From 2e96ea757a45a4e5c7de9075d6a75f0085072a27 Mon Sep 17 00:00:00 2001
From: SJ <76181208+imaginary-person@users.noreply.github.com>
Date: Fri, 5 Feb 2021 23:33:52 -0600
Subject: [PATCH 1/3] Do not add duplicate FDs in duplicate_for_child()

While spawning, if the code built on top of python's multiprocessing module calls DupFd (in  /Lib/multiprocessing/reduction.py) multiple times on the same File descriptor by mistake, then it ends up invoking duplicate_for_child(fd)) of the class Popen, which, in turn, adds a duplicate FD to a list.
This list is then used in spawnv_passfds() in multiprocessing/util.py, which uses that list as an argument in a call of _posixsubprocess.fork_exec().

In Modules/_posixsubprocess.c, checks exist to ensure that all the FDs in the list are unique, positive, and in a sorted order.
If duplicate entries are found, a 'ValueError: bad value(s) in fds_to_keep' exception is raised, and the execution is terminated because this exception is not handled by Python.
Even if it were, a developer can't change the aforementioned FD list in Python without breaking modularity.

It's better to let developers get away with calling DupFd (in  /Lib/multiprocessing/reduction.py) by not accepting duplicate entries in the first place,
so that spawning a process can be successful.
---
 Lib/multiprocessing/popen_spawn_posix.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Lib/multiprocessing/popen_spawn_posix.py b/Lib/multiprocessing/popen_spawn_posix.py
index 24b8634523e5f2..c3c96b52675cd0 100644
--- a/Lib/multiprocessing/popen_spawn_posix.py
+++ b/Lib/multiprocessing/popen_spawn_posix.py
@@ -32,7 +32,8 @@ def __init__(self, process_obj):
         super().__init__(process_obj)
 
     def duplicate_for_child(self, fd):
-        self._fds.append(fd)
+        if fd not in self._fds:
+            self._fds.append(fd)
         return fd
 
     def _launch(self, process_obj):

From f06f482ee906a44c8967672b99da309f9ee7abe4 Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Sat, 6 Feb 2021 06:43:53 +0000
Subject: [PATCH 2/3] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20blu?=
 =?UTF-8?q?rb=5Fit.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst

diff --git a/Misc/NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst b/Misc/NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst
new file mode 100644
index 00000000000000..87ec85faa69209
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst
@@ -0,0 +1 @@
+Do not add duplicate FDs to list in duplicate_for_child() in order to avoid ValueError: bad value(s) in fds_to_keep
\ No newline at end of file

From 31a801be035c0a5dd57791657c68e06496cc7821 Mon Sep 17 00:00:00 2001
From: SJ <76181208+imaginary-person@users.noreply.github.com>
Date: Sat, 6 Feb 2021 01:22:00 -0600
Subject: [PATCH 3/3] Add comment to trigger CI

All tests except the news item addition had passed the first time around, so need to re-run CI.
---
 Lib/multiprocessing/popen_spawn_posix.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Lib/multiprocessing/popen_spawn_posix.py b/Lib/multiprocessing/popen_spawn_posix.py
index c3c96b52675cd0..7c0377035baa6a 100644
--- a/Lib/multiprocessing/popen_spawn_posix.py
+++ b/Lib/multiprocessing/popen_spawn_posix.py
@@ -32,6 +32,7 @@ def __init__(self, process_obj):
         super().__init__(process_obj)
 
     def duplicate_for_child(self, fd):
+        # duplicates in self._fds would later lead to a ValueError
         if fd not in self._fds:
             self._fds.append(fd)
         return fd