refactor: extract EmailksServer as library, thin main.rs to bootstrap-only

- Add EmailksServer / EmailksServerBuilder to lib.rs
- Expose serve() / serve_with_shutdown() for embedding
- main.rs now only handles etcd config and delegates to library

lib.rs

@@ -16,3 +16,156 @@ pub mod pb {
         }
     }
 }
+
+use std::future::Future;
+use std::net::SocketAddr;
+
+use crate::config::AppConfig;
+use crate::email::EmailSender;
+use crate::pb::email::v1::email_service_server::EmailServiceServer;
+use crate::queue::EmailQueue;
+use crate::server::EmailServiceImpl;
+use tonic::transport::Server as TonicServer;
+use tracing::{error, info};
+
+const DEFAULT_QUEUE_CAPACITY: usize = 1_000;
+
+/// A ready-to-run emailks gRPC server.
+///
+/// Use [`EmailksServerBuilder`] to construct an instance from configuration.
+pub struct EmailksServer {
+    service: EmailServiceImpl,
+    worker_handle: tokio::task::JoinHandle<()>,
+    addr: SocketAddr,
+}
+
+/// Builder for [`EmailksServer`].
+///
+/// # Examples
+///
+/// ```no_run
+/// use emailks::{EmailksServer, config::AppConfig};
+///
+/// # async fn example() -> Result<(), Box<dyn std::error::Error>> {
+/// let config = AppConfig::from_env()?;
+/// let server = EmailksServer::builder()
+///     .config(config)
+///     .build()
+///     .await?;
+/// server.serve().await?;
+/// # Ok(())
+/// # }
+/// ```
+pub struct EmailksServerBuilder {
+    config: Option<AppConfig>,
+}
+
+impl EmailksServer {
+    /// Create a new builder.
+    pub fn builder() -> EmailksServerBuilder {
+        EmailksServerBuilder::default()
+    }
+
+    /// Start the gRPC server and block until Ctrl+C (or SIGTERM on Unix).
+    pub async fn serve(self) -> Result<(), Box<dyn std::error::Error>> {
+        self.serve_with_shutdown(shutdown_signal()).await
+    }
+
+    /// Start the gRPC server and block until the provided `shutdown` future resolves.
+    ///
+    /// In-flight requests are drained before the server returns.
+    pub async fn serve_with_shutdown(
+        self,
+        shutdown: impl Future<Output = ()>,
+    ) -> Result<(), Box<dyn std::error::Error>> {
+        let svc = self.service;
+        let addr = self.addr;
+
+        let (health, health_svc) = tonic_health::server::health_reporter();
+        health
+            .set_serving::<EmailServiceServer<EmailServiceImpl>>()
+            .await;
+
+        info!(%addr, "gRPC server starting");
+        TonicServer::builder()
+            .add_service(health_svc)
+            .add_service(EmailServiceServer::new(svc))
+            .serve_with_shutdown(addr, shutdown)
+            .await?;
+
+        info!("server stopped");
+        if let Err(e) = self.worker_handle.await {
+            error!(error = %e, "worker task panicked");
+        }
+
+        Ok(())
+    }
+}
+
+impl EmailksServerBuilder {
+    /// Set the server configuration.
+    pub fn config(mut self, config: AppConfig) -> Self {
+        self.config = Some(config);
+        self
+    }
+
+    /// Build the server, creating the SMTP sender, queue, worker, and gRPC service.
+    pub async fn build(self) -> Result<EmailksServer, Box<dyn std::error::Error>> {
+        let config = self
+            .config
+            .unwrap_or_else(|| AppConfig::from_env().expect("failed to load emailks config"));
+
+        info!(
+            host = %config.smtp.host,
+            port = config.smtp.port,
+            "smtp config loaded"
+        );
+
+        let sender = EmailSender::new(config.smtp)?;
+        let (queue, worker) = match config.queue_capacity {
+            Some(0) => {
+                info!("creating unbounded queue");
+                EmailQueue::unbounded()
+            }
+            Some(cap) => {
+                info!(capacity = cap, "creating bounded queue");
+                EmailQueue::bounded(cap)
+            }
+            None => {
+                info!(
+                    capacity = DEFAULT_QUEUE_CAPACITY,
+                    "creating bounded queue (default)"
+                );
+                EmailQueue::bounded(DEFAULT_QUEUE_CAPACITY)
+            }
+        };
+
+        let store = queue.status_store().clone();
+        let worker_handle = worker.spawn(move |job| {
+            let s = sender.clone();
+            async move { s.send_job(&job).await }
+        });
+
+        let addr = config.listen_addr;
+        let service = EmailServiceImpl::new(queue, store);
+
+        Ok(EmailksServer {
+            service,
+            worker_handle,
+            addr,
+        })
+    }
+}
+
+impl Default for EmailksServerBuilder {
+    fn default() -> Self {
+        Self { config: None }
+    }
+}
+
+async fn shutdown_signal() {
+    match tokio::signal::ctrl_c().await {
+        Ok(()) => info!("shutdown signal received"),
+        Err(err) => error!(%err, "failed to install CTRL+C handler"),
+    }
+}

