gh-134466: Don't run when termios is inaccessible (GH-138911)

Without the ability to set required capabilities, the REPL cannot
function properly (syntax highlighting and multiline editing can't
work).

We refuse to work in this degraded state.

Author: ambv

Lib/_pyrepl/fancy_termios.py

@@ -20,19 +20,25 @@
 import termios
 
 
+TYPE_CHECKING = False
+
+if TYPE_CHECKING:
+    from typing import cast
+else:
+    cast = lambda typ, val: val
+
+
 class TermState:
-    def __init__(self, tuples):
-        (
-            self.iflag,
-            self.oflag,
-            self.cflag,
-            self.lflag,
-            self.ispeed,
-            self.ospeed,
-            self.cc,
-        ) = tuples
+    def __init__(self, attrs: list[int | list[bytes]]) -> None:
+        self.iflag = cast(int, attrs[0])
+        self.oflag = cast(int, attrs[1])
+        self.cflag = cast(int, attrs[2])
+        self.lflag = cast(int, attrs[3])
+        self.ispeed = cast(int, attrs[4])
+        self.ospeed = cast(int, attrs[5])
+        self.cc = cast(list[bytes], attrs[6])
 
-    def as_list(self):
+    def as_list(self) -> list[int | list[bytes]]:
         return [
             self.iflag,
             self.oflag,
@@ -45,32 +51,32 @@ def as_list(self):
             self.cc[:],
         ]
 
-    def copy(self):
+    def copy(self) -> "TermState":
         return self.__class__(self.as_list())
 
 
-def tcgetattr(fd):
+def tcgetattr(fd: int) -> TermState:
     return TermState(termios.tcgetattr(fd))
 
 
-def tcsetattr(fd, when, attrs):
+def tcsetattr(fd: int, when: int, attrs: TermState) -> None:
     termios.tcsetattr(fd, when, attrs.as_list())
 
 
 class Term(TermState):
     TS__init__ = TermState.__init__
 
-    def __init__(self, fd=0):
+    def __init__(self, fd: int = 0) -> None:
         self.TS__init__(termios.tcgetattr(fd))
         self.fd = fd
-        self.stack = []
+        self.stack: list[list[int | list[bytes]]] = []
 
-    def save(self):
+    def save(self) -> None:
         self.stack.append(self.as_list())
 
-    def set(self, when=termios.TCSANOW):
+    def set(self, when: int = termios.TCSANOW) -> None:
         termios.tcsetattr(self.fd, when, self.as_list())
 
-    def restore(self):
+    def restore(self) -> None:
         self.TS__init__(self.stack.pop())
         self.set()

Lib/_pyrepl/unix_console.py

@@ -35,7 +35,7 @@
 
 from . import terminfo
 from .console import Console, Event
-from .fancy_termios import tcgetattr, tcsetattr
+from .fancy_termios import tcgetattr, tcsetattr, TermState
 from .trace import trace
 from .unix_eventqueue import EventQueue
 from .utils import wlen
@@ -51,16 +51,19 @@
 
 # types
 if TYPE_CHECKING:
-    from typing import IO, Literal, overload
+    from typing import AbstractSet, IO, Literal, overload, cast
 else:
     overload = lambda func: None
+    cast = lambda typ, val: val
 
 
 class InvalidTerminal(RuntimeError):
-    pass
+    def __init__(self, message: str) -> None:
+        super().__init__(errno.EIO, message)
 
 
 _error = (termios.error, InvalidTerminal)
+_error_codes_to_ignore = frozenset([errno.EIO, errno.ENXIO, errno.EPERM])
 
 SIGWINCH_EVENT = "repaint"
 
@@ -125,12 +128,13 @@ def __init__(self):
 
         def register(self, fd, flag):
             self.fd = fd
+
         # note: The 'timeout' argument is received as *milliseconds*
         def poll(self, timeout: float | None = None) -> list[int]:
             if timeout is None:
                 r, w, e = select.select([self.fd], [], [])
             else:
-                r, w, e = select.select([self.fd], [], [], timeout/1000)
+                r, w, e = select.select([self.fd], [], [], timeout / 1000)
             return r
 
     poll = MinimalPoll  # type: ignore[assignment]
@@ -164,8 +168,15 @@ def __init__(
             and os.getenv("TERM_PROGRAM") == "Apple_Terminal"
         )
 
