Ryan Mulligan
3 years ago
parent
1eaaeaaabe
commit
5058f9a5df
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# Warnings and Assertions {#sec-assertions} |
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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. |
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Although Nix has the `abort` and `builtins.trace` [functions](https://nixos.org/nix/manual/#ssec-builtins) to perform such tasks, they are not ideally suited for NixOS modules. Instead of these functions, you can declare your warnings and assertions using the NixOS module system. |
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## Warnings {#sec-assertions-warnings} |
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This is an example of using `warnings`. |
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```nix |
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{ config, lib, ... }: |
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{ |
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config = lib.mkIf config.services.foo.enable { |
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warnings = |
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if config.services.foo.bar |
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then [ ''You have enabled the bar feature of the foo service. |
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This is known to cause some specific problems in certain situations. |
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'' ] |
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else []; |
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} |
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} |
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``` |
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## Assertions {#sec-assertions-assetions} |
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This example, extracted from the [`syslogd` module](https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix) shows how to use `assertions`. 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. |
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```nix |
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{ config, lib, ... }: |
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{ |
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config = lib.mkIf config.services.syslogd.enable { |
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assertions = |
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[ { assertion = !config.services.rsyslogd.enable; |
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message = "rsyslogd conflicts with syslogd"; |
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} |
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]; |
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} |
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} |
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``` |
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@ -1,74 +0,0 @@ |
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<section xmlns="http://docbook.org/ns/docbook" |
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xmlns:xlink="http://www.w3.org/1999/xlink" |
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xmlns:xi="http://www.w3.org/2001/XInclude" |
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version="5.0" |
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xml:id="sec-assertions"> |
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<title>Warnings and Assertions</title> |
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|
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<para> |
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When configuration problems are detectable in a module, it is a good idea to |
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write an assertion or warning. Doing so provides clear feedback to the user |
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and prevents errors after the build. |
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</para> |
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|
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<para> |
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Although Nix has the <literal>abort</literal> and |
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<literal>builtins.trace</literal> |
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<link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link> |
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to perform such tasks, they are not ideally suited for NixOS modules. Instead |
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of these functions, you can declare your warnings and assertions using the |
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NixOS module system. |
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</para> |
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|
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<section xml:id="sec-assertions-warnings"> |
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<title>Warnings</title> |
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|
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<para> |
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This is an example of using <literal>warnings</literal>. |
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</para> |
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|
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<programlisting> |
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<![CDATA[ |
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{ config, lib, ... }: |
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{ |
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config = lib.mkIf config.services.foo.enable { |
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warnings = |
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if config.services.foo.bar |
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then [ ''You have enabled the bar feature of the foo service. |
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This is known to cause some specific problems in certain situations. |
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'' ] |
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else []; |
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} |
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} |
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]]> |
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</programlisting> |
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</section> |
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|
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<section xml:id="sec-assertions-assertions"> |
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<title>Assertions</title> |
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<para> |
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This example, extracted from the |
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<link xlink:href="https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix"> |
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<literal>syslogd</literal> module </link> shows how to use |
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<literal>assertions</literal>. Since there can only be one active syslog |
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daemon at a time, an assertion is useful to prevent such a broken system |
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from being built. |
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</para> |
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|
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<programlisting> |
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<![CDATA[ |
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{ config, lib, ... }: |
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{ |
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config = lib.mkIf config.services.syslogd.enable { |
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assertions = |
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[ { assertion = !config.services.rsyslogd.enable; |
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message = "rsyslogd conflicts with syslogd"; |
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} |
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]; |
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} |
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} |
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]]> |
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</programlisting> |
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</section> |
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</section> |
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@ -0,0 +1,58 @@ |
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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-assertions"> |
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<title>Warnings and Assertions</title> |
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<para> |
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When configuration problems are detectable in a module, it is a good |
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idea to write an assertion or warning. Doing so provides clear |
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feedback to the user and prevents errors after the build. |
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</para> |
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<para> |
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Although Nix has the <literal>abort</literal> and |
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<literal>builtins.trace</literal> |
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<link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link> |
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to perform such tasks, they are not ideally suited for NixOS |
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modules. Instead of these functions, you can declare your warnings |
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and assertions using the NixOS module system. |
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</para> |
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<section xml:id="sec-assertions-warnings"> |
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<title>Warnings</title> |
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<para> |
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This is an example of using <literal>warnings</literal>. |
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</para> |
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<programlisting language="bash"> |
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{ config, lib, ... }: |
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{ |
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config = lib.mkIf config.services.foo.enable { |
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warnings = |
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if config.services.foo.bar |
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then [ ''You have enabled the bar feature of the foo service. |
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This is known to cause some specific problems in certain situations. |
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'' ] |
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else []; |
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} |
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} |
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</programlisting> |
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</section> |
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<section xml:id="sec-assertions-assetions"> |
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<title>Assertions</title> |
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<para> |
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This example, extracted from the |
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<link xlink:href="https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix"><literal>syslogd</literal> |
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module</link> shows how to use <literal>assertions</literal>. |
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Since there can only be one active syslog daemon at a time, an |
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assertion is useful to prevent such a broken system from being |
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built. |
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</para> |
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<programlisting language="bash"> |
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{ config, lib, ... }: |
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{ |
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config = lib.mkIf config.services.syslogd.enable { |
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assertions = |
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[ { assertion = !config.services.rsyslogd.enable; |
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message = "rsyslogd conflicts with syslogd"; |
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} |
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]; |
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} |
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} |
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</programlisting> |
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</section> |
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</section> |
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