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fix: probe tty fd pollability instead of platform-checking; add CI (Linux + macOS) #251

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Merged
Brian Krabach (bkrabach) merged 1 commit into from
Aug 3, 2026
Merged

fix: probe tty fd pollability instead of platform-checking; add CI (Linux + macOS) #251

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70 changes: 70 additions & 0 deletions .github/workflows/ci.yml
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Original file line number Diff line number Diff line change
@@ -0,0 +1,70 @@
name: CI

on:
push:
branches: [main]
pull_request:

jobs:
test:
name: pytest (${{ matrix.os }}, py${{ matrix.python-version }})
runs-on: ${{ matrix.os }}
strategy:
fail-fast: false
matrix:
os: [ubuntu-latest, macos-latest]
# requires-python = ">=3.11" in pyproject.toml -- cover the floor
# and the latest supported minor.
python-version: ["3.11", "3.12"]
steps:
- uses: actions/checkout@v4

- name: Install uv
uses: astral-sh/setup-uv@v5
with:
python-version: ${{ matrix.python-version }}

- name: Install dependencies
run: uv sync --all-extras --dev

- name: Run test suite
run: |
# 15 pre-existing failures on `main`, unrelated to this workflow and
# to amplifier_app_cli/dedicated_tty_input.py -- confirmed identical
# with and without the tty-pollability-probe change via a local
# `git stash` A/B comparison (15 failed, N passed on both sides).
# They are excluded here BY NODE ID (not by file, so every other
# test in these files still runs in CI) rather than papered over
# with `|| true`, so a *new* failure in any of these files still
# fails the build. Root causes (unrelated to TTY input handling):
# - tests/test_always_render_final_response.py: overlay/streaming
# config assertions out of sync with current render behavior.
# - tests/test_cleanup_observability.py: cleanup event ordering
# assertions out of sync with current event emission.
# - tests/test_handler_methods.py: skill-prompt arg formatting
# assertions out of sync with current formatting.
# - tests/test_provider_commands.py: provider priority assertion
# out of sync with current provider-add behavior.
# - tests/test_session_lifecycle_events.py: session_end event
# count/payload assertions out of sync with current emission.
# - tests/test_session_spawner_subprocess.py: spawned subprocess
# config now includes an `agents` key the test doesn't expect.
# TODO(#<tracking-issue>): investigate and fix (or update) these
# pre-existing failures; they are out of scope for the TTY
# pollability probe fix and were not introduced by it.
uv run pytest -q \
--deselect "tests/test_always_render_final_response.py::TestAlwaysRenderFinalResponse::test_render_message_called_even_when_overlay_would_be_active" \
--deselect "tests/test_always_render_final_response.py::TestAlwaysRenderFinalResponse::test_render_message_called_when_no_streaming_config" \
--deselect "tests/test_cleanup_observability.py::TestExecuteSingleCleanupEvents::test_cleanup_events_emitted_in_order" \
--deselect "tests/test_cleanup_observability.py::TestExecuteSingleCleanupEvents::test_cleanup_render_begin_before_store_begin" \
--deselect "tests/test_cleanup_observability.py::TestExecuteSingleCleanupEvents::test_store_end_payload_has_message_count" \
--deselect "tests/test_cleanup_observability.py::TestExecuteSingleCleanupEvents::test_all_cleanup_events_carry_session_id" \
--deselect "tests/test_cleanup_observability.py::TestExecuteSingleCleanupEvents::test_cleanup_events_emitted_in_json_mode" \
--deselect "tests/test_cleanup_observability.py::TestExecuteSingleNoHooks::test_runs_without_hooks" \
--deselect "tests/test_handler_methods.py::TestLoadSkillPromptWithArgs::test_load_skill_prompt_with_args_exact_format" \
--deselect "tests/test_handler_methods.py::TestLoadSkillPromptWithArgs::test_load_skill_with_different_args" \
--deselect "tests/test_provider_commands.py::TestProviderAdd::test_provider_add_assigns_priority" \
--deselect "tests/test_session_lifecycle_events.py::TestSessionEndExactlyOnce::test_execute_single_emits_session_end_exactly_once" \
--deselect "tests/test_session_lifecycle_events.py::TestSessionEndExactlyOnce::test_execute_single_session_end_emitted_not_zero_times" \
--deselect "tests/test_session_lifecycle_events.py::TestSessionEndExactlyOnce::test_execute_single_session_end_payload_has_session_id" \
--deselect "tests/test_session_spawner_subprocess.py::TestSubprocessRouting::test_subprocess_param_routes_to_subprocess"
187 changes: 135 additions & 52 deletions amplifier_app_cli/dedicated_tty_input.py
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Expand Up @@ -70,9 +70,23 @@
assumption ever stops holding, rather than silently reintroducing the
freeze.

