From 6556a5d22216a047c99e6c8d85e6ee0cef5d7b74 Mon Sep 17 00:00:00 2001
From: Rudi Grinberg <me@rgrinberg.com>
Date: Sat, 18 Jul 2026 02:05:43 +0100
Subject: [PATCH] test(spawn): reproduce lost exec errors with closed standard
 fds

The spawn error pipe can occupy file descriptors 0 or 1 when standard
descriptors are closed. Child redirection then overwrites the pipe write
end, so an exec failure looks like a clean close and the parent returns
a pid.

Fork a helper with stdin and stdout closed, redirect the spawned process
to /dev/null, and show that spawning a nonexistent executable currently
returns instead of raising.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
---
 otherlibs/stdune/test/spawn/tests.ml | 30 ++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/otherlibs/stdune/test/spawn/tests.ml b/otherlibs/stdune/test/spawn/tests.ml
index ee886e87486..310d4a925dc 100644
--- a/otherlibs/stdune/test/spawn/tests.ml
+++ b/otherlibs/stdune/test/spawn/tests.ml
@@ -27,6 +27,36 @@ let%expect_test "non-existing program" =
   |}]
 ;;
 
+let%expect_test "exec failure returns if standard descriptors are closed" =
+  if Platform.OS.value = Linux
+  then
+    assert (
+      match Unix.fork () with
+      | 0 ->
+        let dev_null = Unix.openfile "/dev/null" [ O_RDWR ] 0 in
+        Unix.close Unix.stdin;
+        Unix.close Unix.stdout;
+        (match
+           Spawn.spawn
+             ()
+             ~prog:"/doesnt-exist"
+             ~argv:[ "blah" ]
+             ~stdin:dev_null
+             ~stdout:dev_null
+             ~stderr:dev_null
+         with
+         | pid ->
+           (match Proc.wait (Pid pid) [] with
+            | Some _ -> Unix._exit 0
+            | None -> Unix._exit 2)
+         | exception Unix.Unix_error _ -> Unix._exit 1)
+      | child ->
+        (match Proc.wait (Pid (Pid.of_int_exn child)) [] with
+         | Some { status = WEXITED 0; _ } -> true
+         | Some _ | None -> false));
+  [%expect {||}]
+;;
+
 let%expect_test "non-existing dir" =
   show_raise (fun () ->
     Spawn.spawn