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[Backport v14.x] module: add isPreloading indicator #36988

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[Backport v14.x] module: add isPreloading indicator #36988

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c38b2ad
module: add isPreloading indicator
jasnell Nov 25, 2020
4569a65
doc: fix module.isPreloading documentation
aduh95 Jan 15, 2021
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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>
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@jasnell @aduh95
jasnell authored and aduh95 committed Jan 18, 2021
commit c38b2ad4ab108f464dfc6cda63f08f8d14c60602
8 changes: 8 additions & 0 deletions doc/api/module.md
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Expand Up @@ -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
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9 changes: 9 additions & 0 deletions lib/internal/modules/cjs/loader.js
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Expand Up @@ -128,6 +128,7 @@ const relativeResolveCache = ObjectCreate(null);

let requireDepth = 0;
let statCache = null;
let isPreloading = false;

function stat(filename) {
filename = path.toNamespacedPath(filename);
Expand Down Expand Up @@ -219,6 +220,10 @@ ObjectDefineProperty(Module, 'wrapper', {
}
});

ObjectDefineProperty(Module.prototype, 'isPreloading', {
get() { return isPreloading; }
});
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@ExE-Boss ExE-Boss Jan 21, 2021

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It might be a good idea to also backport NativeModule.prototype.isPreloading:

node/lib/internal/modules/cjs/loader.js

Lines 236 to 238 in e279304

const isPreloadingDesc = { get() { return isPreloading; } };
ObjectDefineProperty(Module.prototype, 'isPreloading', isPreloadingDesc);
ObjectDefineProperty(NativeModule.prototype, 'isPreloading', isPreloadingDesc);


let debug = require('internal/util/debuglog').debuglog('module', (fn) => {
debug = fn;
});
Expand Down Expand Up @@ -1202,6 +1207,8 @@ 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.
Expand All @@ -1210,11 +1217,13 @@ 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() {
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2 changes: 2 additions & 0 deletions test/fixtures/ispreloading.js
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@@ -0,0 +1,2 @@
const assert = require('assert');
assert(module.isPreloading);
13 changes: 13 additions & 0 deletions test/parallel/test-preload.js
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Expand Up @@ -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}"`,
Expand Down