extensionSystem: Split out loadExtensionMetadata()

There is no need for sharing the functionality, but moving
the code into extensionUtils allows adding unit tests for it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3379>

js/misc/extensionUtils.js

@@ -109,3 +109,64 @@ export function deserializeExtension(variant) {
     res.dir = Gio.File.new_for_path(res.path);
     return res;
 }
+
+/**
+ * Load extension metadata from directory
+ *
+ * @param {string} uuid of the extension
+ * @param {GioFile} dir to load metadata from
+ * @returns {object}
+ */
+export function loadExtensionMetadata(uuid, dir) {
+    const dirName = dir.get_basename();
+    if (dirName !== uuid)
+        throw new Error(`Directory name "${dirName}" does not match UUID "${uuid}"`);
+
+    const metadataFile = dir.get_child('metadata.json');
+    if (!metadataFile.query_exists(null))
+        throw new Error('Missing metadata.json');
+
+    let metadataContents, success_;
+    try {
+        [success_, metadataContents] = metadataFile.load_contents(null);
+        metadataContents = new TextDecoder().decode(metadataContents);
+    } catch (e) {
+        throw new Error(`Failed to load metadata.json: ${e}`);
+    }
+    let meta;
+    try {
+        meta = JSON.parse(metadataContents);
+    } catch (e) {
+        throw new Error(`Failed to parse metadata.json: ${e}`);
+    }
+
+    const requiredProperties = [{
+        prop: 'uuid',
+        typeName: 'string',
+    }, {
+        prop: 'name',
+        typeName: 'string',
+    }, {
+        prop: 'description',
+        typeName: 'string',
+    }, {
+        prop: 'shell-version',
+        typeName: 'string array',
+        typeCheck: v => Array.isArray(v) && v.length > 0 && v.every(e => typeof e === 'string'),
+    }];
+    for (let i = 0; i < requiredProperties.length; i++) {
+        const {
+            prop, typeName, typeCheck = v => typeof v === typeName,
+        } = requiredProperties[i];
+
+        if (!meta[prop])
+            throw new Error(`missing "${prop}" property in metadata.json`);
+        if (!typeCheck(meta[prop]))
+            throw new Error(`property "${prop}" is not of type ${typeName}`);
+    }
+
+    if (uuid !== meta.uuid)
+        throw new Error(`UUID "${meta.uuid}" from metadata.json does not match directory name "${uuid}"`);
+
+    return meta;
+}