Fetch: Object.prototype shouldn't interfere with fetch streams

Test that setting Object.prototype.start to a function that throws
doesn't interfere with fetch operations that create a stream. Similarly,
verify that setting throwing accessors for 'type', 'size', and
'highWaterMark' doesn't cause problems.

See whatwg/fetch#781 for background.

Author: ricea

fetch/api/basic/stream-safe-creation.any.js

@@ -0,0 +1,54 @@
+// META: global=worker
+
+// These tests verify that stream creation is not affected by changes to
+// Object.prototype.
+
+const creationCases = {
+  fetch: async () => fetch(location.href),
+  request: () => new Request(location.href, {method: 'POST', body: 'hi'}),
+  response: () => new Response('bye'),
+  consumeEmptyResponse: () => new Response().text(),
+  consumeNonEmptyResponse: () => new Response(new Uint8Array([64])).text(),
+  consumeEmptyRequest: () => new Request(location.href).text(),
+  consumeNonEmptyRequest: () => new Request(location.href,
+                                            {method: 'POST', body: 'yes'}).arrayBuffer(),
+};
+
+for (creationCase of Object.keys(creationCases)) {
+  for (accessorName of ['start', 'type', 'size', 'highWaterMark']) {
+    promise_test(async t => {
+      Object.defineProperty(Object.prototype, accessorName, {
+        get() { throw Error(`Object.prototype.${accessorName} was accessed`); },
+        configurable: true
+      });
+      t.add_cleanup(() => {
+        delete Object.prototype[accessorName];
+        return Promise.resolve();
+      });
+      await creationCases[creationCase]();
+    }, `throwing Object.prototype.${accessorName} accessor should not affect ` +
+       `stream creation by '${creationCase}'`);
+
+    promise_test(async t => {
+      // -1 is a convenient value which is invalid, and should cause the
+      // constructor to throw, for all four fields.
+      Object.prototype[accessorName] = -1;
+      t.add_cleanup(() => {
+        delete Object.prototype[accessorName];
+        return Promise.resolve();
+      });
+      await creationCases[creationCase]();
+    }, `Object.prototype.${accessorName} accessor returning invalid value ` +
+       `should not affect stream creation by '${creationCase}'`);
+  }
+
+  promise_test(async t => {
+    Object.prototype.start = controller => controller.error(new Error('start'));
+      t.add_cleanup(() => {
+        delete Object.prototype.start;
+        return Promise.resolve();
+      });
+      await creationCases[creationCase]();
+    }, `Object.prototype.start function which errors the stream should not ` +
+       `affect stream creation by '${creationCase}'`);
+}