modules: [modules] config + example service crate + SDK guide

A [modules] block selects which services start (default: NickServ + ChanServ);
main.rs constructs each compiled-in module only when named, so a node can run a
subset and optional modules ship dormant. Add fedserv-example, a minimal
read-only Service that depends on fedserv-api alone -- the template a new
pseudo-client is copied from, enabled by adding "example" to the config.
MODULES.md documents writing and registering a service or protocol module.

src/config.rs

@@ -16,6 +16,28 @@ pub struct Config {
     // directory. Absent = the RPC server does not start.
     #[serde(default)]
     pub grpc: Option<Grpc>,
+    // Which service modules to start. Absent = the built-in NickServ + ChanServ.
+    #[serde(default)]
+    pub modules: Modules,
+}
+
+// The service modules to bring up at burst. Each name maps to a compiled-in
+// module crate the daemon knows how to construct (see main.rs). Names not built
+// in are ignored.
+#[derive(Debug, Deserialize)]
+pub struct Modules {
+    #[serde(default = "default_services")]
+    pub services: Vec<String>,
+}
+
+impl Default for Modules {
+    fn default() -> Self {
+        Modules { services: default_services() }
+    }
+}
+
+fn default_services() -> Vec<String> {
+    vec!["nickserv".to_string(), "chanserv".to_string()]
 }
 
 #[derive(Debug, Deserialize, Clone)]

src/main.rs

@@ -15,6 +15,7 @@ use tokio::sync::Mutex;
 
 use engine::Engine;
 use fedserv_chanserv::ChanServ;
+use fedserv_example::ExampleServ;
 use fedserv_inspircd::InspIrcd;
 use fedserv_nickserv::NickServ;
 
@@ -39,16 +40,28 @@ async fn main() -> Result<()> {
         ts,
     ));
 
-    let services: Vec<Box<dyn engine::service::Service>> = vec![
-        Box::new(NickServ {
+    // Bring up the service modules named in [modules] (default NickServ +
+    // ChanServ). Each keeps a fixed uid suffix so its identity is stable no
+    // matter which others are enabled.
+    let enabled = |name: &str| cfg.modules.services.iter().any(|s| s == name);
+    let mut services: Vec<Box<dyn engine::service::Service>> = Vec::new();
+    if enabled("nickserv") {
+        services.push(Box::new(NickServ {
             uid: format!("{}AAAAAA", cfg.server.sid),
             guest_nick: cfg.server.guest_nick.clone(),
             guest_seq: (ts % 100_000) as u32,
-        }),
-        Box::new(ChanServ {
+        }));
+    }
+    if enabled("chanserv") {
+        services.push(Box::new(ChanServ {
             uid: format!("{}AAAAAB", cfg.server.sid),
-        }),
-    ];
+        }));
+    }
+    if enabled("example") {
+        services.push(Box::new(ExampleServ {
+            uid: format!("{}AAAAAC", cfg.server.sid),
+        }));
+    }
     let (gossip_tx, _) = tokio::sync::broadcast::channel::<engine::db::LogEntry>(1024);
     let mut db = engine::db::Db::open("fedserv.db.jsonl", &cfg.server.sid);
     db.scram_iterations = cfg.server.scram_iterations;