lib.evalModules: add graph attribute

Co-authored-by: Ali Jamadi <jamadi1377@gmail.com>

doc/module-system/module-system.chapter.md

@@ -116,6 +116,16 @@ A nominal type marker, always `"configuration"`.
 
 The [`class` argument](#module-system-lib-evalModules-param-class).
 
+#### `graph` {#module-system-lib-evalModules-return-value-graph}
+
+Represents all the modules that took part in the evaluation.
+It is a list of `ModuleGraph` where `ModuleGraph` is defined as an attribute set with the following attributes:
+
+- `key`: `string` for the purpose of module deduplication and `disabledModules`
+- `file`: `string` for the purpose of error messages and warnings
+- `imports`: `[ ModuleGraph ]`
+- `disabled`: `bool`
+
 ## Module arguments {#module-system-module-arguments}
 
 Module arguments are the attribute values passed to modules when they are evaluated.

doc/redirects.json

@@ -487,6 +487,9 @@
   "module-system-lib-evalModules-return-value-_configurationClass": [
     "index.html#module-system-lib-evalModules-return-value-_configurationClass"
   ],
+  "module-system-lib-evalModules-return-value-graph": [
+    "index.html#module-system-lib-evalModules-return-value-graph"
+  ],
   "part-stdenv": [
     "index.html#part-stdenv"
   ],

lib/modules.nix

@@ -245,11 +245,12 @@ let
         };
       };
 
-      merged =
+      # This function takes an empty attrset as an argument.
-        let
+      # It could theoretically be replaced with its body,
-          collected =
+      # but such a binding is avoided to allow for earlier grabage collection.
-            collectModules class (specialArgs.modulesPath or "") (regularModules ++ [ internalModule ])
+      doCollect =
-              (
+        { }:
+        collectModules class (specialArgs.modulesPath or "") (regularModules ++ [ internalModule ]) (
           {
             inherit
               lib
@@ -262,8 +263,8 @@ let
           }
           // specialArgs
         );
-        in
+
-        mergeModules prefix (reverseList collected);
+      merged = mergeModules prefix (reverseList (doCollect { }).modules);
 
       options = merged.matchedOptions;
 
@@ -359,12 +360,13 @@ let
         options = checked options;
         config = checked (removeAttrs config [ "_module" ]);
         _module = checked (config._module);
+        inherit (doCollect { }) graph;
         inherit extendModules type class;
       };
     in
     result;
 
-  # collectModules :: (class: String) -> (modulesPath: String) -> (modules: [ Module ]) -> (args: Attrs) -> [ Module ]
+  # collectModules :: (class: String) -> (modulesPath: String) -> (modules: [ Module ]) -> (args: Attrs) -> ModulesTree
   #
   # Collects all modules recursively through `import` statements, filtering out
   # all modules in disabledModules.
@@ -529,9 +531,25 @@ let
           operator = attrs: keyFilter attrs.modules;
         });
 
+      toGraph =
+        modulesPath:
+        { disabled, modules }:
+        let
+          isDisabledModule = isDisabled modulesPath disabled;
+
+          toModuleGraph = structuredModule: {
+            disabled = isDisabledModule structuredModule;
+            inherit (structuredModule) key;
+            file = structuredModule.module._file;
+            imports = map toModuleGraph structuredModule.modules;
+          };
         in
-    modulesPath: initialModules: args:
+        map toModuleGraph (filter (x: x.key != "lib/modules.nix") modules);
-    filterModules modulesPath (collectStructuredModules unknownModule "" initialModules args);
+    in
+    modulesPath: initialModules: args: {
+      modules = filterModules modulesPath (collectStructuredModules unknownModule "" initialModules args);
+      graph = toGraph modulesPath (collectStructuredModules unknownModule "" initialModules args);
+    };
 
   /**
     Wrap a module with a default location for reporting errors.

lib/tests/modules.sh

@@ -20,6 +20,10 @@ cd "$DIR"/modules
 pass=0
 fail=0
 
+local-nix-instantiate() {
+    nix-instantiate --timeout 1 --eval-only --show-trace --read-write-mode --json "$@"
+}
+
 # loc
 #   prints the location of the call of to the function that calls it
 # loc n
@@ -55,7 +59,7 @@ evalConfig() {
     local attr=$1
     shift
     local script="import ./default.nix { modules = [ $* ];}"
-    nix-instantiate --timeout 1 -E "$script" -A "$attr" --eval-only --show-trace --read-write-mode --json
+    local-nix-instantiate -E "$script" -A "$attr"
 }
 
 reportFailure() {
@@ -106,6 +110,20 @@ globalErrorLogCheck() {
       }
 }
 
+checkExpression() {
+  local path=$1
+  local output
+  {
+      output="$(local-nix-instantiate --strict "$path" 2>&1)" && ((++pass))
+  } || {
+      logStartFailure
+      echo "$output"
+      ((++fail))
+      logFailure
+      logEndFailure
+  }
+}
+
 checkConfigError() {
     local errorContains=$1
     local err=""
@@ -337,6 +355,9 @@ checkConfigOutput '^12$' config.value ./declare-coerced-value-unsound.nix
 checkConfigError 'A definition for option .* is not of type .*. Definition values:\n\s*- In .*: "1000"' config.value ./declare-coerced-value-unsound.nix ./define-value-string-bigint.nix
 checkConfigError 'toInt: Could not convert .* to int' config.value ./declare-coerced-value-unsound.nix ./define-value-string-arbitrary.nix
 
+# Check `graph` attribute
+checkExpression './graph/test.nix'
+
 # Check mkAliasOptionModule.
 checkConfigOutput '^true$' config.enable ./alias-with-priority.nix
 checkConfigOutput '^true$' config.enableAlias ./alias-with-priority.nix

lib/tests/modules/graph/a.nix

@@ -0,0 +1,8 @@
+{
+  imports = [
+    {
+      imports = [ { } ];
+    }
+  ];
+  disabledModules = [ ./b.nix ];
+}

lib/tests/modules/graph/b.nix

@@ -0,0 +1,3 @@
+args: {
+  imports = [ { key = "explicit-key"; } ];
+}

lib/tests/modules/graph/test.nix

@@ -0,0 +1,64 @@
+let
+  lib = import ../../..;
+
+  evaluation = lib.evalModules {
+    modules = [
+      { }
+      (args: { })
+      ./a.nix
+      ./b.nix
+    ];
+  };
+
+  actual = evaluation.graph;
+
+  expected = [
+    {
+      key = ":anon-1";
+      file = "<unknown-file>";
+      imports = [ ];
+      disabled = false;
+    }
+    {
+      key = ":anon-2";
+      file = "<unknown-file>";
+      imports = [ ];
+      disabled = false;
+    }
+    {
+      key = toString ./a.nix;
+      file = toString ./a.nix;
+      imports = [
+        {
+          key = "${toString ./a.nix}:anon-1";
+          file = toString ./a.nix;
+          imports = [
+            {
+              key = "${toString ./a.nix}:anon-1:anon-1";
+              file = toString ./a.nix;
+              imports = [ ];
+              disabled = false;
+            }
+          ];
+          disabled = false;
+        }
+      ];
+      disabled = false;
+    }
+    {
+      key = toString ./b.nix;
+      file = toString ./b.nix;
+      imports = [
+        {
+          key = "explicit-key";
+          file = toString ./b.nix;
+          imports = [ ];
+          disabled = false;
+        }
+      ];
+      disabled = true;
+    }
+  ];
+in
+assert actual == expected;
+null