Add request handling (#1682)

**Stack**:
- #1686
- #1685
- #1684
- #1683
- #1718
- #1682 ⬅


⚠️ *Part of a stack created by [spr](https://github.com/ejoffe/spr). Do
not merge manually using the UI - doing so may have unexpected results.*

Author: Draggu

scarb/src/ops/proc_macro_server/mod.rs

@@ -1,15 +1,65 @@
-use crate::compiler::plugin::proc_macro::ProcMacroHost;
-use anyhow::Result;
+use std::num::NonZero;
+use std::thread::available_parallelism;
+
+use anyhow::{anyhow, Result};
 use connection::Connection;
+use crossbeam_channel::{Receiver, Sender};
+use scarb_proc_macro_server_types::jsonrpc::{ResponseError, RpcRequest, RpcResponse};
+use serde_json::Value;
+
+use crate::compiler::plugin::proc_macro::ProcMacroHost;
 
 mod connection;
 
 pub fn start_proc_macro_server(_proc_macros: ProcMacroHost) -> Result<()> {
     let connection = Connection::new();
+    let available_parallelism = available_parallelism().map(NonZero::get).unwrap_or(4);
+
+    for i in 0..available_parallelism {
+        let receiver = connection.receiver.clone();
+        let sender = connection.sender.clone();
 
-    //TODO
+        std::thread::Builder::new()
+            .name(format!("proc-macro-server-worker-thread-{i}"))
+            .spawn(move || {
+                handle_requests(receiver, sender);
+            })
+            .expect("failed to spawn thread");
+    }
 
     connection.join();
 
     Ok(())
 }
+
+fn handle_requests(receiver: Receiver<RpcRequest>, sender: Sender<RpcResponse>) {
+    for request in receiver {
+        let id = request.id;
+        let response = route_request(request);
+
+        let response = match response {
+            Ok(result) => RpcResponse {
+                id,
+                result: Some(result),
+                error: None,
+            },
+            Err(error) => RpcResponse {
+                id,
+                result: None,
+                error: Some(ResponseError {
+                    message: error.to_string(),
+                }),
+            },
+        };
+
+        sender.send(response).unwrap();
+    }
+}
+
+fn route_request(request: RpcRequest) -> Result<Value> {
+    #[allow(clippy::match_single_binding)]
+    match request.method.as_str() {
+        //TODO add method handlers
+        _ => Err(anyhow!("method not found")),
+    }
+}