[extensions] Add an initial set of extension variables (#7035)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/cli/src/config/extensions/variableSchema.ts

@@ -0,0 +1,30 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export interface VariableDefinition {
+  type: 'string';
+  description: string;
+  default?: string;
+  required?: boolean;
+}
+
+export interface VariableSchema {
+  [key: string]: VariableDefinition;
+}
+
+const PATH_SEPARATOR_DEFINITION = {
+  type: 'string',
+  description: 'The path separator.',
+} as const;
+
+export const VARIABLE_SCHEMA = {
+  extensionPath: {
+    type: 'string',
+    description: 'The path of the extension in the filesystem.',
+  },
+  '/': PATH_SEPARATOR_DEFINITION,
+  pathSeparator: PATH_SEPARATOR_DEFINITION,
+} as const;

packages/cli/src/config/extensions/variables.test.ts

@@ -0,0 +1,18 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { expect, describe, it } from 'vitest';
+import { hydrateString } from './variables.js';
+
+describe('hydrateString', () => {
+  it('should replace a single variable', () => {
+    const context = {
+      extensionPath: 'path/my-extension',
+    };
+    const result = hydrateString('Hello, ${extensionPath}!', context);
+    expect(result).toBe('Hello, path/my-extension!');
+  });
+});

packages/cli/src/config/extensions/variables.ts

@@ -0,0 +1,65 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { type VariableSchema, VARIABLE_SCHEMA } from './variableSchema.js';
+
+export type JsonObject = { [key: string]: JsonValue };
+export type JsonArray = JsonValue[];
+export type JsonValue =
+  | string
+  | number
+  | boolean
+  | null
+  | JsonObject
+  | JsonArray;
+
+export type VariableContext = {
+  [key in keyof typeof VARIABLE_SCHEMA]?: string;
+};
+
+export function validateVariables(
+  variables: VariableContext,
+  schema: VariableSchema,
+) {
+  for (const key in schema) {
+    const definition = schema[key];
+    if (definition.required && !variables[key as keyof VariableContext]) {
+      throw new Error(`Missing required variable: ${key}`);
+    }
+  }
+}
+
+export function hydrateString(str: string, context: VariableContext): string {
+  validateVariables(context, VARIABLE_SCHEMA);
+  const regex = /\${(.*?)}/g;
+  return str.replace(regex, (match, key) =>
+    context[key as keyof VariableContext] == null
+      ? match
+      : (context[key as keyof VariableContext] as string),
+  );
+}
+
+export function recursivelyHydrateStrings(
+  obj: JsonValue,
+  values: VariableContext,
+): JsonValue {
+  if (typeof obj === 'string') {
+    return hydrateString(obj, values);
+  }
+  if (Array.isArray(obj)) {
+    return obj.map((item) => recursivelyHydrateStrings(item, values));
+  }
+  if (typeof obj === 'object' && obj !== null) {
+    const newObj: JsonObject = {};
+    for (const key in obj) {
+      if (Object.prototype.hasOwnProperty.call(obj, key)) {
+        newObj[key] = recursivelyHydrateStrings(obj[key], values);
+      }
+    }
+    return newObj;
+  }
+  return obj;
+}