[3.12] gh-114328: tty cbreak mode should not alter ICRNL (GH-114335) (#114410)

The terminal CR -> NL mapping setting should be inherited in cbreak mode as OSes do not specify altering it as part of their stty cbreak mode definition.
(cherry picked from commit fd49e226700e2483a452c3c92da6f15d822ae054)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

4 files changed, 30 insertions, 4 deletions

diff --git a/Doc/library/tty.rst b/Doc/library/tty.rst
index 20ba7d7..ed63561 100644
--- a/Doc/library/tty.rst
+++ b/Doc/library/tty.rst
@@ -35,8 +35,15 @@ The :mod:`tty` module defines the following functions:
    Convert the tty attribute list *mode*, which is a list like the one returned
    by :func:`termios.tcgetattr`, to that of a tty in cbreak mode.
 
+   This clears the ``ECHO`` and ``ICANON`` local mode flags in *mode* as well
+   as setting the minimum input to 1 byte with no delay.
+
    .. versionadded:: 3.12
 
+   .. versionchanged:: 3.12.2
+      The ``ICRNL`` flag is no longer cleared. This matches Linux and macOS
+      ``stty cbreak`` behavior and what :func:`setcbreak` historically did.
+
 
 .. function:: setraw(fd, when=termios.TCSAFLUSH)
 
@@ -56,9 +63,17 @@ The :mod:`tty` module defines the following functions:
    :func:`termios.tcsetattr`. The return value of :func:`termios.tcgetattr`
    is saved before setting *fd* to cbreak mode; this value is returned.
 
+   This clears the ``ECHO`` and ``ICANON`` local mode flags as well as setting
+   the minimum input to 1 byte with no delay.
+
    .. versionchanged:: 3.12
       The return value is now the original tty attributes, instead of None.
 
+   .. versionchanged:: 3.12.2
+      The ``ICRNL`` flag is no longer cleared. This restores the behavior
+      of Python 3.11 and earlier as well as matching what Linux, macOS, & BSDs
+      describe in their ``stty(1)`` man pages regarding cbreak mode.
+
 
 .. seealso::
 
diff --git a/Lib/test/test_tty.py b/Lib/test/test_tty.py
index af20864..4cb730c 100644
--- a/Lib/test/test_tty.py
+++ b/Lib/test/test_tty.py
@@ -19,7 +19,6 @@ class TestTty(unittest.TestCase):
         self.addCleanup(termios.tcsetattr, self.fd, termios.TCSAFLUSH, self.mode)
 
     def check_cbreak(self, mode):
-        self.assertEqual(mode[0] & termios.ICRNL, 0)
         self.assertEqual(mode[3] & termios.ECHO, 0)
         self.assertEqual(mode[3] & termios.ICANON, 0)
         self.assertEqual(mode[6][termios.VMIN], 1)
@@ -56,6 +55,14 @@ class TestTty(unittest.TestCase):
         self.assertEqual(mode[2], self.mode[2])
         self.assertEqual(mode[4], self.mode[4])
         self.assertEqual(mode[5], self.mode[5])
+        mode[tty.IFLAG] |= termios.ICRNL
+        tty.cfmakecbreak(mode)
+        self.assertEqual(mode[tty.IFLAG] & termios.ICRNL, termios.ICRNL,
+                         msg="ICRNL should not be cleared by cbreak")
+        mode[tty.IFLAG] &= ~termios.ICRNL
+        tty.cfmakecbreak(mode)
+        self.assertEqual(mode[tty.IFLAG] & termios.ICRNL, 0,
+                         msg="ICRNL should not be set by cbreak")
 
     def test_setraw(self):
         mode0 = termios.tcgetattr(self.fd)
@@ -74,6 +81,9 @@ class TestTty(unittest.TestCase):
         self.assertEqual(mode1, mode0)
         mode2 = termios.tcgetattr(self.fd)
         self.check_cbreak(mode2)
+        ICRNL = termios.ICRNL
+        self.assertEqual(mode2[tty.IFLAG] & ICRNL, mode0[tty.IFLAG] & ICRNL,
+                         msg="ICRNL should not be altered by cbreak")
         mode3 = tty.setcbreak(self.fd, termios.TCSANOW)
         self.assertEqual(mode3, mode2)
         tty.setcbreak(self.stream)
diff --git a/Lib/tty.py b/Lib/tty.py
index 283e5c3..5a49e04 100644
--- a/Lib/tty.py
+++ b/Lib/tty.py
@@ -45,9 +45,6 @@ def cfmakeraw(mode):
 
 def cfmakecbreak(mode):
     """Make termios mode cbreak."""
-    # Do not map CR to NL on input.
-    mode[IFLAG] &= ~(ICRNL)
-
     # Do not echo characters; disable canonical input.
     mode[LFLAG] &= ~(ECHO | ICANON)
 
diff --git a/Misc/NEWS.d/next/Library/2024-01-19-15-48-06.gh-issue-114328.hixxW3.rst b/Misc/NEWS.d/next/Library/2024-01-19-15-48-06.gh-issue-114328.hixxW3.rst
new file mode 100644
index 0000000..42262c0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-01-19-15-48-06.gh-issue-114328.hixxW3.rst
@@ -0,0 +1,4 @@
+The :func:`tty.setcbreak` and new :func:`tty.cfmakecbreak` no longer clears
+the terminal input ICRLF flag. This fixes a regression introduced in 3.12
+that no longer matched how OSes define cbreak mode in their ``stty(1)``
+manual pages.