Add read-only options

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

2 files changed, 10 insertions, 3 deletions

diff --git a/lib/modules.nix b/lib/modules.nix
index d7037abfbef..50827e18f10 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -261,11 +261,16 @@ rec {
   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
diff --git a/lib/options.nix b/lib/options.nix
index 5c543f56bcf..444ec37e6ea 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -19,6 +19,7 @@ rec {
     , 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"; };
@@ -90,6 +91,7 @@ rec {
           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 {})