Address process booting determinism

The method we're using to launch a daemon process does not give us guarantees that `agentmon_loop --path` will be executed by the time the process boots. To address this we can loop and re-check for some period of time.

This method utilizes the output log from agentmon loop as directed by the spawn daemon command.

buildpacks/ruby/src/bin/agentmon_loop.rs

@@ -47,19 +47,22 @@ fn main() {
         });
 
     match agentmon.try_exists() {
-        Ok(true) => loop {
-            match run(&agentmon, &agentmon_args) {
-                Ok(status) => {
-                    eprintln!("Process completed with status={status}, sleeping {SLEEP_FOR:?}");
-                }
-                Err(error) => {
-                    eprintln!(
-                        "Process could not run due to error. {error}, sleeping {SLEEP_FOR:?}"
-                    );
-                }
-            };
-            sleep(SLEEP_FOR);
-        },
+        Ok(true) => {
+            eprintln!("Booting agentmon_loop");
+            loop {
+                match run(&agentmon, &agentmon_args) {
+                    Ok(status) => {
+                        eprintln!("Process completed with status={status}, sleeping {SLEEP_FOR:?}");
+                    }
+                    Err(error) => {
+                        eprintln!(
+                            "Process could not run due to error. {error}, sleeping {SLEEP_FOR:?}"
+                        );
+                    }
+                };
+                sleep(SLEEP_FOR);
+            }
+        }
         Ok(false) => {
             eprintln!("Path does not exist {path}", path = agentmon.display());
             exit(1);

buildpacks/ruby/tests/integration_test.rs

@@ -5,7 +5,7 @@ use libcnb_test::{
     ContainerContext, TestRunner,
 };
 use std::thread;
-use std::time::Duration;
+use std::time::{Duration, Instant};
 use thiserror::__private::DisplayAsDisplay;
 use ureq::Response;
 
@@ -125,17 +125,33 @@ fn test_barnes_app() {
                 ContainerConfig::new()
                     .entrypoint("launcher")
                     .envs(vec![
-                        ("HEROKU_METRICS_URL", "example.com"),
                         ("DYNO", "web.1"),
                         ("PORT", "1234"),
+                        ("AGENTMON_DEBUG", "1"),
+                        ("HEROKU_METRICS_URL", "example.com"),
                     ])
-                    .command(["ps x"]),
+                    .command(["while true; do sleep 1; done"]),
                 |container| {
-                    let log_output = container.logs_wait();
+                    let boot_message = "Booting agentmon_loop";
+                    let mut agentmon_log = String::new();
+
+                    let started = Instant::now();
+                    while started.elapsed() < Duration::from_secs(20) {
+                        if agentmon_log.contains(boot_message) {
+                            break;
+                        }
+
+                        std::thread::sleep(frac_seconds(0.1));
+                        agentmon_log = container
+                            .shell_exec("cat /layers/heroku_ruby/metrics_agent/output.log")
+                            .stdout;
+                    }
+
+                    let log_output = container.logs_now();
                     println!("{}", log_output.stdout);
                     println!("{}", log_output.stderr);
 
-                    assert_contains!(log_output.stdout, "agentmon_loop --path");
+                    assert_contains!(agentmon_log, boot_message);
                 },
             );
         },