Async iterable Endpoint handlers have back pressure

Apply back pressure by pausing invocations of endpoint handler when
part of the written data was queued in user memory. Once the kernel
buffer is free to be written again, invocations will commence again.

packages/adapter-node/src/server.js

@@ -57,9 +57,24 @@ export function createServer({ render }) {
 					typeof rendered.body[Symbol.asyncIterator] === 'function'
 				) {
 					const flush = compressible(rendered.headers['content-type']) ? res.flush : null;
+					const drainers = [];
+					const writer = (event) =>
+						new Promise((resolve) => {
+							if (!res.write(event)) {
+								drainers.push(resolve);
+							} else {
+								resolve();
+							}
+						});
+					// FIXME: upstream bug in compression prevents usage
+					// of res.once('drain', resolve) in writer's curry.
+					// https://github.com/expressjs/compression/pull/153
+					res.on('drain', () => {
+						drainers.splice(0, drainers.length).forEach((resolve) => resolve());
+					});
 					for await (const event of rendered.body) {
 						if (res.connection.destroyed) break;
-						res.write(event);
+						await writer(event);
 						flush?.();
 					}
 					res.end();