Keep large routed timelines connected when the receiver falls behind (#9109)

* fix(server): pace routed output to its receiving client

A routed transfer previously waited only for the destination-to-intermediary write, so a slower final hop overflowed the route queue. Negotiate bounded per-channel acknowledgments after final forwarding drains, preserving concurrent small sends and legacy peers.

* fix(transport): keep small routed writes concurrent

---------

Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>

packages/protocol/src/chunked-transport.test.ts

@@ -390,3 +390,65 @@ it("unpaced transports retain ordinary outbound admission and accept inbound chu
   expect(delivered).toEqual(["A"]);
   await transport.send("small");
 });
+
+it("fills an ordered writer's window while keeping chunk sequences contiguous", async () => {
+  const delivered: (string | Uint8Array)[] = [];
+  const receiver = new ChunkedTransport({
+    frameBytes: 8,
+    send() {},
+    deliver: (frame) => {
+      delivered.push(frame);
+    },
+  });
+  receiver.enabled = true;
+  const drain = Promise.withResolvers<void>();
+  const frames: (string | Uint8Array)[] = [];
+  const sender = new ChunkedTransport({
+    frameBytes: 8,
+    supportsConcurrentWrites: true,
+    send: (frame) => {
+      frames.push(frame);
+      receiver.receive(frame);
+      return drain.promise;
+    },
+    deliver() {},
+  });
+  sender.enabled = true;
+  const first = sender.send("one");
+  const second = sender.send("two");
+  expect(delivered).toEqual(["one", "two"]);
+  expect(sender.queuedByteLength).toBe(0);
+  const large = sender.send("large message");
+  const after = sender.send("after");
+  expect(frames).toHaveLength(3);
+  expect(sender.queuedByteLength).toBe(5);
+  drain.resolve();
+  await Promise.all([first, second, large, after]);
+  expect(delivered).toEqual(["one", "two", "large message", "after"]);
+  expect(sender.queuedByteLength).toBe(0);
+});
+
+it("accounts for independently draining writes and a rejected admission", async () => {
+  const firstDrain = Promise.withResolvers<void>();
+  const secondDrain = Promise.withResolvers<void>();
+  const sender = new ChunkedTransport({
+    frameBytes: 8,
+    supportsConcurrentWrites: true,
+    send: (frame) => {
+      if (frame === "reject") throw new Error("capacity");
+      return frame === "one" ? firstDrain.promise : secondDrain.promise;
+    },
+    deliver() {},
+  });
+  const first = sender.send("one");
+  const second = sender.send("two");
+  expect(() => sender.send("reject")).toThrow("capacity");
+  expect(sender.queuedByteLength).toBe(0);
+  secondDrain.resolve();
+  await second;
+  expect(sender.queuedByteLength).toBe(0);
+  firstDrain.resolve();
+  await first;
+  expect(sender.queuedByteLength).toBe(0);
+  await sender.send("usable");
+});

packages/protocol/src/chunked-transport.ts

@@ -23,6 +23,8 @@ export class ChunkedTransport {
     private readonly io: {
       frameBytes: number;
       supportsDrain?: boolean;
+      /** Writes are admitted synchronously in order; promises track only their drain. */
+      supportsConcurrentWrites?: boolean;
       send: (frame: Frame, waitForDrain: boolean) => void | Promise<void>;
       deliver: (frame: Frame) => void;
       onActivity?: () => void;
@@ -46,9 +48,11 @@ export class ChunkedTransport {
     const chunk =
       this.enabled && this.io.supportsDrain !== false && byteLength > this.io.frameBytes;
     if (this.closed) throw new Error("Transport closed");
+    let started = false;
     const write = () => {
       if (this.closed) throw new Error("Transport closed");
-      this.activeBytes = byteLength;
+      started = true;
+      this.activeBytes += byteLength;
       // A synchronous rejection did not admit a frame. Only accepted writes can
       // fail the connection; queued admission failures still belong to their sender.
       const result = chunk
@@ -76,27 +80,31 @@ export class ChunkedTransport {
       result = this.pending ? this.pending.then(write) : write();
     } catch (error) {
       this.queuedBytes -= byteLength;
-      this.activeBytes = 0;
+      if (started) this.activeBytes -= byteLength;
       throw error;
     }
     if (!result) {
       this.queuedBytes -= byteLength;
-      this.activeBytes = 0;
+      if (started) this.activeBytes -= byteLength;
       return Promise.resolve();
     }
     const completion = Promise.resolve(result).finally(() => {
       this.queuedBytes -= byteLength;
-      this.activeBytes = 0;
-    });
-    const tail = completion.then(
-      () => undefined,
-      () => undefined,
-    );
-    this.pending = tail;
-    void tail.then(() => {
-      if (this.pending === tail) this.pending = undefined;
-      return undefined;
+      if (started) this.activeBytes -= byteLength;
     });
+    // Ordinary writes already entered an ordered writer and can use its bounded
+    // window. Chunk sequences remain a barrier so their frames never interleave.
+    if (chunk || !this.io.supportsConcurrentWrites) {
+      const tail = completion.then(
+        () => undefined,
+        () => undefined,
+      );
+      this.pending = tail;
+      void tail.then(() => {
+        if (this.pending === tail) this.pending = undefined;
+        return undefined;
+      });
+    }
     return completion;
   }
 

packages/protocol/src/hosts.ts

@@ -116,8 +116,19 @@ export const HostsLinkOpenRequestSchema = z.object({
   type: z.literal("hosts.link.open.request"),
   requestId: z.string(),
   channelId: z.string().min(1).max(256),
+  flowControl: z.boolean().optional(),
 });
 export const HostsLinkOpenResponseSchema = z.object({
   type: z.literal("hosts.link.open.response"),
-  payload: z.object({ requestId: z.string(), routeId: z.string() }),
+  payload: z.object({
+    requestId: z.string(),
+    routeId: z.string(),
+    flowControl: z.boolean().optional(),
+  }),
+});
+
+export const HostsLinkAckSchema = z.object({
+  type: z.literal("hosts.link.ack.request"),
+  routeId: z.string(),
+  sequence: z.number().int().positive(),
 });

packages/protocol/src/messages.ts

@@ -5,6 +5,7 @@ import {
   HostsUpdateRequestSchema,
   HostsRemoveRequestSchema,
   HostsLinkOpenRequestSchema,
+  HostsLinkAckSchema,
   HostsRouteOpenRequestSchema,
   HostsRouteFrameRequestSchema,
   HostsRouteCloseRequestSchema,
@@ -3397,6 +3398,7 @@ export const SessionInboundMessageSchema = z.discriminatedUnion("type", [
   HostsUpdateRequestSchema,
   HostsRemoveRequestSchema,
   HostsLinkOpenRequestSchema,
+  HostsLinkAckSchema,
   HostsRouteOpenRequestSchema,
   HostsRouteFrameRequestSchema,
   HostsRouteCloseRequestSchema,