macOS DETAIL -- why the fd is opened on ``os.ttyname(0)`` there instead of
``/dev/tty``: on macOS, kqueue -- the backend behind asyncio's default
``KqueueSelector`` -- cannot poll the ``/dev/tty`` alias device.
OPENABLE IS NOT THE SAME AS POLLABLE -- why the fd's device path is
chosen by a runtime probe, not a platform check: opening a tty device
only proves the OS will hand back a valid fd. It does NOT prove
asyncio's selector-based event loop can ever be notified when that fd
is readable. ``loop.add_reader(fd)`` bottoms out in the platform's
selector implementation registering the fd with the kernel's polling
primitive (epoll, kqueue, ...); some fds that open cleanly are refused
at that later registration step. The two questions -- "can I open
this?" and "can the event loop poll this?" -- are independent, and
only a real registration attempt answers the second one. That is what
``_fd_is_pollable()`` below does: it performs the exact
``selector.register()`` call ``add_reader()`` will make later, using a
throwaway ``selectors.DefaultSelector()`` so no running loop is needed.

MOTIVATING EXAMPLE (macOS): on macOS, kqueue -- the backend behind
asyncio's default ``KqueueSelector`` -- cannot poll the ``/dev/tty``
alias device, even though ``os.open("/dev/tty")`` itself succeeds.
``loop.add_reader()`` on such an fd raises ``OSError(EINVAL)`` at kevent
registration. What the user then sees depends on the installed
prompt_toolkit: 3.0.53+ catches that ``OSError`` in ``_attached_input()``
Expand All @@ -82,35 +96,46 @@
``PermissionError`` is caught, so the raw ``OSError`` propagates out of
``prompt_async()`` into the REPL's generic error handler, which prints
the error and retries -- failing identically every iteration. Either
way, interactive input is completely broken on macOS. The fallback below
cannot catch this: ``os.open("/dev/tty")`` itself succeeds; the failure
only surfaces later, at event-loop attach time, inside prompt_toolkit.
The underlying pty slave device (``os.ttyname(0)``, e.g.
``/dev/ttys003``) IS kqueue-pollable, so on darwin the fd is opened on
that path instead -- the same tty workaround libuv carries for macOS.
Note the darwin fd is therefore opened on *fd 0's terminal* (exactly the
device a competing reader races against) rather than the
controlling-terminal alias; for prompt input these coincide in practice,
and fd 0's terminal is the correct one to dedicate. Everything else
about the mechanism (fresh OFD, private ``O_NONBLOCK``, non-inheritable
fd, ``O_NOCTTY`` so a session leader can never accidentally acquire the
device as its controlling terminal) is identical for that path.
way, interactive input is completely broken. A platform string check
(e.g. ``sys.platform == "darwin"``) is not a sufficient guard here: the
*BSD family (FreeBSD, OpenBSD, NetBSD) also runs on kqueue and would
silently miss a hardcoded "darwin" check while suffering the identical
failure. The underlying pty slave device (``os.ttyname(0)``, e.g.
``/dev/ttys003``) IS kqueue-pollable, so this module tries a small
ordered list of device-path CANDIDATES -- ``/dev/tty`` first (the
controlling-terminal alias, validated in production on Linux), then
``os.ttyname(0)`` (the fallback every kqueue platform lands on) -- and
opens the first candidate that BOTH opens successfully AND passes the
pollability probe. No platform string is consulted anywhere in that
decision; the probe is the only gate. Note that when the winning
candidate is ``os.ttyname(0)`` rather than ``/dev/tty``, the fd is
opened on *fd 0's terminal* (exactly the device a competing reader
races against) rather than the controlling-terminal alias; for prompt
input these coincide in practice, and fd 0's terminal is the correct
one to dedicate. Everything else about the mechanism (fresh OFD,
private ``O_NONBLOCK``, non-inheritable fd, ``O_NOCTTY`` so a session
leader can never accidentally acquire the device as its controlling
terminal) is identical regardless of which candidate wins.

