home-assistant fails to load when component doesn't have dependencies

[peterhoeg]

Issue description

Before HA 0.69, the upstream requirements file listed notify.matrix however since 0.69 that is no longer the case. As a consequence when trying to add a matrix notifier, things will fail with:

error: attribute 'notify.matrix' missing, at /home/peter/src/active/nixpkgs_unstable/pkgs/servers/home-assistant/default.nix:69:28

Adding in a dummy like this in component-packages.nix makes things work again:

    "notify.matrix" = ps: with ps; [  ];

Cc: @dotlambda

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.16.9, NixOS, 18.09.git.3864e6e9515M (Jellyfish)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.0.2`
 - channels(root): `""`
 - channels(peter): `""`
 - nixpkgs: `/home/peter/src/active/nixpkgs_unstable`

[dotlambda]

Are you setting extraComponents by hand? The module should only add available ones.
We could avoid this problem by filtering extraComponents to only include the ones that are also in availableComponents. However, we should print a warning that the user has added an unavailable component.
EDIT: "Unavailable" is probably not the best description. There is either no component of that name or it isn't listed in requirements_all.txt.

[peterhoeg]

Sorry, I should have been clearer. There is a notify.matrix component but it is not listed in requirements.txt because the functionality is provided using the matrix component (matrix vs notify.matrix) - see https://www.home-assistant.io/components/matrix/ and https://www.home-assistant.io/components/notify.matrix/

[dotlambda]

I know, but there was only notify.matrix until recently (and it was listed in requirements_all.txt).

Since the notify.matrix component now requires the matrix component, you should be just fine if you're using autoExtraComponents.

However, using something along the lines of

pkgs.home-assistant.override {
  extraComponents = [ "notify.matrix" ];
}

indeed gives you an error because notify.matrix was removed from requirements_all.txt.
That has a somewhat positive effect of making the user check why it was removed/what it was replaced by.

In your opinion, what would be the ideal way to deal with this case?

[dotlambda]

I think we have three options (ordered by difficulty of implementation):

1. Leave it as is and maybe add some bit of documentation on which modules can/should be added to extraComponents.
2. As described above, ignore modules that are in extraComponents but not in availableComponents, i.e. that don't occur in requirements_all.txt. Maybe print a warning that the module name might be misspelled.
3. Store a list of all Home Assistant components, not just those in requirements_all.txt, and allow any of these components to be in extraComponents. Abort with an error if a member of extraComponents is not in the list of all components. If it is done this way, some people might be confused as to which components to add to extraComponents (e.g. matrix and not notify.matrix because only the former has dependencies).

[peterhoeg]

How about a mapping of components to other component dependencies so that the parse script can generate the correct entry?

I don't know if there are others than the matrix component but it wouldn't be too far fetched to assume that this was the case.

So notify.matrix gets written out to the components file with the dependencies of matrix.

[dotlambda]

There are definitely others, with the most notable one being the history component which depends on the recorder component which requires the SQLAlchemy package.
There is currently no way of detecting these dependencies in parse-requirements.py. However, it is possible using the DEPENDENCIES list exported by each component, see e.g. https://github.com/home-assistant/home-assistant/blob/0.69.1/homeassistant/components/notify/matrix.py#L21. This would require downloading the Home Assistant source tarball, importing each component and looking up its dependencies in parse-requirements.py. There is https://github.com/home-assistant/home-assistant/blob/0.69.1/script/gen_requirements_all.py which can serve as a guide on how to do that.

[peterhoeg]

The alternative is to define in parse-requirements.py which components have dependencies on others. That would have to be manually maintained (which isn't ideal) but would be a quick way to get things going.