module: add isPreloading indicator

Adds a `module.isPreloading` property that is `true` only during the
preload (`-r`) phase of Node.js bootstrap. This provides modules an
easy, non-hacky way of knowing if they are being loaded during preload.

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: #36263
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/module.md

@@ -76,6 +76,14 @@ const requireUtil = createRequireFromPath('../src/utils/');
 requireUtil('./some-tool');
 ```
 
+### `module.isPreloading`
+<!-- YAML
+added: REPLACEME
+-->
+
+* Type: {boolean} `true` if the module is running during the Node.js preload
+  phase.
+
 ### `module.syncBuiltinESMExports()`
 <!-- YAML
 added: v12.12.0

lib/internal/modules/cjs/loader.js

@@ -139,6 +139,7 @@ const relativeResolveCache = ObjectCreate(null);
 
 let requireDepth = 0;
 let statCache = null;
+let isPreloading = false;
 
 function stat(filename) {
   filename = path.toNamespacedPath(filename);
@@ -231,6 +232,10 @@ ObjectDefineProperty(Module, 'wrapper', {
   }
 });
 
+ObjectDefineProperty(Module.prototype, 'isPreloading', {
+  get() { return isPreloading; }
+});
+
 function getModuleParent() {
   return moduleParentCache.get(this);
 }
@@ -1243,6 +1248,8 @@ Module._preloadModules = function(requests) {
   if (!ArrayIsArray(requests))
     return;
 
+  isPreloading = true;
+
   // Preloaded modules have a dummy parent module which is deemed to exist
   // in the current working directory. This seeds the search path for
   // preloaded modules.
@@ -1251,11 +1258,13 @@ Module._preloadModules = function(requests) {
     parent.paths = Module._nodeModulePaths(process.cwd());
   } catch (e) {
     if (e.code !== 'ENOENT') {
+      isPreloading = false;
       throw e;
     }
   }
   for (let n = 0; n < requests.length; n++)
     parent.require(requests[n]);
+  isPreloading = false;
 };
 
 Module.syncBuiltinESMExports = function syncBuiltinESMExports() {

test/fixtures/ispreloading.js

@@ -0,0 +1,2 @@
+const assert = require('assert');
+assert(module.isPreloading);

test/parallel/test-preload.js

@@ -27,6 +27,19 @@ const fixtureE = fixtures.path('intrinsic-mutation.js');
 const fixtureF = fixtures.path('print-intrinsic-mutation-name.js');
 const fixtureG = fixtures.path('worker-from-argv.js');
 const fixtureThrows = fixtures.path('throws_error4.js');
+const fixtureIsPreloading = fixtures.path('ispreloading.js');
+
+// Assert that module.isPreloading is false here
+assert(!module.isPreloading);
+
+// Test that module.isPreloading is set in preloaded module
+// Test preloading a single module works
+childProcess.exec(
+  `"${nodeBinary}" ${preloadOption([fixtureIsPreloading])} "${fixtureB}"`,
+  function(err, stdout, stderr) {
+    assert.ifError(err);
+    assert.strictEqual(stdout, 'B\n');
+  });
 
 // Test preloading a single module works
 childProcess.exec(`"${nodeBinary}" ${preloadOption([fixtureA])} "${fixtureB}"`,