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nixos/docs: Update assertion docs for new module-builtin ones

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Silvan Mosberger 2020-09-02 19:23:39 +02:00
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131
nixos/doc/manual/development/assertions.xml
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@ -8,7 +8,7 @@
<para>
When configuration problems are detectable in a module, it is a good idea to
write an assertion or warning. Doing so provides clear feedback to the user
and prevents errors after the build.
and can prevent errors before the build.
</para>
<para>
@ -20,55 +20,114 @@
NixOS module system.
</para>
<section xml:id="sec-assertions-warnings">
<title>Warnings</title>
<section xml:id="sec-assertions-define">
<title>Defining Warnings and Assertions</title>
<para>
This is an example of using <literal>warnings</literal>.
Both warnings and assertions can be defined using the <xref linkend="opt-_module.assertions"/> option. Each assertion needs an attribute name, under which you have to define an enable condition using <xref linkend="opt-_module.assertions._name_.enable"/> and a message using <xref linkend="opt-_module.assertions._name_.message"/>. Note that the enable condition is <emphasis>inverse</emphasis> of what an assertion would be: To assert a value being true, the enable condition should be false in that case, so that it isn't triggered. For the assertion message, you can add <literal>options</literal> to the module arguments and use <literal>${options.path.to.option}</literal> to print a context-aware string representation of the option path. Here is an example showing how this can be done.
</para>
<programlisting>
<![CDATA[
{ config, lib, ... }:
{
config = lib.mkIf config.services.foo.enable {
warnings =
if config.services.foo.bar
then [ ''You have enabled the bar feature of the foo service.
This is known to cause some specific problems in certain situations.
'' ]
else [];
}
{ config, options, ... }: {
_module.assertions.gpgSshAgent = {
enable = config.programs.gnupg.agent.enableSSHSupport &amp;&amp; config.programs.ssh.startAgent;
message = "You can't enable both ${options.programs.ssh.startAgent}"
+ " and ${options.programs.gnupg.agent.enableSSHSupport}!";
};
_module.assertions.grafanaPassword = {
enable = config.services.grafana.database.password != "";
message = "The grafana password defined with ${options.services.grafana.database.password}"
+ " will be stored as plaintext in the Nix store!";
# This is a non-fatal warning
type = "warning";
};
}
]]>
</programlisting>
</section>
<section xml:id="sec-assertions-assertions">
<title>Assertions</title>
<section xml:id="sec-assertions-ignoring">
<title>Ignoring Warnings and Assertions</title>
<para>
This example, extracted from the
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix">
<literal>syslogd</literal> module </link> shows how to use
<literal>assertions</literal>. Since there can only be one active syslog
daemon at a time, an assertion is useful to prevent such a broken system
from being built.
Sometimes you can get warnings or assertions that don't apply to your specific case and you wish to ignore them, or at least make assertions non-fatal. You can do so for all assertions defined using <xref linkend="opt-_module.assertions"/> by using the attribute name of the definition, which is conveniently printed using <literal>[...]</literal> when the assertion is triggered. For above example, the evaluation output when the assertions are triggered looks as follows:
</para>
<programlisting>
<![CDATA[
{ config, lib, ... }:
{
config = lib.mkIf config.services.syslogd.enable {
assertions =
[ { assertion = !config.services.rsyslogd.enable;
message = "rsyslogd conflicts with syslogd";
}
];
}
}
]]>
trace: warning: [grafanaPassword] The grafana password defined with
services.grafana.database.password will be stored as plaintext in the Nix store!
error: Failed assertions:
- [gpgSshAgent] You can't enable both programs.ssh.startAgent and
programs.gnupg.agent.enableSSHSupport!
</programlisting>
<para>
The <literal>[grafanaPassword]</literal> and <literal>[gpgSshAgent]</literal> strings tell you that these were defined under the <literal>grafanaPassword</literal> and <literal>gpgSshAgent</literal> attributes of <xref linkend="opt-_module.assertions"/> respectively. With this knowledge you can adjust them to your liking:
</para>
<programlisting>
{ lib, ... }: {
# Change the assertion into a non-fatal warning
_module.assertions.gpgSshAgent.type = "warning";
# We don't care about this warning, disable it
_module.assertions.grafanaPassword.enable = lib.mkForce false;
}
</programlisting>
</section>
<section xml:id="sec-assertions-submodules">
<title>Warnings and Assertions in Submodules</title>
<para>
Warnings and assertions can be defined within submodules in the same way. Here is an example:
</para>
<programlisting>
{ lib, ... }: {
options.myServices = lib.mkOption {
type = lib.types.attrsOf (lib.types.submodule ({ config, options, ... }: {
options.port = lib.mkOption {};
config._module.assertions.portConflict = {
enable = config.port == 80;
message = "Port ${toString config.port} defined using"
+ " ${options.port} is usually used for HTTP";
type = "warning";
};
}));
};
}
</programlisting>
<para>
When this assertion is triggered, it shows both the submodule path along with the assertion attribute within that submodule, joined by a <literal>/</literal>. Note also how <literal>${options.port}</literal> correctly shows the context of the option.
</para>
<programlisting>
trace: warning: [myServices.foo/portConflict] Port 80 defined using
myServices.foo.port is usually used for HTTP
</programlisting>
<para>
Therefore to disable such an assertion, you can do so by changing the <xref linkend="opt-_module.assertions"/> option within the <literal>myServices.foo</literal> submodule:
</para>
<programlisting>
{ lib, ... }: {
myServices.foo._module.assertions.portConflict.enable = lib.mkForce false;
}
</programlisting>
<note>
<para>
Assertions defined in submodules under <literal>types.listOf</literal> can't be ignored, since there's no way to change previously defined list items.
</para>
</note>
</section>
</section>