Improve SIGINT handling in uv run

[zanieb]

There should be two functional changes here:

• If we receive SIGINT twice, forward it to the child process
• If the uv run child process changes its PGID, then forward SIGINT

Previously, we never forwarded SIGINT to a child process. Instead, we
relied on shell to do so.

On Windows, we still do nothing but eat the Ctrl-C events we receive.
I cannot see an easy way to send them to the child.

The motivation for these changes should be explained in the comments.

Closes #10952 (in which Ray changes its PGID)
Replaces the (much simpler) #10989 with a more comprehensive approach.

See #6738 (comment) for some previous context.

[zanieb]

Frankly, I'm still not entirely sure why we wouldn't forward all signals in the same way that we do with SIGTERM. If it wasn't a huge pain, I would probably do it today? But since it is I might just wait until someone complains that something isn't working?

[zanieb]

A downside of forwarding Ctrl-C on the second send is that a process can receive it three times... but I don't know of programs that have special-casing for that.

[zanieb]

The first SIGINT is not forwarded (but is received because the shell sends it directly). The second SIGINT is forwarded

import signal, os; 

print(f'PID: {os.getpid()}');
print(f'GPID: {os.getpgid(os.getpid())}');

# We display a newline before SIGINT to separate from ^C in shells
signal.signal(signal.SIGINT, lambda s, _: print(f'{signal.Signals(s).name}', flush=True))
signal.signal(signal.SIGTERM, lambda s, _: print(f'{signal.Signals(s).name}', flush=True))

signal.valid_signals

# Hang until we receive an input, e.g., Ctrl-D
try:
    input()
except EOFError:
    pass

❯ uv run -v example.py
DEBUG uv 0.5.24+8 (adf534009 2025-01-27)
DEBUG Found project root: `/Users/zb/workspace/uv`
DEBUG Project `uv` is marked as unmanaged
DEBUG No project found; searching for Python interpreter
DEBUG Reading Python requests from version file at `/Users/zb/workspace/uv/.python-version`
DEBUG Searching for Python 3.12 in virtual environments, managed installations, or search path
DEBUG Found `cpython-3.12.8-macos-aarch64-none` at `/Users/zb/workspace/uv/.venv/bin/python3` (virtual environment)
DEBUG Using Python 3.12.8 interpreter at: /Users/zb/workspace/uv/.venv/bin/python3
DEBUG Running `python example.py`
PID: 84848
GPID: 84847
^CSIGINT
DEBUG Received SIGINT, assuming the child received it as part of the process group
^CSIGINT
DEBUG Received SIGINT, forwarding to child at 84848
SIGINT

SIGTERM is forwarded immediately (and only received once)

❯ uv run -v example.py
DEBUG uv 0.5.24+8 (adf534009 2025-01-27)
DEBUG Found project root: `/Users/zb/workspace/uv`
DEBUG Project `uv` is marked as unmanaged
DEBUG No project found; searching for Python interpreter
DEBUG Reading Python requests from version file at `/Users/zb/workspace/uv/.python-version`
DEBUG Searching for Python 3.12 in virtual environments, managed installations, or search path
DEBUG Found `cpython-3.12.8-macos-aarch64-none` at `/Users/zb/workspace/uv/.venv/bin/python3` (virtual environment)
DEBUG Using Python 3.12.8 interpreter at: /Users/zb/workspace/uv/.venv/bin/python3
DEBUG Running `python example.py`
PID: 85335
GPID: 85334
DEBUG Received SIGTERM, forwarding to child at 85335
SIGTERM

When the PGID of the child is changed, we forward the first SIGINT

import signal, os; 

print(f'PID: {os.getpid()}');

# Change PGID
os.setpgid(0, 0)

print(f'GPID: {os.getpgid(os.getpid())}');

signal.signal(signal.SIGINT, lambda s, _: print(f'{signal.Signals(s).name}', flush=True))
signal.signal(signal.SIGTERM, lambda s, _: print(f'{signal.Signals(s).name}', flush=True))

# Sleep for 20s; we can't use `input()` or it breaks output for some reason
try:
    import time
    time.sleep(20)
except EOFError:
    pass

❯ uv run -v example.py
DEBUG uv 0.5.24+8 (adf534009 2025-01-27)
DEBUG Found project root: `/Users/zb/workspace/uv`
DEBUG Project `uv` is marked as unmanaged
DEBUG No project found; searching for Python interpreter
DEBUG Reading Python requests from version file at `/Users/zb/workspace/uv/.python-version`
DEBUG Searching for Python 3.12 in virtual environments, managed installations, or search path
DEBUG Found `cpython-3.12.8-macos-aarch64-none` at `/Users/zb/workspace/uv/.venv/bin/python3` (virtual environment)
DEBUG Using Python 3.12.8 interpreter at: /Users/zb/workspace/uv/.venv/bin/python3
DEBUG Running `python example.py`
PID: 84317
GPID: 84317
^CDEBUG Received SIGINT, forwarding to child at 84317
SIGINT

[zanieb]

Test coverage pretty painful to add here.

[BurntSushi]

Nice comment. I buy this. I'm unsure about Windows though.

[BurntSushi]

Ah now I get it. Thanks for explaining this here!

[BurntSushi]

Does it make sense to log an error here if one occurs?

[zanieb]

Probably yeah. This is the existing behavior so I prefer to retain but we could add logging in a separate patch.

[BurntSushi]

Does the signal actually get sent to the child? Or maybe my general question here is, will ^C still work to terminate the child?

[zanieb]

Yeah so this should retain our existing behavior — it's just moved from a platform-wide block to a Windows-specific block. The note is just that we can't implement the more complicated behavior we do above for Unix.

[zanieb]

I also had Aria test this assumption in PowerShell and the initial change adding the Ctrl-C handling fixed various bugs.

[BurntSushi]

Ah nice! On the ball.