Extract out local plugin check

packages/gatsby/src/bootstrap/load-plugins/resolve-plugin.ts

@@ -1,14 +1,14 @@
 import path from "path"
 import fs from "fs"
 import { slash, createRequireFromPath } from "gatsby-core-utils"
-import { sync as existsSync } from "fs-exists-cached"
 import { warnOnIncompatiblePeerDependency } from "./validate"
 import { PackageJson } from "../../.."
 import { IPluginInfo, PluginRef } from "./types"
 import { createPluginId } from "./utils/create-id"
 import { createFileContentHash } from "./utils/create-hash"
 import reporter from "gatsby-cli/lib/reporter"
 import { isString } from "lodash"
+import { checkLocalPlugin } from "./utils/check-local-plugin"
 
 /**
  * @param plugin
@@ -24,30 +24,25 @@ import { isString } from "lodash"
 export function resolvePlugin(plugin: PluginRef, rootDir: string): IPluginInfo {
   const pluginName = isString(plugin) ? plugin : plugin.resolve
 
-  // Only find plugins when we're not given an absolute path
-  if (!existsSync(pluginName) && rootDir) {
-    // Find the plugin in the local plugins folder
-    const resolvedPath = slash(path.join(rootDir, `plugins/${pluginName}`))
+  // Handle local plugins
+  const { validLocalPlugin, localPluginPath = `` } = checkLocalPlugin(
+    plugin,
+    rootDir
+  )
 
-    if (existsSync(resolvedPath)) {
-      if (existsSync(`${resolvedPath}/package.json`)) {
-        const packageJSON = JSON.parse(
-          fs.readFileSync(`${resolvedPath}/package.json`, `utf-8`)
-        ) as PackageJson
-        const name = packageJSON.name || pluginName
-        warnOnIncompatiblePeerDependency(name, packageJSON)
+  if (validLocalPlugin && localPluginPath) {
+    const packageJSON = JSON.parse(
+      fs.readFileSync(`${localPluginPath}/package.json`, `utf-8`)
+    ) as PackageJson
+    const name = packageJSON.name || pluginName
+    warnOnIncompatiblePeerDependency(name, packageJSON)
 
-        return {
-          resolve: resolvedPath,
-          name,
-          id: createPluginId(name),
-          version:
-            packageJSON.version || createFileContentHash(resolvedPath, `**`),
-        }
-      } else {
-        // Make package.json a requirement for local plugins too
-        throw new Error(`Plugin ${pluginName} requires a package.json file`)
-      }
+    return {
+      resolve: localPluginPath,
+      name,
+      id: createPluginId(name),
+      version:
+        packageJSON.version || createFileContentHash(localPluginPath, `**`),
     }
   }
 

packages/gatsby/src/bootstrap/load-plugins/utils/check-local-plugin.ts

@@ -0,0 +1,41 @@
+import { PluginRef } from "../types"
+import { sync as existsSync } from "fs-exists-cached"
+import { slash } from "gatsby-core-utils"
+import path from "path"
+
+/**
+ * Checks if a plugin is a valid local plugin and returns the resolved path if it is.
+ */
+export function checkLocalPlugin(
+  plugin: PluginRef,
+  rootDir: string
+): { validLocalPlugin: boolean; localPluginPath?: string } {
+  const pluginName = typeof plugin === `string` ? plugin : plugin.resolve
+
+  // Make sure the plugin exists relatively
+  if (existsSync(pluginName) || !rootDir) {
+    return {
+      validLocalPlugin: false,
+    }
+  }
+
+  const resolvedPath = slash(path.join(rootDir, `plugins/${pluginName}`))
+
+  if (!existsSync(resolvedPath)) {
+    return {
+      validLocalPlugin: false,
+    }
+  }
+
+  const resolvedPackageJson = existsSync(`${resolvedPath}/package.json`)
+
+  // package.json is a requirement for local plugins
+  if (!resolvedPackageJson) {
+    throw new Error(`Local plugin ${pluginName} requires a package.json file`)
+  }
+
+  return {
+    validLocalPlugin: true,
+    localPluginPath: resolvedPath,
+  }
+}