Bun.spawn: close stdout/stderr pipes after a timeout kill

src/runtime/api/bun/subprocess.rs

@@ -705,9 +705,10 @@
         sp.on_max_buffer(kind);
     }
 
-    /// Close any still-open stdout/stderr pipe readers so the sync wait loop
-    /// stops waiting for EOF after timeout/maxBuffer. Matches Node.js
-    /// `SyncProcessRunner::Kill()`. Called outside any reader callback.
+    /// Close any still-open stdout/stderr pipe readers so we stop waiting for
+    /// EOF after a timeout/maxBuffer kill. A grandchild may still hold the
+    /// pipe's write end. Matches Node.js `SyncProcessRunner::Kill()`. Called
+    /// outside any reader callback.
     pub fn close_readable_pipes(&self) {
         if matches!(self.stdout.get(), Readable::Pipe(_)) {
             self.stdout.with_mut(|s| s.close());
@@ -1033,6 +1034,18 @@
             }
         }
 
+        // When Bun itself killed the child (timeout/maxBuffer), stop waiting on
+        // pipe EOF: a grandchild may still hold the write end and the caller
+        // has already opted into a bounded wait. The drain above buffers
+        // whatever was readable at exit; closing now delivers that buffer
+        // instead of blocking `proc.stdout` until the grandchild exits. Same
+        // semantics as the spawnSync wait loop's `close_readable_pipes()` call.
+        if self.event_loop_timer.get().state == EventLoopTimerState::FIRED
+            || self.exited_due_to_maxbuf.get().is_some()
+        {
+            self.close_readable_pipes();
+        }
+
         if let Some(pipe_ptr) = stdin {
             self.weak_file_sink_stdin_ptr.set(None);
             self.update_flags(|f| f.insert(Flags::HAS_STDIN_DESTRUCTOR_CALLED));

test/js/bun/spawn/spawn-maxbuf.test.ts

@@ -187,6 +187,34 @@
     expect(stderr).toBe("");
   });
 
+  // When Bun kills the child on timeout, a grandchild that inherited the pipe
+  // may still hold the write end. Reading stdout/stderr after `proc.exited`
+  // must not wait for that grandchild to exit; Bun closes its read end and
+  // delivers whatever was buffered (same as `spawnSync`).
+  test.skipIf(isWindows)(
+    "Bun.spawn stdout does not hang when a grandchild outlives the timeout",
+    async () => {
+      // `sh` starts fast enough that the background `sleep` is running before
+      // the timeout fires even under a debug build; the signal is delivered
+      // to `sh` alone, so `sleep` survives holding the pipe's write end.
+      await using proc = Bun.spawn({
+        cmd: ["sh", "-c", "echo from-child; sleep 60 & read _"],
+        env: bunEnv,
+        timeout: 200,
+        killSignal: "SIGTERM",
+        stdio: ["pipe", "pipe", "pipe"],
+      });
+      await proc.exited;
+      const [stdout, stderr] = await Promise.all([proc.stdout.text(), proc.stderr.text()]);
+      expect({ stdout, stderr, exitCode: proc.exitCode, signalCode: proc.signalCode }).toEqual({
+        stdout: "from-child\n",
+        stderr: "",
+        exitCode: null,
+        signalCode: "SIGTERM",
+      });
+    },
+  );
+
   test("Bun.spawnSync", () => {
     const timeStart = Date.now();
     const proc = Bun.spawnSync([bunExe(), "exec", "sleep 5"], {