[Python-checkins] gh-102179: Fix `os.dup2` error reporting for negative fds (#102180)

[kumaraditya303]

https://github.com/python/cpython/commit/c2bd55d26f8eb2850eb9f9026b5d7f0ed1420b65
commit: c2bd55d26f8eb2850eb9f9026b5d7f0ed1420b65
branch: main
author: Alexey Izbyshev <izbyshev at ispras.ru>
committer: kumaraditya303 <59607654+kumaraditya303 at users.noreply.github.com>
date: 2023-03-04T19:54:08+05:30
summary:

gh-102179: Fix `os.dup2` error reporting for negative fds (#102180)

files:
A Misc/NEWS.d/next/Library/2023-02-23-15-06-01.gh-issue-102179.P6KQ4c.rst
M Lib/test/test_os.py
M Modules/posixmodule.c

diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index deea207bfdad..792794ca1094 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -2221,6 +2221,26 @@ 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 000000000000..f77493e267ac
--- /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 937233a3bd07..7beb2cee64a0 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -9795,11 +9795,6 @@ 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.