From 4ae023d4828b246ef2bea98d20d4cee848364bf0 Mon Sep 17 00:00:00 2001
From: Alexey Izbyshev <izbyshev@ispras.ru>
Date: Sat, 4 Mar 2023 17:24:08 +0300
Subject: [PATCH] gh-102179: Fix `os.dup2` error reporting for negative fds
 (GH-102180) (cherry picked from commit
 c2bd55d26f8eb2850eb9f9026b5d7f0ed1420b65)

Co-authored-by: Alexey Izbyshev <izbyshev@ispras.ru>
---
 Lib/test/test_os.py                           | 20 +++++++++++++++++++
 ...-02-23-15-06-01.gh-issue-102179.P6KQ4c.rst |  1 +
 Modules/posixmodule.c                         |  5 -----
 3 files changed, 21 insertions(+), 5 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2023-02-23-15-06-01.gh-issue-102179.P6KQ4c.rst

diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 9970b234b4831b..58e83f658c4445 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -2176,6 +2176,26 @@ def test_closerange(self):
     def test_dup2(self):
         self.check(os.dup2, 20)
 
+    @unittest.skipUnless(hasattr(os, 'dup2'), 'test needs os.dup2()')
+    @unittest.skipIf(
+        support.is_emscripten,
+        "dup2() with negative fds is broken on Emscripten (see gh-102179)"
+    )
+    def test_dup2_negative_fd(self):
+        valid_fd = os.open(__file__, os.O_RDONLY)
+        self.addCleanup(os.close, valid_fd)
+        fds = [
+            valid_fd,
+            -1,
+            -2**31,
+        ]
+        for fd, fd2 in itertools.product(fds, repeat=2):
+            if fd != fd2:
+                with self.subTest(fd=fd, fd2=fd2):
+                    with self.assertRaises(OSError) as ctx:
+                        os.dup2(fd, fd2)
+                    self.assertEqual(ctx.exception.errno, errno.EBADF)
+
     @unittest.skipUnless(hasattr(os, 'fchmod'), 'test needs os.fchmod()')
     def test_fchmod(self):
         self.check(os.fchmod, 0)
diff --git a/Misc/NEWS.d/next/Library/2023-02-23-15-06-01.gh-issue-102179.P6KQ4c.rst b/Misc/NEWS.d/next/Library/2023-02-23-15-06-01.gh-issue-102179.P6KQ4c.rst
new file mode 100644
index 00000000000000..f77493e267ac7e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-02-23-15-06-01.gh-issue-102179.P6KQ4c.rst
@@ -0,0 +1 @@
+Fix :func:`os.dup2` error message for negative fds.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 150bb78cb41c81..613b86f09c9eaf 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -9387,11 +9387,6 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
     static int dup3_works = -1;
 #endif
 
-    if (fd < 0 || fd2 < 0) {
-        posix_error();
-        return -1;
-    }
-
     /* dup2() can fail with EINTR if the target FD is already open, because it
      * then has to be closed. See os_close_impl() for why we don't handle EINTR
      * upon close(), and therefore below.