From 7839b5a35992fde0ae92752b8d3ddabf22ee7235 Mon Sep 17 00:00:00 2001
From: Brandon Payton <brandon@happycode.net>
Date: Tue, 14 Jul 2026 01:16:57 -0400
Subject: [PATCH] Runner: preserve complete guest output on short writes

---
 examples/run-example-output.ts       | 24 ++++++++++++++++++++++++
 examples/run-example.ts              |  5 +++--
 host/test/run-example-output.test.ts | 26 ++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 2 deletions(-)
 create mode 100644 examples/run-example-output.ts
 create mode 100644 host/test/run-example-output.test.ts

diff --git a/examples/run-example-output.ts b/examples/run-example-output.ts
new file mode 100644
index 0000000000..8895d9594d
--- /dev/null
+++ b/examples/run-example-output.ts
@@ -0,0 +1,24 @@
+import { writeSync } from "fs";
+
+export type WriteBytes = (
+    fd: number,
+    data: Uint8Array,
+    offset: number,
+    length: number,
+) => number;
+
+const writeBytes: WriteBytes = (fd, data, offset, length) =>
+    writeSync(fd, data, offset, length, null);
+
+export function writeAllSync(
+    fd: number,
+    data: Uint8Array,
+    write: WriteBytes = writeBytes,
+): void {
+    let offset = 0;
+    while (offset < data.byteLength) {
+        const written = write(fd, data, offset, data.byteLength - offset);
+        if (written <= 0) throw new Error("short write to guest output sink");
+        offset += written;
+    }
+}
diff --git a/examples/run-example.ts b/examples/run-example.ts
index 93fbbcdbf4..98ca394225 100644
--- a/examples/run-example.ts
+++ b/examples/run-example.ts
@@ -12,10 +12,11 @@
  *   npx tsx examples/run-example.ts /path/to/test.wasm
  */
 
-import { closeSync, existsSync, openSync, readFileSync, statSync, writeSync } from "fs";
+import { closeSync, existsSync, openSync, readFileSync, statSync } from "fs";
 import { resolve, dirname, isAbsolute } from "path";
 import { NodeKernelHost } from "../host/src/node-kernel-host";
 import { tryResolveBinary } from "../host/src/binary-resolver";
+import { writeAllSync } from "./run-example-output";
 import { isWithinRealDirectory } from "./run-example-paths";
 
 const repoRoot = resolve(dirname(new URL(import.meta.url).pathname), "..");
@@ -362,7 +363,7 @@ async function main() {
         if (guestOutputFd === null) {
             fallback.write(data);
         } else {
-            writeSync(guestOutputFd, data);
+            writeAllSync(guestOutputFd, data);
         }
     };
 
diff --git a/host/test/run-example-output.test.ts b/host/test/run-example-output.test.ts
new file mode 100644
index 0000000000..9b8a83e3bb
--- /dev/null
+++ b/host/test/run-example-output.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+import { writeAllSync } from "../../examples/run-example-output";
+
+describe("run-example guest output", () => {
+  it("preserves every byte across short synchronous writes", () => {
+    const chunks: Uint8Array[] = [];
+    const offsets: number[] = [];
+    const input = new Uint8Array([0, 255, 1, 128, 2]);
+
+    writeAllSync(7, input, (_fd, data, offset, length) => {
+      const written = Math.min(2, length);
+      offsets.push(offset);
+      chunks.push(new Uint8Array(data.subarray(offset, offset + written)));
+      return written;
+    });
+
+    expect(offsets).toEqual([0, 2, 4]);
+    expect(new Uint8Array(Buffer.concat(chunks))).toEqual(input);
+  });
+
+  it("fails instead of spinning when a write makes no progress", () => {
+    expect(() =>
+      writeAllSync(7, new Uint8Array([1]), () => 0),
+    ).toThrow("short write to guest output sink");
+  });
+});