Bun.serve: cancel the piped upstream fetch when the client aborts while backpressured

src/runtime/server/RequestContext.rs

@@ -1496,6 +1496,14 @@ where
             return;
         }
 
+        if let Some(byte_stream) = this.byte_stream.take() {
+            bun_ptr::BackRef::from(byte_stream).cancel_from_sink(None);
+            any_js_calls.set(true);
+            this.response_body_readable_stream_ref
+                .with_mut(|s| s.deinit());
+            this.deref();
+        }
+
         // if we can, free the request now.
         if this.is_dead_request() {
             this.finalize_without_deinit();
@@ -3385,6 +3393,7 @@ where
         // SAFETY: caller passes the live `*mut RequestContext` stored as the
         // sink ctx; `_ref` keeps it alive for this call.
         let this = unsafe { &*this };
+        this.byte_stream.set(None);
 
         if this.is_aborted_or_ended() {
             return;
@@ -3398,7 +3407,6 @@ where
             if !this.flags.has_written_status() {
                 let global_this = this.server().global_this();
                 let js_err = err.to_js(global_this);
-                this.byte_stream.set(None);
                 this.response_body_readable_stream_ref
                     .with_mut(|s| s.deinit());
                 this.run_error_handler(js_err);

test/js/bun/http/serve-fetch-body-abort-backpressure.test.ts

@@ -0,0 +1,102 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+import net from "node:net";
+import { join } from "node:path";
+
+// A client that aborts while Bun.serve is holding an upstream fetch() body
+// paused for backpressure must not leave the upstream parked: on_abort has
+// to cancel the ByteStream sink so the proxy tears down its connection to
+// the upstream.
+test("client abort while a fetch() body is backpressured cancels the upstream", async () => {
+  const CHUNK = 256 * 1024;
+  const CAP_CHUNKS = 256;
+
+  let pulls = 0;
+  let cancelled = false;
+
+  await using upstream = Bun.serve({
+    port: 0,
+    idleTimeout: 255,
+    fetch() {
+      return new Response(
+        new ReadableStream({
+          async pull(controller) {
+            controller.enqueue(new Uint8Array(CHUNK));
+            pulls++;
+            if (pulls >= CAP_CHUNKS) return controller.close();
+            if (pulls % 32 === 0) await Bun.sleep(0);
+          },
+          cancel() {
+            cancelled = true;
+          },
+        }),
+        { headers: { "content-length": String(CHUNK * CAP_CHUNKS) } },
+      );
+    },
+  });
+
+  await using proxy = Bun.spawn({
+    cmd: [
+      bunExe(),
+      join(import.meta.dir, "serve-fetch-body-abort-backpressure-fixture.ts"),
+      `http://127.0.0.1:${upstream.port}/`,
+    ],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "inherit",
+  });
+
+  const reader = proxy.stdout.getReader();
+  let head = "";
+  while (!head.includes("\n")) {
+    const { value, done } = await reader.read();
+    if (done) throw new Error("proxy exited before reporting port");
+    head += Buffer.from(value).toString("utf8");
+  }
+  reader.releaseLock();
+  const { proxyPort } = JSON.parse(head.slice(0, head.indexOf("\n")));
+
+  const failed = Promise.withResolvers<never>();
+  proxy.exited.then(code => failed.reject(new Error(`proxy exited early (code ${code})`)));
+
+  const socket = net.connect(proxyPort, "127.0.0.1");
+  try {
+    const stalled = Promise.withResolvers<void>();
+    socket.on("error", e => failed.reject(e));
+    socket.on("connect", () => socket.write("GET / HTTP/1.1\r\nHost: x\r\nConnection: close\r\n\r\n"));
+    socket.once("data", () => {
+      socket.pause();
+      stalled.resolve();
+    });
+    await Promise.race([stalled.promise, failed.promise]);
+
+    // Wait until the upstream pull count stops growing (backpressure engaged)
+    // or the upstream produces the whole capped body (no backpressure at all).
+    let lastPulls = -1;
+    let stableTurns = 0;
+    while (pulls < CAP_CHUNKS && stableTurns < 12) {
+      await Bun.sleep(25);
+      if (pulls === lastPulls) stableTurns++;
+      else {
+        stableTurns = 0;
+        lastPulls = pulls;
+      }
+    }
+    expect(pulls).toBeLessThan(CAP_CHUNKS / 2);
+  } finally {
+    socket.removeAllListeners("error");
+    socket.on("error", () => {});
+    socket.destroy();
+  }
+
+  // The proxy's on_abort should cancel its fetch to the upstream; the
+  // upstream's serve then cancels the ReadableStream. Poll for that signal.
+  for (let i = 0; i < 200 && !cancelled; i++) await Bun.sleep(10);
+
+  expect({ cancelled, pullsUnderCap: pulls < CAP_CHUNKS, pulls }).toMatchObject({
+    cancelled: true,
+    pullsUnderCap: true,
+  });
+  expect(proxy.exitCode).toBeNull();
+  expect(proxy.signalCode).toBeNull();
+});