Add read-only options

These are options that can have only one definition, regardless of
priority.

lib/modules.nix

···
   evalOptionValue = loc: opt: defs:
     let
       # Add in the default value for this option, if any.
-      defs' = (optional (opt ? default)
-        { file = head opt.declarations; value = mkOptionDefault opt.default; }) ++ defs;
+      defs' =
+          (optional (opt ? default)
+            { file = head opt.declarations; value = mkOptionDefault opt.default; }) ++ defs;
 
       # Handle properties, check types, and merge everything together.
-      res = mergeDefinitions loc opt.type defs';
+      res =
+        if opt.readOnly or false && length defs' > 1 then
+          throw "The option `${showOption loc}' is read-only, but it's set multiple times."
+        else
+          mergeDefinitions loc opt.type defs';
 
       # Check whether the option is defined, and apply the ‘apply’
       # function to the merged value.  This allows options to yield a

lib/options.nix

···
     , apply ? null # Function that converts the option value to something else.
     , internal ? null # Whether the option is for NixOS developers only.
     , visible ? null # Whether the option shows up in the manual.
+    , readOnly ? null # Whether the option can be set only once
     , options ? null # Obsolete, used by types.optionSet.
     } @ attrs:
     attrs // { _type = "option"; };
···
           declarations = filter (x: x != unknownModule) opt.declarations;
           internal = opt.internal or false;
           visible = opt.visible or true;
+          readOnly = opt.readOnly or false;
           type = opt.type.name or null;
         }
         // (if opt ? example then { example = scrubOptionValue opt.example; } else {})

nixos/doc/manual/options-to-docbook.xsl

···
                   <emphasis>Type:</emphasis>
                   <xsl:text> </xsl:text>
                   <xsl:apply-templates select="attr[@name = 'type']" mode="top" />
+                  <xsl:if test="attr[@name = 'readOnly']/bool/@value = 'true'">
+                    <xsl:text> </xsl:text>
+                    <emphasis>(read only)</emphasis>
+                  </xsl:if>
                 </para>
               </xsl:if>
 

nixos/modules/misc/version.nix

···
     };
 
     system.nixosRelease = mkOption {
-      internal = true;
+      readOnly = true;
       type = types.str;
       default = readFile "${toString pkgs.path}/.version";
       description = "NixOS release.";
···
     };
 
     system.nixosCodeName = mkOption {
-      internal = true;
+      readOnly = true;
       type = types.str;
       description = "NixOS release code name.";
     };