From 08804c361ccc0e05b6daedd704ce78401fd5ab0e Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Fri, 26 Jan 2024 18:18:54 +0100
Subject: [PATCH] cli: fix module discovery race

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
---
 .changeset/two-geese-explain.md                |  5 +++++
 packages/cli/package.json                      |  1 +
 .../cli/src/lib/bundler/packageDetection.ts    | 18 ++++++++++++------
 yarn.lock                                      |  1 +
 4 files changed, 19 insertions(+), 6 deletions(-)
 create mode 100644 .changeset/two-geese-explain.md

diff --git a/.changeset/two-geese-explain.md b/.changeset/two-geese-explain.md
new file mode 100644
index 0000000000..12ae307cfe
--- /dev/null
+++ b/.changeset/two-geese-explain.md
@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Fixed an issue that would cause an invalid `__backstage-autodetected-plugins__.js` to be written when using experimental module discovery.
diff --git a/packages/cli/package.json b/packages/cli/package.json
index 5ca7d675d3..96dc2e5db8 100644
--- a/packages/cli/package.json
+++ b/packages/cli/package.json
@@ -112,6 +112,7 @@
     "node-libs-browser": "^2.2.1",
     "npm-packlist": "^5.0.0",
     "ora": "^5.3.0",
+    "p-queue": "^6.6.2",
     "postcss": "^8.1.0",
     "process": "^0.11.10",
     "react-dev-utils": "^12.0.0-next.60",
diff --git a/packages/cli/src/lib/bundler/packageDetection.ts b/packages/cli/src/lib/bundler/packageDetection.ts
index 3aa0955c06..0c357fe79e 100644
--- a/packages/cli/src/lib/bundler/packageDetection.ts
+++ b/packages/cli/src/lib/bundler/packageDetection.ts
@@ -18,6 +18,7 @@ import { BackstagePackageJson } from '@backstage/cli-node';
 import { Config, ConfigReader } from '@backstage/config';
 import chokidar from 'chokidar';
 import fs from 'fs-extra';
+import PQueue from 'p-queue';
 import { join as joinPath, resolve as resolvePath } from 'path';
 import { paths as cliPaths } from '../paths';
 
@@ -104,6 +105,9 @@ async function detectPackages(
   });
 }
 
+// Make sure we're not issuing multiple writes at the same time, which can cause partial overwrites
+const writeQueue = new PQueue({ concurrency: 1 });
+
 async function writeDetectedPackagesModule(
   pkgs: { name: string; export?: string; import: string }[],
 ) {
@@ -116,13 +120,15 @@ async function writeDetectedPackagesModule(
     )
     .join(',');
 
-  await fs.writeFile(
-    joinPath(
-      cliPaths.targetRoot,
-      'node_modules',
-      `${DETECTED_MODULES_MODULE_NAME}.js`,
+  await writeQueue.add(() =>
+    fs.writeFile(
+      joinPath(
+        cliPaths.targetRoot,
+        'node_modules',
+        `${DETECTED_MODULES_MODULE_NAME}.js`,
+      ),
+      `window['__@backstage/discovered__'] = { modules: [${requirePackageScript}] };`,
     ),
-    `window['__@backstage/discovered__'] = { modules: [${requirePackageScript}] };`,
   );
 }
 
diff --git a/yarn.lock b/yarn.lock
index 1c83813e10..ad55841387 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -3649,6 +3649,7 @@ __metadata:
     nodemon: ^3.0.1
     npm-packlist: ^5.0.0
     ora: ^5.3.0
+    p-queue: ^6.6.2
     postcss: ^8.1.0
     process: ^0.11.10
     react-dev-utils: ^12.0.0-next.60