Fix node.js stream blockage in mxstream channels

When the node.js implementation of a `MultiplexingStream` channel receives more data than the highWatermark (16KB) limit, a flowing stream stops flowing permanently, blocking all communication in that direction.

This fixes the problem by calling `resume()` on the stream when flowing has been stopped by that particular data buffer.

src/nerdbank-streams/src/Channel.ts

@@ -216,8 +216,17 @@ export class ChannelClass extends Channel {
 	}
 
 	public onContent(buffer: Buffer | null) {
+		const priorReadableFlowing = this._duplex.readableFlowing
+
 		this._duplex.push(buffer)
 
+		// Large buffer pushes can switch a stream from flowing to non-flowing
+		// when it meets or exceeds the highWaterMark. We need to resume the stream
+		// in this case so that the user can continue to receive data.
+		if (priorReadableFlowing && this._duplex.readableFlowing === false) {
+			this._duplex.resume()
+		}
+
 		// We should find a way to detect when we *actually* share the received buffer with the Channel's user
 		// and only report consumption when they receive the buffer from us so that we effectively apply
 		// backpressure to the remote party based on our user's actual consumption rather than continually allocating memory.

src/nerdbank-streams/src/tests/MultiplexingStream.spec.ts

@@ -8,6 +8,59 @@ import { Channel } from '../Channel'
 import CancellationToken from 'cancellationtoken'
 import * as assert from 'assert'
 import { nextTick } from 'process'
+import { Duplex } from 'stream'
+
+it('highWatermark threshold does not clog', async () => {
+	// Brokered service
+	let bytesToReceive = 0
+	let receivedAllBytes = new Deferred()
+	function receiver(pipe: Duplex) {
+		let lengths: number[] = []
+		pipe.on('data', (data: Buffer) => {
+			lengths.push(data.length)
+
+			bytesToReceive -= data.length
+			// console.log(`recv ${data.length}. ${bytesToReceive} remaining`)
+			if (bytesToReceive <= 0) {
+				receivedAllBytes.resolve(undefined)
+			}
+		})
+	}
+
+	// IServiceBroker
+	const { first: localServicePipe, second: servicePipe } = FullDuplexStream.CreatePair()
+	receiver(localServicePipe)
+
+	// MultiplexingStreamServiceBroker
+	const simulatedMxStream = FullDuplexStream.CreatePair()
+	const [mx1, mx2] = await Promise.all([MultiplexingStream.CreateAsync(simulatedMxStream.first), MultiplexingStream.CreateAsync(simulatedMxStream.second)])
+	const [local, remote] = await Promise.all([mx1.offerChannelAsync(''), mx2.acceptChannelAsync('')])
+	servicePipe.pipe(local.stream)
+	local.stream.pipe(servicePipe)
+
+	global.test_servicePipe = servicePipe
+	global.test_d = local.stream
+	global.test_localServicePipe = localServicePipe
+
+	// brokered service client
+	function writeHelper(buffer: Buffer): boolean {
+		bytesToReceive += buffer.length
+		const result = remote.stream.write(buffer)
+		// console.log('written', buffer.length, result)
+		return result
+	}
+	for (let i = 15; i < 20; i++) {
+		const buffer = Buffer.alloc(i * 1024)
+		writeHelper(buffer)
+		await nextTickAsync()
+		writeHelper(Buffer.alloc(10))
+		await nextTickAsync()
+	}
+
+	if (bytesToReceive > 0) {
+		await receivedAllBytes.promise
+	}
+})
 ;[1, 2, 3].forEach(protocolMajorVersion => {
 	describe(`MultiplexingStream v${protocolMajorVersion}`, () => {
 		let mx1: MultiplexingStream