feat: add isAvailable method (#23)

src/index.ts

@@ -74,3 +74,18 @@ export function instance(options?: string|Options) {
 export function project(options?: string|Options) {
   return metadataAccessor('project', options);
 }
+
+/**
+ * Determine if the metadata server is currently available.
+ */
+export async function isAvailable() {
+  try {
+    // Attempt to read instance metadata. As configured, this will
+    // retry 3 times if there is a valid response, and fail fast
+    // if there is an ETIMEDOUT or ENOTFOUND error.
+    await instance();
+    return true;
+  } catch (err) {
+    return false;
+  }
+}

test/index.test.ts

@@ -120,3 +120,23 @@ test.serial('should not retry on DNS errors', async t => {
   await t.throws(gcp.instance());
   scope.done();
 });
+
+test.serial(
+    'should report isGCE if the server returns a 500 first', async t => {
+      const scopes = [
+        nock(HOST).get(`${PATH}/${TYPE}`).reply(500),
+        nock(HOST).get(`${PATH}/${TYPE}`).reply(200, {}, HEADERS)
+      ];
+      const isGCE = await gcp.isAvailable();
+      scopes.forEach(s => s.done());
+      t.true(isGCE);
+    });
+
+test.serial(
+    'should fail fast on isAvailable if a network err is returned', async t => {
+      const scope =
+          nock(HOST).get(`${PATH}/${TYPE}`).replyWithError({code: 'ENOTFOUND'});
+      const isGCE = await gcp.isAvailable();
+      scope.done();
+      t.false(isGCE);
+    });