mkOverride int value, which has a convenient short-cut
named mkForce value, with the force int-priority.

This way, you mention that one definition override all the others, as
thus ignore all the definitions with a default priority.  These
properties are briefly described in the Manual [1], and in the Wiki
[2].

[1] 
https://nixos.org/nixos/manual/sec-writing-modules.html#sec-option-definitions
[2] https://nixos.org/wiki/NixOS:Modules

-- 
Nicolas Pierron
http://www.linkedin.com/in/nicolasbpierron - http://nbp.name/
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[Nix-dev] How to override/replace part of a service definition?

[Michael Alan Dorman]

I'm very new to NixOS; perhaps this isn't the correct venue to ask this
sort of question, in which case I would appreciate guidance about where
better to ask.

In trying to use the couchdb package, I found a couple of issues with
its pre-start script[1].  My PR
(https://github.com/NixOS/nixpkgs/pull/6283) has not yet been merged,
and until such time as it is, I would like to override things locally so
it will work without manual intervention after each reboot.

All the documentation I can find about overriding things seems focuses
on packages, but what I'm interested in overriding is the systemd
pre-start-script.  I tried what seemed obvious:

{ config, ... }:

let
  cfg = config.services.couchdb;

in {
  systemd.services.couchdb.preStart =
  ''
  mkdir -p `dirname ${cfg.uriFile}`;
  mkdir -p `dirname ${cfg.logFile}`;
  mkdir -p ${cfg.databaseDir};
  mkdir -p ${cfg.viewIndexDir};
  touch ${cfg.configFile}
  touch -a ${cfg.logFile}

  if [ $(id -u) = 0 ]; then
chown ${cfg.user}:${cfg.group} `dirname ${cfg.uriFile}`;
(-f ${cfg.uriFile}  chown ${cfg.user}:${cfg.group}
${cfg.uriFile}) || true
chown ${cfg.user}:${cfg.group} ${cfg.databaseDir}
chown ${cfg.user}:${cfg.group} ${cfg.viewIndexDir}
chown ${cfg.user}:${cfg.group} ${cfg.configFile}
chown ${cfg.user}:${cfg.group} ${cfg.logFile}
  fi
  '';
}

But that appears to have had an *additive* effect---that is, a copy of
the modified text appears *after* the existing, non-working, definition.

Any specific remedies, or pointers to specific bits of documentation
I've missed---I believe I've combed the wiki pretty thoroughly---would
be much appreciated.

Mike.

1. After rebooting, checking the service status provides you with:

Feb 22 08:42:49 aching systemd[1]: Starting CouchDB Server...
Feb 22 08:42:49 aching systemd[1]: couchdb.service: control process
exited, code=exited status=1
Feb 22 08:42:49 aching systemd[1]: Failed to start CouchDB Server.
Feb 22 08:42:49 aching systemd[1]: Unit couchdb.service entered failed
state.
Feb 22 08:42:49 aching systemd[1]: couchdb.service failed.
Feb 22 08:42:49 aching couchdb-pre-start[1464]: chown: cannot access
‘/var/run/couchdb/couchdb.uri’: No such file or directory

Simple inspection reveals commands being run under -e that are not being
careful in examining their environment.
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