[core] Move stream reconnect logic to getReadable level

.changeset/huge-guests-smash.md

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+---
+"@workflow/world-vercel": patch
+---
+
+Propagate client stream cancellation

.changeset/stream-reconnect-at-getreadable.md

@@ -0,0 +1,5 @@
+---
+'@workflow/core': patch
+---
+
+Auto-reconnect `getReadable()` streams on server close or transient errors

packages/core/src/reconnecting-framed-stream.test.ts

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+import type { World } from '@workflow/world';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('./version.js', () => ({ version: '0.0.0-test' }));
+
+import { setWorld } from './runtime/world.js';
+import { createReconnectingFramedStream } from './serialization.js';
+
+const FRAME_HEADER_SIZE = 4;
+
+function encodeFrame(payload: Uint8Array): Uint8Array {
+  const out = new Uint8Array(FRAME_HEADER_SIZE + payload.length);
+  new DataView(out.buffer).setUint32(0, payload.length, false);
+  out.set(payload, FRAME_HEADER_SIZE);
+  return out;
+}
+
+function payloadFrame(n: number): Uint8Array {
+  return encodeFrame(new Uint8Array([n]));
+}
+
+/**
+ * Build a stream from a scripted pull sequence. Each entry either
+ * enqueues a value or errors — this keeps the stream from transitioning
+ * to the errored state before earlier values are actually read (which
+ * `start()`-time `controller.error` does immediately).
+ */
+function scriptedStream(
+  steps: Array<
+    | { kind: 'value'; value: Uint8Array }
+    | { kind: 'error'; err: unknown }
+    | { kind: 'close' }
+  >,
+  onCancel?: (reason?: unknown) => void
+): ReadableStream<Uint8Array> {
+  let i = 0;
+  return new ReadableStream<Uint8Array>({
+    pull(controller) {
+      const step = steps[i++];
+      if (!step) {
+        controller.close();
+        return;
+      }
+      if (step.kind === 'value') controller.enqueue(step.value);
+      else if (step.kind === 'error') controller.error(step.err);
+      else controller.close();
+    },
+    cancel(reason) {
+      onCancel?.(reason);
+    },
+  });
+}
+
+async function readAll(
+  stream: ReadableStream<Uint8Array>
+): Promise<Uint8Array[]> {
+  const reader = stream.getReader();
+  const chunks: Uint8Array[] = [];
+  for (;;) {
+    const r = await reader.read();
+    if (r.done) break;
+    if (r.value) chunks.push(r.value);
+  }
+  return chunks;
+}
+
+/**
+ * Builds a mock world whose readFromStream returns a prepared
+ * sequence per `startIndex`. Each call records the requested startIndex
+ * so assertions can check reconnect positioning.
+ */
+function makeWorldWithScriptedStreams(
+  scripts: Record<number, () => ReadableStream<Uint8Array>>
+): { world: World; calls: number[] } {
+  const calls: number[] = [];
+  const world = {
+    readFromStream: vi.fn(async (_name: string, startIndex?: number) => {
+      const idx = startIndex ?? 0;
+      calls.push(idx);
+      const factory = scripts[idx];
+      if (!factory) {
+        throw new Error(`unexpected startIndex ${idx}`);
+      }
+      return factory();
+    }),
+  } as unknown as World;
+  return { world, calls };
+}
+
+describe('createReconnectingFramedStream', () => {
+  afterEach(() => {
+    setWorld(undefined as unknown as World);
+  });
+
+  it('passes through complete frames and closes cleanly on EOF', async () => {
+    const { world, calls } = makeWorldWithScriptedStreams({
+      0: () =>
+        scriptedStream([
+          { kind: 'value', value: payloadFrame(1) },
+          { kind: 'value', value: payloadFrame(2) },
+          { kind: 'value', value: payloadFrame(3) },
+          { kind: 'close' },
+        ]),
+    });
+    setWorld(world);
+
+    const stream = createReconnectingFramedStream('s', 0);
+    const chunks = await readAll(stream);
+
+    expect(chunks).toEqual([payloadFrame(1), payloadFrame(2), payloadFrame(3)]);
+    expect(calls).toEqual([0]);
+  });
+
+  it('forwards a frame delivered across multiple reads', async () => {
+    const full = payloadFrame(42);
+    const { world } = makeWorldWithScriptedStreams({
+      0: () =>
+        scriptedStream([
