…instances

Python 3.12+ issues a DeprecationWarning if `os.fork()` is called while
there are multiple threads, and in fact this is not strictly safe on any
version of Python, although on Linux with glibc it mostly works well
enough that we don't notice. The man page for `fork()` says:

> After a fork() in a multithreaded program, the child can safely call
only async-signal-safe functions (see signal-safety(7)) until such time
as it calls execve(2).

This is incompatible with executing any Python code, hence the warning.
IPython is always multi-threaded because of HistorySavingThread, so it's
never safe to call `os.fork()` or use multiprocessing's fork context.
However, I think we can easily resolve this by stopping the history
saving thread before fork, and starting it again after, so at the moment
of fork it's a single-threaded process. In principle, I think it's safe
to start it again in the `after_in_parent` handler, but Python's check
for threads to issue the warning runs after that callback, so to avoid
triggering the warning, it's better to start it again when there's
something for it to do.

This unfortunately only solves the issue for IPython in the terminal,
since IPykernel uses several more threads. But fixing it in the terminal
is already useful, and this would also be one small step towards fixing
it in IPykernel.

-----

Testing:

```python
import os
import time

def func_with_fork():
    if (pid := os.fork()) > 0:
        print("Parent")
        t = os.waitpid(pid, 0)
        print("Parent: child finished", t)
    else:
        print("Child")
        time.sleep(0.5)
        print("Child finishing")
        os._exit(0)

if __name__ == "__main__":
    func_with_fork()
```

```shell
PYTHONWARNINGS=default ipython
```

# Fix doctest paste: strip prompts and dedent (#15089)

## Summary

This PR improves how IPython handles pasting doctest and xdoctest code
snippets. When pasting code like:

```
>>>     print(1)
```

IPython now strips the `>>>` prompt and also removes the extra
indentation, so the result is:

```
print(1)
```

Previously, the result would have been:

```
    print(1)
```

which can cause an `IndentationError` at top-level.

## What changed
- `inputtransformer2.py`
    - Enhanced PromptStripper to:
        - Detect doctest-style pastes when any line matches `^\s*>>>`.
- In doctest-mode, strip `^\s*>>>\s?` and `^\s*\.\.\.\s?`
(continuation).
- After stripping, apply `textwrap.dedent()` to the joined block, then
return dedented lines.
- Only apply dedent when doctest prompts were actually stripped (to
avoid changing non-doctest pastes).
- Only treat `...` as a continuation prompt when a `>>>` is present in
the same block (avoids confusing Python Ellipsis with doctest
continuation).
- `test_inputtransformer2_line.py`
- Added/updated tests covering single-line doctest, indented `>>>`,
multi-line `>>>` + `...` doctest, and standalone ... (unchanged when no
`>>>`).


## Description of changes

- Update `IPython/core/inputtransformer2.py` to enhance the
`PromptStripper`:
- Detect doctest-mode for a pasted block when any line matches
`^\s*>>>`.
  - In doctest-mode, strip indented doctest prompts:
    - `^\s*>>>\s?` (primary prompt)
    - `^\s*\.\.\.\s?` (continuation prompt)
- After stripping doctest prompts, apply `textwrap.dedent()` to the
joined block and return the dedented lines.
- Only apply `dedent()` when doctest prompts were actually stripped to
avoid changing normal paste semantics.
- Update/add tests in `tests/test_inputtransformer2_line.py` for the new
behavior.


## Motivation and context

Users often copy doctest examples from documentation or tools that show
doctest prompts and indentation (for example, xdoctest-style indented
examples). Previously IPython removed the `>>>` prompt but preserved the
remaining indentation, which could turn a pasted, single-line doctest
into an indented top-level statement and raise `IndentationError`. This
PR makes doctest pastes behave as users expect: prompts are removed and
the code block is normalized by dedenting.


## Examples (Before → After)

### Single-line doctest

Input:

```py
>>>     print(1)
```

Before:

```py
    print(1)
```

After:

```py
print(1)
```


### Indented xdoctest-style prompt

Input:

```py
    >>>     print(1)
```

After:

```py
print(1)
```


### Multi-line doctest with continuation

Input:

```py
>>> for i in range(2):
...     print(i)
```

After:

```py
for i in range(2):
    print(i)
```


### Standalone continuation prompt (unchanged)

Input:

```py
...     print(1)
```

After (unchanged because no `>>>` present):

```py
...     print(1)
```

## Implementation notes

- `dedent()` is applied only after doctest prompts were detected and
stripped. This avoids affecting normal paste behavior for non-doctest
code, and prevents accidental removal of meaningful indentation in
user-pasted code.
- `...` is considered a continuation prompt only when the pasted block
contains at least one `>>>`, to avoid confusing Python's `Ellipsis`
literal with doctest continuations.
- Regexes used are tolerant of leading whitespace to support indented
`>>>` prompts (xdoctest-style).


## Related issues

- Fixes #15089


## Testing performed

- Added and updated tests in `tests/test_inputtransformer2_line.py` to
cover:
- Single-line doctest with extra indentation: `">>> print(1)\n" ->
"print(1)\n"`
- Leading whitespace before `>>>`: `" >>> print(1)\n" -> "print(1)\n"`
  - Multiline doctest using `>>>` and `...` continuation lines.
  - Standalone `...` only lines remain unchanged.

## Doc

Contribution by Gittensor, see my contribution statistics at
https://gittensor.io/miners/details?githubId=191372963

See issue #15116

See issue #15116