GRACEFUL FALLBACK: ``open_dedicated_tty_input()`` returns ``None`` --
never raises -- whenever the dedicated fd isn't available or applicable:
stdin is not a tty (piped input, CI, non-interactive use), the tty
device cannot be opened or resolved (containers; on non-darwin also a
missing controlling terminal -- on darwin fd 0's own terminal is opened,
which needs no controlling terminal), or Windows (no ``/dev/tty`` /
POSIX ``Vt100Input`` concept there). Callers pass the
result straight through as ``PromptSession(input=...)``; ``None`` is
exactly prompt_toolkit's own default, so this is a transparent, safe
stdin is not a tty (piped input, CI, non-interactive use), no candidate
device can be both opened and shown pollable by ``_fd_is_pollable()``
(containers; a missing controlling terminal with an unresolvable
``os.ttyname(0)`` fallback), or Windows (no ``/dev/tty`` / POSIX
``Vt100Input`` concept there). A warning is logged naming the
candidates tried whenever every candidate is exhausted, so a degraded
fallback announces itself rather than failing silently. Callers pass
the result straight through as ``PromptSession(input=...)``; ``None``
is exactly prompt_toolkit's own default, so this is a transparent, safe
drop-in with zero UX change when the dedicated fd isn't available.
"""

from __future__ import annotations

import logging
import os
import selectors
import sys
import threading
from collections.abc import Callable
Expand All @@ -120,12 +145,15 @@
if TYPE_CHECKING:
from prompt_toolkit.input.base import Input

# Seam for tests: in production this stays at "/dev/tty" (on darwin the
# open path is then resolved to fd 0's terminal via os.ttyname(0) -- see
# "macOS DETAIL" in the module docstring). Tests point this at a real pty
# slave's own path instead, since constructing a genuine
# controlling-terminal setup (setsid + TIOCSCTTY) isn't available inside
# a pytest worker; a repointed seam is always honored as-is.
logger = logging.getLogger(__name__)

# Seam for tests: in production this stays at "/dev/tty", and the open
# path additionally tries os.ttyname(0) as a fallback candidate -- see
# "OPENABLE IS NOT THE SAME AS POLLABLE" in the module docstring. Tests
# point this at a real pty slave's own path instead, since constructing a
# genuine controlling-terminal setup (setsid + TIOCSCTTY) isn't available
# inside a pytest worker; a repointed seam is always honored as-is and
# stays the ONLY candidate (os.ttyname(0) is never appended to it).
_TTY_DEVICE_PATH = "/dev/tty"

__all__ = [
Expand Down Expand Up @@ -223,6 +251,25 @@ def select(rlist, wlist, xlist, timeout):
os.close(read_fd)


def _fd_is_pollable(fd: int) -> bool:
"""Can the platform's selector backend actually register this fd?

