nixos/make-options-doc: treat missing descriptions as errors by default

this partially solves the problem of "missing description" warnings of the
options doc build being lost by nix build, at the cost of failing builds that
previously ran. an option to disable this behaviour is provided.

nixos/doc/manual/default.nix

@@ -1,4 +1,13 @@
-{ pkgs, options, config, version, revision, extraSources ? [], baseOptionsJSON ? null, prefix ? ../../.. }:
+{ pkgs
+, options
+, config
+, version
+, revision
+, extraSources ? []
+, baseOptionsJSON ? null
+, warningsAreErrors ? true
+, prefix ? ../../..
+}:
 
 with pkgs;
 
@@ -15,7 +24,7 @@ let
   stripAnyPrefixes = lib.flip (lib.foldr lib.removePrefix) prefixesToStrip;
 
   optionsDoc = buildPackages.nixosOptionsDoc {
-    inherit options revision baseOptionsJSON;
+    inherit options revision baseOptionsJSON warningsAreErrors;
     transformOptions = opt: opt // {
       # Clean up declaration sites to not refer to the NixOS source tree.
       declarations = map stripAnyPrefixes opt.declarations;

nixos/lib/make-options-doc/default.nix

@@ -25,6 +25,9 @@
 # used to split the options doc build into a static part (nixos/modules) and a dynamic part
 # (non-nixos modules imported via configuration.nix, other module sources).
 , baseOptionsJSON ? null
+# instead of printing warnings for eg options with missing descriptions (which may be lost
+# by nix build unless -L is given), emit errors instead and fail the build
+, warningsAreErrors ? true
 }:
 
 let
@@ -121,6 +124,7 @@ in rec {
           then "cp $options $dst/options.json"
           else ''
             ${pkgs.python3Minimal}/bin/python ${./mergeJSON.py} \
+              ${lib.optionalString warningsAreErrors "--warnings-are-errors"} \
               ${baseOptionsJSON} $options \
               > $dst/options.json
           ''

nixos/lib/make-options-doc/mergeJSON.py

@@ -41,8 +41,10 @@ def unpivot(options: Dict[Key, Option]) -> Dict[str, JSON]:
         result[opt.name] = opt.value
     return result
 
-options = pivot(json.load(open(sys.argv[1], 'r')))
-overrides = pivot(json.load(open(sys.argv[2], 'r')))
+warningsAreErrors = sys.argv[1] == "--warnings-are-errors"
+optOffset = 1 if warningsAreErrors else 0
+options = pivot(json.load(open(sys.argv[1 + optOffset], 'r')))
+overrides = pivot(json.load(open(sys.argv[2 + optOffset], 'r')))
 
 # fix up declaration paths in lazy options, since we don't eval them from a full nixpkgs dir
 for (k, v) in options.items():
@@ -65,10 +67,20 @@ for (k, v) in overrides.items():
             cur[ok] = ov
 
 # check that every option has a description
-# TODO: nixos-rebuild with flakes may hide the warning, maybe turn on -L by default for those?
+hasWarnings = False
 for (k, v) in options.items():
     if v.value.get('description', None) is None:
-        print(f"\x1b[1;31mwarning: option {v.name} has no description\x1b[0m", file=sys.stderr)
+        severity = "error" if warningsAreErrors else "warning"
+        hasWarnings = True
+        print(f"\x1b[1;31m{severity}: option {v.name} has no description\x1b[0m", file=sys.stderr)
         v.value['description'] = "This option has no description."
+if hasWarnings and warningsAreErrors:
+    print(
+        "\x1b[1;31m" +
+        "Treating warnings as errors. Set documentation.nixos.options.warningsAreErrors " +
+        "to false to ignore these warnings." +
+        "\x1b[0m",
+        file=sys.stderr)
+    sys.exit(1)
 
 json.dump(unpivot(options), fp=sys.stdout)

nixos/modules/misc/documentation.nix

@@ -15,7 +15,7 @@ let
       f = import m;
       instance = f (mapAttrs (n: _: abort "evaluating ${n} for `meta` failed") (functionArgs f));
     in
-      cfg.nixos.splitOptionDocBuild
+      cfg.nixos.options.splitBuild
         && builtins.isPath m
         && isFunction f
         && instance ? options
@@ -101,6 +101,7 @@ let
               exit 1
             } >&2
         '';
+    inherit (cfg.nixos.options) warningsAreErrors;
   };
 
 
@@ -256,7 +257,7 @@ in
         '';
       };
 
-      nixos.splitOptionDocBuild = mkOption {
+      nixos.options.splitBuild = mkOption {
         type = types.bool;
         default = true;
         description = ''
@@ -266,6 +267,15 @@ in
         '';
       };
 
+      nixos.options.warningsAreErrors = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Treat warning emitted during the option documentation build (eg for missing option
+          descriptions) as errors.
+        '';
+      };
+
       nixos.includeAllModules = mkOption {
         type = types.bool;
         default = false;