pythongh-114328: tty cbreak mode should not alter ICRNL.

CR -> NL mapping should be inherited in cbreak mode as OSes do not
specify altering it as part of their `stty` cbreak mode definition.

Doc/library/tty.rst

@@ -37,6 +37,9 @@ The :mod:`tty` module defines the following functions:
 
    .. versionadded:: 3.12
 
+   .. versionchanged:: 3.12.2
+      The ``ICRNL`` flag is no longer cleared.
+
 
 .. function:: setraw(fd, when=termios.TCSAFLUSH)
 
@@ -59,6 +62,11 @@ The :mod:`tty` module defines the following functions:
    .. 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 matches both the behavior
+      of 3.11 and earlier as well as what Linux, macOS, and BSDs describe in
+      their ``stty(1)`` man pages regarding cbreak mode.
+
 
 .. seealso::
 

Lib/test/test_tty.py

@@ -19,7 +19,6 @@ def setUp(self):
         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 @@ def test_cfmakecbreak(self):
         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 @@ def test_setcbreak(self):
         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)

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)