cli: add local linking of backend packages too

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/cli/config/nodeTransform.cjs

@@ -16,6 +16,39 @@
 
 const { transformSync } = require('@swc/core');
 const { addHook } = require('pirates');
+const { Module } = require('module');
+
+// This hooks into module resolution and overrides imports of packages that
+// exist in the linked workspace to instead be resolved from the linked workspace.
+if (process.env.LINKED_WORKSPACE) {
+  const { join: joinPath } = require('path');
+  const { getPackagesSync } = require('@manypkg/get-packages');
+  const { packages: linkedPackages, root: linkedRoot } = getPackagesSync(
+    process.env.LINKED_WORKSPACE,
+  );
+
+  // Matches all packages in the linked workspaces, as well as sub-path exports from them
+  const replacementRegex = new RegExp(
+    `^(?:${linkedPackages
+      .map(pkg => pkg.packageJson.name)
+      .join('|')})(?:/.*)?$`,
+  );
+
+  const origLoad = Module._load;
+  Module._load = function requireHook(request, parent) {
+    if (!replacementRegex.test(request)) {
+      return origLoad.call(this, request, parent);
+    }
+
+    // The package import that we're overriding will always existing in the root
+    // node_modules of the linked workspace, so it's enough to override the the
+    // parent paths with that single entry
+    return origLoad.call(this, request, {
+      ...parent,
+      paths: [joinPath(linkedRoot.dir, 'node_modules')],
+    });
+  };
+}
 
 addHook(
   (code, filename) => {

packages/cli/src/commands/start/command.ts

@@ -34,20 +34,10 @@ export async function command(opts: OptionValues): Promise<void> {
 
   switch (role) {
     case 'backend':
-      if (options.linkedWorkspace) {
-        throw new Error(
-          'The --link flag is not supported for this package role',
-        );
-      }
       return startBackend(options);
     case 'backend-plugin':
     case 'backend-plugin-module':
     case 'node-library':
-      if (options.linkedWorkspace) {
-        throw new Error(
-          'The --link flag is not supported for this package role',
-        );
-      }
       return startBackendPlugin(options);
     case 'frontend':
       return startFrontend({

packages/cli/src/commands/start/startBackend.ts

@@ -22,6 +22,7 @@ interface StartBackendOptions {
   checksEnabled: boolean;
   inspectEnabled: boolean;
   inspectBrkEnabled: boolean;
+  linkedWorkspace?: string;
   require?: string;
 }
 
@@ -30,6 +31,7 @@ export async function startBackend(options: StartBackendOptions) {
     entry: 'src/index',
     inspectEnabled: options.inspectEnabled,
     inspectBrkEnabled: options.inspectBrkEnabled,
+    linkedWorkspace: options.linkedWorkspace,
     require: options.require,
   });
 
@@ -52,6 +54,7 @@ export async function startBackendPlugin(options: StartBackendOptions) {
     inspectEnabled: options.inspectEnabled,
     inspectBrkEnabled: options.inspectBrkEnabled,
     require: options.require,
+    linkedWorkspace: options.linkedWorkspace,
   });
 
   await waitForExit();

packages/cli/src/lib/runner/runBackend.ts

@@ -40,6 +40,8 @@ export type RunBackendOptions = {
   inspectBrkEnabled: boolean;
   /** Additional module to require via the --require flag to the node process */
   require?: string;
+  /** An external linked workspace to override module resolution towards */
+  linkedWorkspace?: string;
 };
 
 export async function runBackend(options: RunBackendOptions) {
@@ -119,6 +121,7 @@ export async function runBackend(options: RunBackendOptions) {
         stdio: ['ignore', 'inherit', 'inherit', 'ipc'],
         env: {
           ...process.env,
+          LINKED_WORKSPACE: options.linkedWorkspace,
           BACKSTAGE_CLI_CHANNEL: '1',
           ESBK_TSCONFIG_PATH: paths.resolveTargetRoot('tsconfig.json'),
         },