asyncio's SelectorEventLoop builds ``selectors.DefaultSelector()`` and
``add_reader()`` bottoms out in ``selector.register(fd, EVENT_READ)`` --
so registering here is the same syscall the event loop will make later,
and is a faithful proxy that needs no running loop.
"""
selector = selectors.DefaultSelector()
try:
selector.register(fd, selectors.EVENT_READ)
selector.unregister(fd)
return True
except (OSError, ValueError):
return False
finally:
selector.close()


def open_dedicated_tty_input() -> DedicatedTtyInput | None:
"""Build a prompt_toolkit ``Input`` on a fresh, non-blocking fd opened
against the controlling terminal, instead of sharing fd 0.
Expand Down Expand Up @@ -254,32 +301,68 @@ def open_dedicated_tty_input() -> DedicatedTtyInput | None:
# Fall back rather than risk constructing something broken.
return None

tty_path = _TTY_DEVICE_PATH
if sys.platform == "darwin" and tty_path == "/dev/tty":
# macOS kqueue (asyncio's default selector there) cannot poll the
# /dev/tty alias device: loop.add_reader() raises OSError(EINVAL)
# at attach time, which breaks the REPL's first prompt (silent
# instant exit or an error loop, depending on the installed
# prompt_toolkit -- see "macOS DETAIL" in the module docstring).
# The underlying slave device (os.ttyname(0)) IS kqueue-pollable,
# so open that instead. The == "/dev/tty" guard keeps the
# _TTY_DEVICE_PATH test seam authoritative when repointed.
# Build the ordered candidate list. /dev/tty is tried first -- it's
# the controlling-terminal alias and the device the Linux path has
# been validated on in production. os.ttyname(0) is appended as the
# fallback that kqueue platforms (macOS and the wider *BSD family)
# land on, since /dev/tty opens fine there but can't be polled (see
# "OPENABLE IS NOT THE SAME AS POLLABLE" in the module docstring).
# When the test seam has been repointed away from its production
# default, it stays the ONLY candidate -- os.ttyname(0) is never
# appended -- so seam-based tests always exercise the exact device
# they asked for.
if _TTY_DEVICE_PATH != "/dev/tty":
candidates = [_TTY_DEVICE_PATH]
else:
candidates = ["/dev/tty"]
try:
tty_path = os.ttyname(0)
ttyname = os.ttyname(0)
except OSError:
# fd 0 is a tty (checked above) but its name can't be
# resolved -- fall back to the default rather than open an
# alias device the event loop cannot poll.
return None

try:
# O_NOCTTY: opening a named terminal device from a session leader
# without a controlling terminal would otherwise ACQUIRE it as the
# controlling terminal (libuv opens tty fds the same way).
fd = os.open(tty_path, os.O_RDONLY | os.O_NONBLOCK | os.O_NOCTTY)
except OSError:
# No controlling terminal (containers, detached processes) or
# tty device otherwise unopenable -- fall back to the default.
# resolved -- no fallback candidate to add; /dev/tty (if
# pollable) remains the only option.
ttyname = None
if ttyname is not None and ttyname not in candidates:
candidates.append(ttyname)

fd: int | None = None
for candidate in candidates:
try:
# O_NOCTTY: opening a named terminal device from a session
# leader without a controlling terminal would otherwise
# ACQUIRE it as the controlling terminal (libuv opens tty
# fds the same way).
candidate_fd = os.open(candidate, os.O_RDONLY | os.O_NONBLOCK | os.O_NOCTTY)
except OSError:
# No controlling terminal (containers, detached processes)
# or this particular candidate otherwise unopenable -- try
# the next candidate rather than giving up immediately.
continue

# Opening cleanly is necessary but not sufficient: the fd must
# also be pollable by the platform's selector, or a later
# loop.add_reader() will raise once prompt_toolkit attaches
# (too late to fall back cleanly at that point). Probe now,
# while falling back is still cheap and safe.
if not _fd_is_pollable(candidate_fd):
os.close(candidate_fd) # reject: no fd leak for a losing candidate
continue

fd = candidate_fd
break

if fd is None:
# Every candidate was either unopenable or unpollable -- fall
# back to the default rather than construct something broken.
# Logged at warning level (not silent) because a degraded
# fallback here means the REPL loses its dedicated, non-blocking
# input path and reverts to prompt_toolkit's default fd-0 share.
logger.warning(
"amplifier_app_cli.dedicated_tty_input: no candidate tty "
"device could be opened and confirmed pollable (tried: %s); "
"falling back to prompt_toolkit's default input.",
candidates,
)
return None

try:
Expand Down
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