main.rs

@@ -1,12 +1,9 @@
 use emailks::{
-    config::AppConfig, email::EmailSender, etcd::{EtcdConfig, ServiceRegistry},
-    pb::email::v1::email_service_server::EmailServiceServer, queue::EmailQueue,
-    server::EmailServiceImpl,
+    config::AppConfig,
+    etcd::{EtcdConfig, ServiceRegistry},
+    EmailksServer,
 };
-use tonic::transport::Server;
-use tracing::{error, info};
-
-const DEFAULT_QUEUE_CAPACITY: usize = 1_000;
+use tracing::info;
 
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
@@ -19,7 +16,6 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     dotenvy::dotenv().ok();
 
-    // Phase 1: read etcd endpoints from env (required to bootstrap etcd)
     let etcd_endpoints: Vec<String> = std::env::var("ETCD_ENDPOINTS")
         .unwrap_or_else(|_| "http://localhost:2379".to_string())
         .split(',')
@@ -29,15 +25,12 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let etcd_prefix = std::env::var("ETCD_KEY_PREFIX")
         .unwrap_or_else(|_| "/appks/".to_string());
 
-    // Phase 2: connect etcd, create config overlay (etcd > env > default)
     let etcd = EtcdConfig::connect(etcd_endpoints, &etcd_prefix).await?;
     let listen_addr_str = etcd.get("EMAILKS_LISTEN_ADDR", "127.0.0.1:50051").await;
 
-    // Phase 3: register this service so other services (appks) can discover us
     let registry = ServiceRegistry::new(etcd.client(), &etcd_prefix);
     registry.register("emailks", &listen_addr_str).await?;
 
-    // Phase 4: load SMTP config — each key: etcd first, then env, then default
     let smtp_host = etcd.get("APP_SMTP_HOST", "").await;
     if smtp_host.is_empty() {
         return Err("APP_SMTP_HOST is required (set via etcd or env)".into());
@@ -68,54 +61,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     info!(host = %config.smtp.host, port = config.smtp.port, "smtp config loaded (etcd priority)");
 
-    let sender = EmailSender::new(config.smtp)?;
-    let (queue, worker) = match config.queue_capacity {
-        Some(0) => {
-            info!("creating unbounded queue");
-            EmailQueue::unbounded()
-        }
-        Some(cap) => {
-            info!(capacity = cap, "creating bounded queue");
-            EmailQueue::bounded(cap)
-        }
-        None => {
-            info!(capacity = DEFAULT_QUEUE_CAPACITY, "creating bounded queue (default)");
-            EmailQueue::bounded(DEFAULT_QUEUE_CAPACITY)
-        }
-    };
-
-    let store = queue.status_store().clone();
-    let worker_handle = worker.spawn(move |job| {
-        let s = sender.clone();
-        async move { s.send_job(&job).await }
-    });
-
-    let addr = config.listen_addr;
-    let svc = EmailServiceImpl::new(queue, store);
-
-    let (health, health_svc) = tonic_health::server::health_reporter();
-    health
-        .set_serving::<EmailServiceServer<EmailServiceImpl>>()
-        .await;
-
-    info!(%addr, "gRPC server starting");
-    Server::builder()
-        .add_service(health_svc)
-        .add_service(EmailServiceServer::new(svc))
-        .serve_with_shutdown(addr, shutdown_signal())
-        .await?;
-
-    info!("server stopped");
-    if let Err(e) = worker_handle.await {
-        tracing::error!(error = %e, "worker task panicked");
-    }
+    let server = EmailksServer::builder().config(config).build().await?;
+    server.serve().await?;
 
     Ok(())
 }
-
-async fn shutdown_signal() {
-    match tokio::signal::ctrl_c().await {
-        Ok(()) => info!("shutdown signal received"),
-        Err(err) => error!(%err, "failed to install CTRL+C handler"),
-    }
-}