Capture recent output for nicer error messages

This will include recent output in the Slack error messages.

src/cli.rs

@@ -3,6 +3,7 @@ use parking_lot::Mutex;
 use reqwest::Url;
 use signal_hook::consts::{SIGINT, SIGTERM};
 use std::{
+    collections::VecDeque,
     io::{Read, Write},
     process::{Child, Command, ExitStatus, Stdio},
     str::FromStr,
@@ -48,6 +49,9 @@ pub(crate) struct Cli {
     /// Arguments to the process
     #[arg(required = false)]
     pub(crate) args: Vec<String>,
+    /// How many lines of output should we store for error messages?
+    #[arg(long, default_value_t = 10)]
+    pub(crate) output_lines: usize,
 }
 
 #[derive(Debug)]
@@ -82,45 +86,59 @@ impl Cli {
         let mut command = Command::new(&self.command);
         command.args(&self.args[..]);
 
-        if self.task_output_timeout.is_some() {
-            command.stdout(Stdio::piped()).stderr(Stdio::piped());
-        }
+        command.stdout(Stdio::piped()).stderr(Stdio::piped());
 
         let mut child = command
             .spawn()
             .context(format!("Failed to spawn {}", self.command))?;
 
         let (send, recv) = mpsc::channel::<MainMessage>();
         let send = SendMainMessage(send);
+        let max_recent_output = self.output_lines;
+        let recent_output = Arc::new(Mutex::new(VecDeque::with_capacity(max_recent_output)));
 
-        match self.task_output_timeout {
-            Some(task_output_timeout) => {
-                let last_output = Arc::new(Mutex::new(Instant::now()));
-                let child_stdout = child.stdout.take().context("child stdout is None")?;
-                let child_stderr = child.stderr.take().context("child stderr is None")?;
-                let send_clone = send.clone();
-                let last_output_clone = last_output.clone();
-                std::thread::spawn(|| {
-                    process_std_handle(child_stdout, send_clone, StdType::Stdout, last_output_clone)
-                });
-                let send_clone = send.clone();
-                let last_output_clone = last_output.clone();
-                std::thread::spawn(|| {
-                    process_std_handle(child_stderr, send_clone, StdType::Stderr, last_output_clone)
-                });
-                let send_clone = send.clone();
-                std::thread::spawn(move || {
-                    detect_deadlock(
-                        last_output,
-                        send_clone,
-                        Duration::from_secs(task_output_timeout),
-                    )
-                });
-            }
-            None => {
-                anyhow::ensure!(child.stdout.is_none());
-                anyhow::ensure!(child.stderr.is_none());
-            }
+        // Always capture output so we can keep recent output available for error messages.
+        let last_output = Arc::new(Mutex::new(Instant::now()));
+        {
+            let child_stdout = child.stdout.take().context("child stdout is None")?;
+            let child_stderr = child.stderr.take().context("child stderr is None")?;
+            let send_clone = send.clone();
+            let last_output_clone = last_output.clone();
+            let recent_output_clone = recent_output.clone();
+            std::thread::spawn(move || {
+                process_std_handle(
+                    child_stdout,
+                    send_clone,
+                    StdType::Stdout,
+                    last_output_clone,
+                    recent_output_clone,
+                    max_recent_output,
+                )
+            });
+            let send_clone = send.clone();
+            let last_output_clone = last_output.clone();
+            let recent_output_clone = recent_output.clone();
+            std::thread::spawn(move || {
+                process_std_handle(
+                    child_stderr,
+                    send_clone,
+                    StdType::Stderr,
+                    last_output_clone,
+                    recent_output_clone,
+                    max_recent_output,
+                )
+            });
+        }
+
+        if let Some(task_output_timeout) = self.task_output_timeout {
+            let send_clone = send.clone();
+            std::thread::spawn(move || {
+                detect_deadlock(
+                    last_output,
+                    send_clone,
+                    Duration::from_secs(task_output_timeout),
+                )
+            });
         }
 
         let send_clone = send.clone();
@@ -174,7 +192,12 @@ impl Cli {
                         self.app_version,
                         self.image_url,
                     );
-                    let result = slack_app.send_notification(&e);
+                    let mut msg = String::new();
+                    for line in &*recent_output.lock() {
+                        msg.push_str(line);
+                        msg.push('\n');
+                    }
+                    let result = slack_app.send_notification(&e, &msg);
                     if let Err(err) = result {
                         eprintln!("Slack notification failed: {err:?}");
                     }
@@ -190,6 +213,8 @@ fn process_std_handle(
     send: SendMainMessage,
     std_type: StdType,
     last_output: Arc<Mutex<Instant>>,
+    recent_output: Arc<Mutex<VecDeque<String>>>,
+    max_recent_output: usize,
 ) {
     let mut buffer = [0u8; 4096];
     loop {
@@ -217,6 +242,15 @@ fn process_std_handle(
                     send.send(MainMessage::Error(e));
                     break;
                 }
+
+                let mut guard = recent_output.lock();
+                for line in buffer.split(|x| *x == b'\n') {
+                    if guard.len() >= max_recent_output {
+                        guard.pop_front();
+                    }
+                    let line = line.strip_suffix(&[b'\r']).unwrap_or(line);
+                    guard.push_back(String::from_utf8_lossy(line).into_owned());
+                }
             }
             Err(e) => {
                 send.send(MainMessage::Error(e));

src/slack.rs

@@ -49,7 +49,11 @@ impl SlackApp {
         )
     }
 
-    pub(crate) fn send_notification(&self, message: &anyhow::Error) -> Result<()> {
+    pub(crate) fn send_notification(
+        &self,
+        message: &anyhow::Error,
+        latest_output: &str,
+    ) -> Result<()> {
         let description = self.compute_description();
         let mut value = serde_json::json!(
         {
@@ -69,7 +73,14 @@ impl SlackApp {
                         "type": "mrkdwn",
                         "text": description
                     },
-                }
+                },
+                {
+                    "type": "section",
+                    "text": {
+                        "type": "plain_text",
+                        "text": latest_output,
+                    }
+                },
             ]
         });
         if let Some(image_url) = &self.app_info.image_url {