+        try:
+            self.__input_fd_set(tcgetattr(self.input_fd), ignore=frozenset())
+        except _error as e:
+            raise RuntimeError(f"termios failure ({e.args[1]})")
+
         @overload
-        def _my_getstr(cap: str, optional: Literal[False] = False) -> bytes: ...
+        def _my_getstr(
+            cap: str, optional: Literal[False] = False
+        ) -> bytes: ...
 
         @overload
         def _my_getstr(cap: str, optional: bool) -> bytes | None: ...
@@ -205,7 +216,9 @@ def _my_getstr(cap: str, optional: bool = False) -> bytes | None:
 
         self.__setup_movement()
 
-        self.event_queue = EventQueue(self.input_fd, self.encoding, self.terminfo)
+        self.event_queue = EventQueue(
+            self.input_fd, self.encoding, self.terminfo
+        )
         self.cursor_visible = 1
 
         signal.signal(signal.SIGCONT, self._sigcont_handler)
@@ -217,7 +230,6 @@ def _sigcont_handler(self, signum, frame):
     def __read(self, n: int) -> bytes:
         return os.read(self.input_fd, n)
 
-
     def change_encoding(self, encoding: str) -> None:
         """
         Change the encoding used for I/O operations.
@@ -329,6 +341,8 @@ def prepare(self):
         """
         Prepare the console for input/output operations.
         """
+        self.__buffer = []
+
         self.__svtermstate = tcgetattr(self.input_fd)
         raw = self.__svtermstate.copy()
         raw.iflag &= ~(termios.INPCK | termios.ISTRIP | termios.IXON)
@@ -340,14 +354,7 @@ def prepare(self):
         raw.lflag |= termios.ISIG
         raw.cc[termios.VMIN] = 1
         raw.cc[termios.VTIME] = 0
-        try:
-            tcsetattr(self.input_fd, termios.TCSADRAIN, raw)
-        except termios.error as e:
-            if e.args[0] != errno.EIO:
-                # gh-135329: when running under external programs (like strace),
-                # tcsetattr may fail with EIO. We can safely ignore this
-                # and continue with default terminal settings.
-                raise
+        self.__input_fd_set(raw)
 
         # In macOS terminal we need to deactivate line wrap via ANSI escape code
         if self.is_apple_terminal:
@@ -356,8 +363,6 @@ def prepare(self):
         self.screen = []
         self.height, self.width = self.getheightwidth()
 
-        self.__buffer = []
-
         self.posxy = 0, 0
         self.__gone_tall = 0
         self.__move = self.__move_short
@@ -379,11 +384,7 @@ def restore(self):
         self.__disable_bracketed_paste()
         self.__maybe_write_code(self._rmkx)
         self.flushoutput()
-        try:
-            tcsetattr(self.input_fd, termios.TCSADRAIN, self.__svtermstate)
-        except termios.error as e:
-            if e.args[0] != errno.EIO:
-                raise
+        self.__input_fd_set(self.__svtermstate)
 
         if self.is_apple_terminal:
             os.write(self.output_fd, b"\033[?7h")
@@ -820,3 +821,17 @@ def __tputs(self, fmt, prog=delayprog):
                 os.write(self.output_fd, self._pad * nchars)
             else:
                 time.sleep(float(delay) / 1000.0)
+
+    def __input_fd_set(
+        self,
+        state: TermState,
+        ignore: AbstractSet[int] = _error_codes_to_ignore,
+    ) -> bool:
+        try:
+            tcsetattr(self.input_fd, termios.TCSADRAIN, state)
+        except termios.error as te:
+            if te.args[0] not in ignore:
+                raise
+            return False
+        else:
+            return True

Misc/NEWS.d/next/Core_and_Builtins/2025-09-15-14-04-56.gh-issue-134466.yR4fYW.rst

@@ -0,0 +1,2 @@
+Don't run PyREPL in a degraded environment where setting termios attributes
+is not allowed.