+          // Split frame into 3 byte-level reads to prove boundary-aware
+          // buffering works regardless of transport chunking.
+          { kind: 'value', value: full.slice(0, 2) },
+          { kind: 'value', value: full.slice(2, 4) },
+          { kind: 'value', value: full.slice(4) },
+          { kind: 'close' },
+        ]),
+    });
+    setWorld(world);
+
+    const stream = createReconnectingFramedStream('s', 0);
+    const chunks = await readAll(stream);
+
+    expect(chunks).toHaveLength(1);
+    expect(chunks[0]).toEqual(full);
+  });
+
+  it('reconnects with startIndex = consumed count on upstream error', async () => {
+    const { world, calls } = makeWorldWithScriptedStreams({
+      0: () =>
+        scriptedStream([
+          { kind: 'value', value: payloadFrame(1) },
+          { kind: 'value', value: payloadFrame(2) },
+          // Simulate server 2-minute abort mid-frame: deliver the first
+          // 3 bytes of a frame then error. The wrapper should discard
+          // those partial bytes and reopen at the right index.
+          { kind: 'value', value: payloadFrame(3).slice(0, 3) },
+          { kind: 'error', err: new Error('max-duration abort') },
+        ]),
+      2: () =>
+        scriptedStream([
+          { kind: 'value', value: payloadFrame(3) },
+          { kind: 'value', value: payloadFrame(4) },
+          { kind: 'close' },
+        ]),
+    });
+    setWorld(world);
+
+    const stream = createReconnectingFramedStream('s', 0);
+    const chunks = await readAll(stream);
+
+    expect(chunks).toEqual([
+      payloadFrame(1),
+      payloadFrame(2),
+      payloadFrame(3),
+      payloadFrame(4),
+    ]);
+    // First connection: startIndex=0. After 2 frames consumed, reconnect
+    // opens a fresh stream at startIndex=2.
+    expect(calls).toEqual([0, 2]);
+  });
+
+  it('respects an initial non-zero startIndex on reconnect', async () => {
+    const { world, calls } = makeWorldWithScriptedStreams({
+      10: () =>
+        scriptedStream([
+          { kind: 'value', value: payloadFrame(10) },
+          { kind: 'error', err: new Error('abort') },
+        ]),
+      11: () =>
+        scriptedStream([
+          { kind: 'value', value: payloadFrame(11) },
+          { kind: 'close' },
+        ]),
+    });
+    setWorld(world);
+
+    const stream = createReconnectingFramedStream('s', 10);
+    const chunks = await readAll(stream);
+
+    expect(chunks).toEqual([payloadFrame(10), payloadFrame(11)]);
+    expect(calls).toEqual([10, 11]);
+  });
+
+  it('does not reconnect when startIndex is negative', async () => {
+    const { world, calls } = makeWorldWithScriptedStreams({
+      [-5]: () =>
+        scriptedStream([
+          { kind: 'value', value: payloadFrame(99) },
+          { kind: 'error', err: new Error('abort') },
+        ]),
+    });
+    setWorld(world);
+
+    const stream = createReconnectingFramedStream('s', -5);
+    await expect(readAll(stream)).rejects.toThrow(/abort/);
+    expect(calls).toEqual([-5]);
+  });
+
+  it('cancel aborts the upstream reader', async () => {
+    const cancelSpy = vi.fn();
+    const { world } = makeWorldWithScriptedStreams({
+      0: () => {
+        // Keep the upstream pending after the first value so cancel
+        // actually has a live stream to abort; an auto-closed upstream
+        // would swallow the cancel per web-streams spec.
+        let pulls = 0;
+        return new ReadableStream<Uint8Array>({
+          async pull(controller) {
+            if (pulls++ === 0) {
+              controller.enqueue(payloadFrame(1));
+              return;
+            }
+            await new Promise(() => {}); // hang forever
+          },
+          cancel(reason) {
+            cancelSpy(reason);
+          },
+        });
+      },
+    });
+    setWorld(world);
+
+    const stream = createReconnectingFramedStream('s', 0);
+    const reader = stream.getReader();
+    const first = await reader.read();
+    expect(first.done).toBe(false);
+
+    await reader.cancel('client abort');
+
+    expect(cancelSpy).toHaveBeenCalled();
+  });
+});