_pyrepl: Reader.suspend() discards buffer, pos, and kill_ring

[Tony123-tech]

Bug description:

Reader.suspend() captures all 29 fields of the reader, but only restores
6 of them (msg, ps1–ps4, paste_mode). The self.prepare() call in
the finally block clears self.buffer, resets self.pos, and clears
self.kill_ring, and those are not restored.

Reproducer:

from _pyrepl.reader import Reader

class FakeConsole:
    height = 24
    width = 80
    input_hook = None
    def prepare(self): pass
    def restore(self): pass
    def refresh(self, screen): pass
    def move_cursor(self, x, y): pass
    def finish(self): pass
    def wait(self, timeout): return False
    def get_event(self, block=False): return None
    def push_char(self, c): pass
    def getheightwidth(self): return (self.height, self.width)

r = Reader(FakeConsole())
r.buffer[:] = list("hello world")
r.pos = 5
r.kill_ring.append(list("saved"))

before = (list(r.buffer), r.pos, [list(k) for k in r.kill_ring])
with r.suspend():
    pass
after = (list(r.buffer), r.pos, [list(k) for k in r.kill_ring])

print(f"before: {before}")
print(f"after:  {after}")

Expected:
suspend() should be transparent: after the context manager exits, the
reader should be in the same state as before, apart from the fields that
the nested reader is allowed to change.

Actual:
The buffer is empty, pos is 0, and kill_ring is cleared.

Reachability:
Reader.suspend() is called by help (F1) in Lib/_pyrepl/commands.py:

python
class help(Command):
    def do(self) -> None:
        import _sitebuiltins
        with self.reader.suspend():
            self.reader.msg = _sitebuiltins._Helper()()
        self.reader.invalidate_prompt()
and wrapped by HistoricalReader.suspend() in
Lib/_pyrepl/historical_reader.py:

python
@contextmanager
def suspend(self) -> SimpleContextManager:
    with super().suspend(), self.suspend_history():
        yield

A user who types something and then presses F1 loses their input.

CPython versions tested on:
CPython main branch

Operating systems tested on:
macOS

Linked PRs

• gh-158273: _pyrepl: preserve reader state across suspend() #158274

[StanFromIreland]

@Tony123-tech, can this be hit in practice? Please share real REPL steps to reproduce.

[Tony123-tech]

Yes, it can be hit in practice. Real REPL steps:

1. Start the REPL: ./python.exe
2. Type some text, e.g. 'hello world' (do not press Enter)
3. Press F1 (fn+F1 on macOS) to open the help viewer
4. Type 'q' to close the help viewer
5. Observe that the buffer is empty instead of still containing 'hello world'

The F1 key is bound to the 'help' command in default_keymap, which calls Reader.suspend().

[picnixz]

It may be useful if you are currently writing code and don't remember a method for instance (and thus enter F1 mode to look at it) and then continue.

So I'm neutral on this change. I'm just unusure about whether it's what users expect in general (F1 is like writing help but "reader suspension" rings like "I'm going to restore the state afterwards" so it's not necessarily wrong).

cc @pablogsal

[StanFromIreland]

Also, in the future please prefer real reproducers (like the one above) rather than contrived synthetic examples.