Split code out into smaller functions and add tests

packages/gatsby/src/bootstrap/__mocks__/resolve-module-exports.js

@@ -0,0 +1,19 @@
+'use strict'
+
+let mockResults = {}
+
+module.exports = input => {
+  // return a mocked result
+  if (typeof input === `string`) {
+    return mockResults[input]
+  }
+
+  // return default result
+  if (typeof input !== `object`) {
+    return []
+  }
+
+  // set mock results
+  mockResults = Object.assign({}, input)
+  return undefined
+}

packages/gatsby/src/bootstrap/index.js

@@ -176,7 +176,12 @@ module.exports = async (args: BootstrapArgs) => {
   // Find plugins which implement gatsby-browser and gatsby-ssr and write
   // out api-runners for them.
   const hasAPIFile = (env, plugin) => {
-    if (plugin[`${env}APIs`].length > 0 ) {
+    // The plugin loader has disabled SSR APIs for this plugin. Usually due to
+    // multiple implementations of an API that can only be implemented once
+    if (env === `ssr` && plugin.skipSSR === true) return undefined
+
+    const envAPIs = plugin[`${env}APIs`]
+    if (envAPIs && Array.isArray(envAPIs) && envAPIs.length > 0 ) {
       return path.join(plugin.resolve, `gatsby-${env}.js`)
     }
     return undefined