One less listen address race (#5718)

* One less listen address race

watch/src/server/mod.rs

@@ -31,7 +31,7 @@ pub async fn serve(config: FullConfig) -> Result<(), Error> {
             )
         })?;
 
-    let server = start_server(&config, slots_per_epoch as u64, db)?;
+    let (_addr, server) = start_server(&config, slots_per_epoch as u64, db)?;
 
     server.await?;
 
@@ -58,7 +58,13 @@ pub fn start_server(
     config: &FullConfig,
     slots_per_epoch: u64,
     pool: PgPool,
-) -> Result<impl Future<Output = Result<(), std::io::Error>> + 'static, Error> {
+) -> Result<
+    (
+        SocketAddr,
+        impl Future<Output = Result<(), std::io::Error>> + 'static,
+    ),
+    Error,
+> {
     let mut routes = Router::new()
         .route("/v1/slots", get(handler::get_slots_by_range))
         .route("/v1/slots/:slot", get(handler::get_slot))
@@ -106,11 +112,15 @@ pub fn start_server(
     let addr = SocketAddr::new(config.server.listen_addr, config.server.listen_port);
     let listener = TcpListener::bind(addr)?;
     listener.set_nonblocking(true)?;
+
+    // Read the socket address (it may be different from `addr` if listening on port 0).
+    let socket_addr = listener.local_addr()?;
+
     let serve = axum::serve(tokio::net::TcpListener::from_std(listener)?, app);
 
     info!("HTTP server listening on {}", addr);
 
-    Ok(serve.into_future())
+    Ok((socket_addr, serve.into_future()))
 }
 
 // The default route indicating that no available routes matched the request.

watch/tests/tests.rs

@@ -14,7 +14,6 @@ use rand::distributions::Alphanumeric;
 use rand::{thread_rng, Rng};
 use std::collections::HashMap;
 use std::env;
-use std::net::SocketAddr;
 use std::time::Duration;
 use testcontainers::{clients::Cli, core::WaitFor, Image, RunnableImage};
 use tokio::{runtime, task::JoinHandle};
@@ -154,7 +153,7 @@ impl TesterBuilder {
          * Create a watch configuration
          */
         let database_port = unused_tcp4_port().expect("Unable to find unused port.");
-        let server_port = unused_tcp4_port().expect("Unable to find unused port.");
+        let server_port = 0;
         let config = Config {
             database: DatabaseConfig {
                 dbname: random_dbname(),
@@ -187,14 +186,9 @@ impl TesterBuilder {
         /*
          * Spawn a Watch HTTP API.
          */
-        let watch_server = start_server(&self.config, SLOTS_PER_EPOCH, pool).unwrap();
+        let (addr, watch_server) = start_server(&self.config, SLOTS_PER_EPOCH, pool).unwrap();
         tokio::spawn(watch_server);
 
-        let addr = SocketAddr::new(
-            self.config.server.listen_addr,
-            self.config.server.listen_port,
-        );
-
         /*
          * Create a HTTP client to talk to the watch HTTP API.
          */