From 9f59cdde4b9a85d6d45e5b6d455743e968b56659 Mon Sep 17 00:00:00 2001
From: broken-circle <252359939+broken-circle@users.noreply.github.com>
Date: Wed, 10 Jun 2026 13:04:52 -0700
Subject: [PATCH] Spawn `cat` in its own session in
 `testSuspendResumeProcess()`

The test spawned `/bin/cat` with default `PlatformOptions`, so `cat`
inherited the test runner's process group, then sent it `SIGSTOP`,
leaving a stopped process in the runner's own process group.

On POSIX, when a process group becomes orphaned and any member is
stopped, `SIGHUP` followed by `SIGCONT` is sent to every process in the
group. Under the full suite's concurrent process churn, that group
intermittently became orphaned while `cat` was stopped, and the kernel
delivered `SIGHUP` to every member of the group, including the test
process, which exited on the signal. This surfaced only when the suite
ran in parallel, as a bare signal-1 exit with no recorded test failure.

Spawn `cat` with `createSession` so it runs in its own session, and
stopping it no longer leaves a stopped member in the runner's group.
`cat`'s own group is orphaned at `setsid` time, which POSIX exempts
from the SIGHUP, so the suspend/resume assertions are unaffected.
---
 Tests/SubprocessTests/UnixTests.swift | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Tests/SubprocessTests/UnixTests.swift b/Tests/SubprocessTests/UnixTests.swift
index a6032aa7..6d3a1fae 100644
--- a/Tests/SubprocessTests/UnixTests.swift
+++ b/Tests/SubprocessTests/UnixTests.swift
@@ -378,10 +378,19 @@ extension SubprocessUnixTests {
         let flags = fcntl(outputPipe.readEnd.rawValue, F_GETFL)
         try #require(fcntl(outputPipe.readEnd.rawValue, F_SETFL, flags | O_NONBLOCK) == 0)
 
+        // Isolate cat in its own session: the SIGSTOP below would otherwise
+        // leave a stopped process in the test runner's process group, and a
+        // concurrent child exit that orphans that group makes the kernel
+        // deliver SIGHUP+SIGCONT to every member of the group, including this
+        // test process, killing the run.
+        var platformOptions = PlatformOptions()
+        platformOptions.createSession = true
+
         try await outputPipe.readEnd.closeAfter {
             try await inputPipe.writeEnd.closeAfter {
                 _ = try await Subprocess.run(
                     .path("/bin/cat"),
+                    platformOptions: platformOptions,
                     // cat reads from inputPipe.readEnd. The parent keeps writeEnd to feed it.
                     input: .fileDescriptor(inputPipe.readEnd, closeAfterSpawningProcess: true),
                     // cat writes to outputPipe.writeEnd. The parent keeps readEnd to drain it.