From dd00c6b0c12f8e0a032168b4832b0cba622bf131 Mon Sep 17 00:00:00 2001
From: Ben Newman <ben@meteor.com>
Date: Wed, 3 Oct 2018 13:56:27 -0400
Subject: [PATCH] Warn about duplicate api.mainModule paths, like api.addFiles
 does.

This would have helped catch the underlying problem in #10234.
---
 tools/isobuild/package-api.js | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/isobuild/package-api.js b/tools/isobuild/package-api.js
index 6239cfa7c17..4fff4a2d8ca 100644
--- a/tools/isobuild/package-api.js
+++ b/tools/isobuild/package-api.js
@@ -366,6 +366,8 @@ export class PackageAPI {
   mainModule(path, arch, fileOptions = {}) {
     arch = toArchArray(arch);
 
+    const errors = [];
+
     forAllMatchingArchs(arch, a => {
       const filesForArch = this.files[a];
       const source = {
@@ -392,11 +394,19 @@ export class PackageAPI {
         }
       }
 
+      if (filesForArch.sources.some(old => source.relPath === old.relPath)) {
+        errors.push(`Duplicate api.mainModule: ${path}`);
+      }
+
       filesForArch.main = source;
       filesForArch.sources.push(source);
 
       this._forbidExportWithLazyMain(a);
     });
+
+    errors.forEach(error => {
+      buildmessage.error(error, { useMyCaller: 1 });
+    });
   }
 
   _forbidExportWithLazyMain(arch) {