brokerd, memoryd, gatewayd: a lock file keeps each to one instance

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/brokerd/src/main.rs

@@ -11,6 +11,7 @@ use brokerd::container::Podman;
 use brokerd::memory_port::{MEMORY_WAIT, MemoryPort, Routed};
 use brokerd::runner::{Refusing, Runtime};
 use brokerd::serve::{self, ServeError};
+use bxcore::instance;
 
 const USAGE: &str = "usage: brokerd serve --config <path> [--accept-break]";
 /// Where the owner looks when brokerd cannot start: its config, a directory or a socket.
@@ -42,6 +43,18 @@ fn main() -> ExitCode {
         }
     };
 
+    // One brokerd per home (M5b spec, section 9a). The lock comes before anything is made, the
+    // audit log's own lock included, so a second copy never takes the sockets of the one that
+    // runs. Held until the process ends.
+    let lock_path = cfg.paths.home.join("run/brokerd.lock");
+    let _instance = match instance::lock(&lock_path) {
+        Ok(held) => held,
+        Err(e) => {
+            eprintln!("{}", e.line("brokerd", &lock_path));
+            return ExitCode::from(1);
+        }
+    };
+
     // Keep the socket paths for the serving notice, before `cfg` is moved into `start`.
     let broker_path = cfg.broker_socket();
     let admin_path = cfg.admin_socket();