From 9d365ba4b32166ddc67758bc2ccd7703a408c55d Mon Sep 17 00:00:00 2001
From: "Gregory P. Smith" <greg@krypto.org>
Date: Thu, 24 Dec 2020 22:26:17 -0800
Subject: [PATCH] bpo-38435: Detect preexec_fn=os.setsid as
 start_new_session=True

This is a somewhat common use case, this makes it more reliable.
---
 Lib/subprocess.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index e259dc3a8e5..c39d98104b3 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -937,6 +937,10 @@ class Popen(object):
             if uid < 0:
                 raise ValueError(f"User ID cannot be negative, got {uid}")
 
+        if preexec_fn == os.setsid:  # A common unnecessary legacy use.
+            start_new_session = True
+            preexec_fn = None
+
         try:
             if p2cwrite != -1:
                 self.stdin = io.open(p2cwrite, 'wb', bufsize)