review: loopd prints every startup notice before it binds; tests wait for "serving on"

The serve tests waited for the socket file, then killed `loopd` and read its stderr, but the
socket is bound before its mode is set and before "serving on" and the broker notice are printed,
so a kill in between lost those lines (`pointers` failed once under load in M5c-1 task 15). The
broker notice now comes before the bind, so "serving on" is the last startup line, and the tests
wait for it (`tests/support/serving.rs`).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/loopd/tests/pointers.rs

@@ -2,6 +2,8 @@
 //! `scripts/check-runbook.sh` checks that the entries exist; these tests check that the messages
 //! name them. Do not edit.
 
+#[path = "support/serving.rs"]
+mod serving;
 mod support;
 
 use std::path::Path;
@@ -162,20 +164,10 @@ fn healthy_routes(server: &FakeServer) {
     );
 }
 
-/// Waits for `loopd serve` to bind its socket, stops it, and returns what it printed.
-fn stderr_once_serving(mut child: Child, socket: &Path) -> String {
-    let mut up = false;
-    for _ in 0..200 {
-        if socket.exists() {
-            up = true;
-            break;
-        }
-        std::thread::sleep(Duration::from_millis(50));
-    }
-    child.kill().unwrap();
-    let output = child.wait_with_output().unwrap();
-    let stderr = String::from_utf8_lossy(&output.stderr).to_string();
-    assert!(up, "the socket never appeared; stderr: {stderr}");
+/// Waits for `loopd serve` to print "serving on", stops it, and returns what it printed.
+fn stderr_once_serving(child: Child) -> String {
+    let (up, stderr) = serving::stderr_once_serving(child);
+    assert!(up, "loopd never said it was serving; stderr: {stderr}");
     stderr
 }
 
@@ -218,8 +210,7 @@ fn serve_without_a_broker_says_so_once_with_the_entry() {
     let server = FakeServer::start();
     healthy_routes(&server);
     let config = config_file(&home, &server, 2, "");
-    let socket = home.dir.join("run/loop/loop.sock");
-    let stderr = stderr_once_serving(serve(&config), &socket);
+    let stderr = stderr_once_serving(serve(&config));
     let lines: Vec<&str> = stderr
         .lines()
         .filter(|l| l.contains("no tool broker is configured"))
@@ -240,8 +231,7 @@ fn serve_with_a_broker_socket_does_not_say_it() {
         home.dir.join("run/loop-broker/broker.sock").display()
     );
     let config = config_file(&home, &server, 2, &extra);
-    let socket = home.dir.join("run/loop/loop.sock");
-    let stderr = stderr_once_serving(serve(&config), &socket);
+    let stderr = stderr_once_serving(serve(&config));
     assert!(!stderr.contains("no tool broker"), "{stderr}");
     assert!(stderr.contains("serving on"), "{stderr}");
 }

crates/loopd/tests/serve.rs

@@ -1,10 +1,11 @@
 //! Tests for the `loopd serve` command. Do not edit.
 
+#[path = "support/serving.rs"]
+mod serving;
 mod support;
 
 use std::os::unix::fs::PermissionsExt;
 use std::process::{Command, Stdio};
-use std::time::Duration;
 use support::{FakeServer, Home, Reply, tokenizer};
 
 fn config_file(home: &Home, server: &FakeServer, expect_slots: u32) -> std::path::PathBuf {
@@ -56,25 +57,16 @@ fn serve_runs_the_self_test_then_binds_the_socket_with_mode_0600() {
     healthy_routes(&server);
     let config = config_file(&home, &server, 2);
     let socket = home.dir.join("run").join("loop").join("loop.sock");
-    let mut child = Command::new(env!("CARGO_BIN_EXE_loopd"))
+    let child = Command::new(env!("CARGO_BIN_EXE_loopd"))
         .args(["serve", "--config"])
         .arg(&config)
         .stderr(Stdio::piped())
         .spawn()
         .unwrap();
-    let mut up = false;
-    for _ in 0..200 {
-        if socket.exists() {
-            up = true;
-            break;
-        }
-        std::thread::sleep(Duration::from_millis(50));
-    }
+    // "serving on" comes after the mode is set; the socket stays after loopd is killed.
+    let (up, stderr) = serving::stderr_once_serving(child);
+    assert!(up, "loopd never said it was serving; stderr: {stderr}");
     let mode = std::fs::metadata(&socket).map(|m| m.permissions().mode() & 0o777);
-    let _ = child.kill();
-    let output = child.wait_with_output().unwrap();
-    let stderr = String::from_utf8_lossy(&output.stderr);
-    assert!(up, "the socket never appeared; stderr: {stderr}");
     assert_eq!(mode.unwrap(), 0o600, "{stderr}");
     assert!(stderr.contains("selftest: ok"), "{stderr}");
     assert!(stderr.contains("serving on"), "{stderr}");

crates/loopd/tests/support/serving.rs

@@ -0,0 +1,63 @@
+//! Waiting for `loopd serve` to be up, by the line it prints last before it serves.
+//!
+//! The socket appears before `loopd` has set its mode and printed "serving on", so a test that
+//! waits for the socket and then stops `loopd` can lose those lines (M5c-1 review, finding 7).
+//! Every startup notice comes before "serving on", so once it is read, the rest has been printed.
+
+use std::io::{BufRead, BufReader, Read};
+use std::process::Child;
+use std::sync::mpsc;
+use std::time::{Duration, Instant};
+
+/// How long `loopd` may take to pass its self-test and bind, on a loaded machine.
+const UP: Duration = Duration::from_secs(60);
+
+/// Reads `child`'s stderr until the "serving on" line, stops it, and returns whether that line came
+/// and everything it printed. `child` must have been spawned with `stderr(Stdio::piped())`.
+pub fn stderr_once_serving(mut child: Child) -> (bool, String) {
+    let stderr = child.stderr.take().expect("stderr is piped");
+    let (tx, rx) = mpsc::channel::<String>();
+    let reader = std::thread::spawn(move || {
+        let mut lines = BufReader::new(stderr);
+        let mut line = String::new();
+        loop {
+            line.clear();
+            match lines.read_line(&mut line) {
+                Ok(0) | Err(_) => break,
+                Ok(_) => {
+                    if tx.send(line.clone()).is_err() {
+                        break;
+                    }
+                }
+            }
+        }
+        let mut rest = String::new();
+        let _ = lines.read_to_string(&mut rest);
+        rest
+    });
+    let deadline = Instant::now() + UP;
+    let mut text = String::new();
+    let mut up = false;
+    while let Some(left) = deadline.checked_duration_since(Instant::now()) {
+        match rx.recv_timeout(left) {
+            Ok(line) => {
+                up = line.starts_with("loopd: serving on ");
+                text.push_str(&line);
+                if up {
+                    break;
+                }
+            }
+            Err(_) => break,
+        }
+    }
+    let _ = child.kill();
+    let _ = child.wait();
+    // The reader ends at the pipe's end once `loopd` is gone; join it before draining, so no line
+    // it sends after the drain is lost.
+    let rest = reader.join().unwrap_or_default();
+    while let Ok(line) = rx.try_recv() {
+        text.push_str(&line);
+    }
+    text.push_str(&rest);
+    (up, text)
+}