[v2] Don't allow unknown keys on node.internal object (#5770)

* [v2] Don't allow unknown fields on node.internal object

Validate against changes like #5557

* Document the change

docs/docs/migrating-from-v1-to-v2.md

@@ -24,6 +24,7 @@ This is a reference for upgrading your site from Gatsby v1 to Gatsby v2. While t
 - [Change `modifyBabelrc` to `onCreateBabelConfig`](#change-modifybabelrc-to-oncreatebabelconfig)
 - [Change `modifyWebpackConfig` to `onCreateWebpackConfig`](#change-modifywebpackconfig-to-oncreatewebpackconfig)
 - [Remove inlined CSS in `html.js`](#remove-inlined-css-in-htmljs)
+- [Only allow defined keys on node.internal object](#only-allow-defined-keys-on-the-node-internal-object)
 
 You can start with a few of the most important steps - install Gatsby v2 dependencies and update your layout components.
 
@@ -378,3 +379,8 @@ See [Gatsby's webpack docs for more details](/docs/add-custom-webpack-config) ab
 Gatsby v2 automatically inlines CSS. You can remove any custom CSS inlining from your custom `html.js`.
 
 See an example in [this PR that upgrades the `using-remark` site to Gatsby v2](https://github.com/gatsbyjs/gatsby/commit/765b679cbc222fd5f527690427ee431cca7ccd61#diff-637c76e3c059ed8efacedf6e30de2d61).
+
+## Only allow defined keys on the node internal object
+
+The node internal object isn't meant for adding node data. Those should be added to the top-level object. We
+didn't document this in v1 nor validate against it but are now for v2.

packages/gatsby/src/joi-schemas/joi.js

@@ -39,14 +39,16 @@ export const nodeSchema = Joi.object()
           `"parent" must be the "id" of another node or if there is no parent (common), "null"`
       ),
     fields: Joi.object(),
-    internal: Joi.object().keys({
-      contentDigest: Joi.string().required(),
-      mediaType: Joi.string(),
-      type: Joi.string().required(),
-      owner: Joi.string().required(),
-      fieldOwners: Joi.array(),
-      content: Joi.string().allow(``),
-      description: Joi.string(),
-    }),
+    internal: Joi.object()
+      .keys({
+        contentDigest: Joi.string().required(),
+        mediaType: Joi.string(),
+        type: Joi.string().required(),
+        owner: Joi.string().required(),
+        fieldOwners: Joi.array(),
+        content: Joi.string().allow(``),
+        description: Joi.string(),
+      })
+      .unknown({ allow: false }), // Don't allow non-standard fields
   })
   .unknown()