Implement send_signal for unix child processes

[Qelxiros]

Tracking issue: #141975

There are two main differences between my implementation and the Public API section of the tracking issue. First, send_signal requires a mutable reference, like Child::kill. Second, ChildExt has Sealed as a supertrait, bringing it more in line with other extension traits like CommandExt.

[rustbot]

r? @ChrisDenton

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[ChrisDenton]

cc @tgross35 as this modifies the proposed API slightly

- fn send_signal(&self, signal: i32) -> Result<()>;
+ fn send_signal(&mut self, signal: i32) -> Result<()>;

[tgross35]

I didn't put too much thought into what I wrote there, so it's fine if it needs to change. That being said, why does this need to be &mut? kill should be MT-Safe https://www.gnu.org/software/libc/manual/2.28/html_node/Signaling-Another-Process.html

@Qelxiros could you add documentation and examples here?

[Qelxiros]

You're right that it doesn't need to be mutable, so I changed it back. I do wonder why e.g. Child::kill takes a mutable reference. Is it required on windows? If so, is it worth adding a kill-specific method to the unix ChildExt?

[ChrisDenton]

No it's not to do with Windows. It seems to have been a deliberate decision in the early days of rust, though I can't find any record of the thinking behind it. It was implemented in #22119 and all the implementations use &self behind the scenes, only the public interface used &mut self.

[Qelxiros]

@tgross35 could you add an unresolved question to the tracking issue regarding & vs &mut? I want to make sure it doesn't get forgotten before stabilization.

[tgross35]

@tgross35 could you add an unresolved question to the tracking issue regarding & vs &mut? I want to make sure it doesn't get forgotten before stabilization.

Added.

I think this implementation looks fine, just needs a squash (I'll let Chris do the final review)

[Qelxiros]

@rustbot ready

[ChrisDenton]

Thanks!

@bors r+

[bors]

📌 Commit 269ee72 has been approved by ChrisDenton

It is now in the queue for this repository.

[tgross35]

@bors r-

Failed the rollup at #142558 (comment). You'll need to make sure that all OSs in https://github.com/rust-lang/rust/tree/269ee72c330c21a9e0526e69fc0f85d762043d2d/library/std/src/sys/process/unix / https://github.com/rust-lang/rust/tree/68ac5abb067806a88464ddbfbd3c7eec877b488d/library/std/src/sys/process/unix have an implementation.