feat: make ctrlc available to plugins (#13181)

# Description

This PR adds a new method to `EngineInterface`: `register_ctrlc_handler`
which takes a closure to run when the plugin's driving engine receives a
ctrlc-signal. It also adds a mirror of the `signals` attribute from the
main shell `EngineState`.

This is an example of how a plugin which makes a long poll http request
can end the request on ctrlc:
https://github.com/cablehead/nu_plugin_http/blob/main/src/commands/request.rs#L68-L77

To facilitate the feature, a new attribute has been added to
`EngineState`: `ctrlc_handlers`. This is a Vec of closures that will be
run when the engine's process receives a ctrlc signal.

When plugins are added to an `engine_state` during a `merge_delta`, the
engine passes the ctrlc_handlers to the plugin's
`.configure_ctrlc_handler` method, which gives the plugin a chance to
register a handler that sends a ctrlc packet through the
`PluginInterface`, if an instance of the plugin is currently running.

On the plugin side: `EngineInterface` also has a ctrlc_handlers Vec of
closures. Plugin calls can use `register_ctrlc_handler` to register a
closure that will be called in the plugin process when the
PluginInput::Ctrlc command is received.

For future reference these are some alternate places that were
investigated for tying the ctrlc trigger to transmitting a Ctrlc packet
through the `PluginInterface`:

- Directly from `src/signals.rs`: the handler there would need a
reference to the Vec<Arc<RegisteredPlugins>>, which would require us to
wrap the plugins in a Mutex, which we don't want to do.

- have `PersistentPlugin.get_plugin` pass down the engine's
CtrlcHandlers to .get and then to .spawn (if the plugin isn't already
running). Once we have CtrlcHandlers in spawn, we can register a handler
to write directly to PluginInterface. We don't want to double down on
passing engine_state to spawn this way though, as it's unpredictable
because it would depend on whether the plugin has already been spawned
or not.

- pass `ctrlc_handlers` to PersistentPlugin::new so it can store it on
itself so it's available to spawn.

- in `PersistentPlugin.spawn`, create a handler that sends to a clone of
the GC event loop's tx. this has the same issues with regards to how to
get CtrlcHandlers to the spawn method, and is more complicated than a
handler that writes directly to PluginInterface

# User-Facing Changes

No breaking changes

---------

Co-authored-by: Ian Manske <ian.manske@pm.me>

crates/nu_plugin_example/src/commands/ctrlc.rs

@@ -0,0 +1,50 @@
+use std::sync::mpsc;
+
+use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
+use nu_protocol::{Category, LabeledError, PipelineData, Signature};
+
+use crate::ExamplePlugin;
+
+/// `example ctrlc`
+pub struct Ctrlc;
+
+impl PluginCommand for Ctrlc {
+    type Plugin = ExamplePlugin;
+
+    fn name(&self) -> &str {
+        "example ctrlc"
+    }
+
+    fn usage(&self) -> &str {
+        "Example command that demonstrates registering a ctrl-c handler"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name()).category(Category::Experimental)
+    }
+
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["example"]
+    }
+
+    fn run(
+        &self,
+        _plugin: &ExamplePlugin,
+        engine: &EngineInterface,
+        _call: &EvaluatedCall,
+        _input: PipelineData,
+    ) -> Result<PipelineData, LabeledError> {
+        let (sender, receiver) = mpsc::channel::<()>();
+        let _guard = engine.register_ctrlc_handler(Box::new(move || {
+            let _ = sender.send(());
+        }));
+
+        eprintln!("interrupt status: {:?}", engine.signals().interrupted());
+        eprintln!("waiting for ctrl-c signal...");
+        receiver.recv().expect("handler went away");
+        eprintln!("interrupt status: {:?}", engine.signals().interrupted());
+        eprintln!("peace.");
+
+        Ok(PipelineData::Empty)
+    }
+}

crates/nu_plugin_example/src/commands/mod.rs

@@ -15,12 +15,14 @@ pub use two::Two;
 // Engine interface demos
 mod call_decl;
 mod config;
+mod ctrlc;
 mod disable_gc;
 mod env;
 mod view_span;
 
 pub use call_decl::CallDecl;
 pub use config::Config;
+pub use ctrlc::Ctrlc;
 pub use disable_gc::DisableGc;
 pub use env::Env;
 pub use view_span::ViewSpan;

crates/nu_plugin_example/src/lib.rs

@@ -27,6 +27,7 @@ impl Plugin for ExamplePlugin {
             Box::new(Env),
             Box::new(ViewSpan),
             Box::new(DisableGc),
+            Box::new(Ctrlc),
             Box::new(CallDecl),
             // Stream demos
             Box::new(CollectBytes),