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/src/plugin/interface/mod.rs

@@ -1,7 +1,7 @@
 //! Interface used by the plugin to communicate with the engine.
 
 use nu_plugin_core::{
-    util::{Sequence, Waitable, WaitableMut},
+    util::{Waitable, WaitableMut},
     Interface, InterfaceManager, PipelineDataWriter, PluginRead, PluginWrite, StreamManager,
     StreamManagerHandle,
 };
@@ -11,13 +11,14 @@ use nu_plugin_protocol::{
     PluginOutput, ProtocolInfo,
 };
 use nu_protocol::{
-    engine::Closure, Config, DeclId, LabeledError, PipelineData, PluginMetadata, PluginSignature,
-    ShellError, Signals, Span, Spanned, Value,
+    engine::{ctrlc, Closure, Sequence},
+    Config, DeclId, LabeledError, PipelineData, PluginMetadata, PluginSignature, ShellError,
+    Signals, Span, Spanned, Value,
 };
 use nu_utils::SharedCow;
 use std::{
     collections::{btree_map, BTreeMap, HashMap},
-    sync::{mpsc, Arc},
+    sync::{atomic::AtomicBool, mpsc, Arc},
 };
 
 /// Plugin calls that are received by the [`EngineInterfaceManager`] for handling.
@@ -63,6 +64,10 @@ struct EngineInterfaceState {
         mpsc::Sender<(EngineCallId, mpsc::Sender<EngineCallResponse<PipelineData>>)>,
     /// The synchronized output writer
     writer: Box<dyn PluginWrite<PluginOutput>>,
+    // Mirror signals from `EngineState`
+    signals: Signals,
+    /// Registered Ctrl-C handlers
+    ctrlc_handlers: ctrlc::Handlers,
 }
 
 impl std::fmt::Debug for EngineInterfaceState {
@@ -116,6 +121,8 @@ impl EngineInterfaceManager {
                 stream_id_sequence: Sequence::default(),
                 engine_call_subscription_sender: subscription_tx,
                 writer: Box::new(writer),
+                signals: Signals::new(Arc::new(AtomicBool::new(false))),
+                ctrlc_handlers: ctrlc::Handlers::new(),
             }),
             protocol_info_mut,
             plugin_call_sender: Some(plug_tx),
@@ -235,7 +242,6 @@ impl InterfaceManager for EngineInterfaceManager {
 
     fn consume(&mut self, input: Self::Input) -> Result<(), ShellError> {
         log::trace!("from engine: {:?}", input);
-
         match input {
             PluginInput::Hello(info) => {
                 let info = Arc::new(info);
@@ -331,6 +337,11 @@ impl InterfaceManager for EngineInterfaceManager {
                     });
                 self.send_engine_call_response(id, response)
             }
+            PluginInput::Ctrlc => {
+                self.state.signals.trigger();
+                self.state.ctrlc_handlers.run();
+                Ok(())
+            }
         }
     }
 
@@ -510,6 +521,15 @@ impl EngineInterface {
         self.state.writer.is_stdout()
     }
 
+    /// Register a closure which will be called when the engine receives a Ctrl-C signal. Returns a
+    /// RAII guard that will keep the closure alive until it is dropped.
+    pub fn register_ctrlc_handler(
+        &self,
+        handler: ctrlc::Handler,
+    ) -> Result<ctrlc::Guard, ShellError> {
+        self.state.ctrlc_handlers.register(handler)
+    }
+
     /// Get the full shell configuration from the engine. As this is quite a large object, it is
     /// provided on request only.
     ///
@@ -959,6 +979,10 @@ impl EngineInterface {
         self.write(PluginOutput::CallResponse(self.context()?, response))?;
         self.flush()
     }
+
+    pub fn signals(&self) -> &Signals {
+        &self.state.signals
+    }
 }
 
 impl Interface for EngineInterface {