refactor: format error messages without allocating a string for each line

I added a test to confirm the output matches the old.

Fixes clippy warning clippy::format_collect
https://rust-lang.github.io/rust-clippy/master/index.html#/format_collect

binaries/coordinator/src/lib.rs

@@ -575,6 +575,19 @@ async fn stop_dataflow_by_uuid(
     Ok(())
 }
 
+fn format_error(machine: &str, err: &str) -> String {
+    let mut error = err
+        .lines()
+        .fold(format!("- machine `{machine}`:\n"), |mut output, line| {
+            output.push_str("    ");
+            output.push_str(line);
+            output.push('\n');
+            output
+        });
+    error.push('\n');
+    error
+}
+
 fn dataflow_result(
     results: &BTreeMap<String, Result<(), String>>,
     dataflow_uuid: Uuid,
@@ -582,8 +595,7 @@ fn dataflow_result(
     let mut errors = Vec::new();
     for (machine, result) in results {
         if let Err(err) = result {
-            let err: String = err.lines().map(|line| format!("    {line}\n")).collect();
-            errors.push(format!("- machine `{machine}`:\n{err}\n"));
+            errors.push(format_error(machine, err));
         }
     }
 
@@ -941,3 +953,21 @@ fn set_up_ctrlc_handler() -> Result<impl Stream<Item = Event>, eyre::ErrReport>
 
     Ok(ReceiverStream::new(ctrlc_rx))
 }
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_format_error() {
+        let machine = "machine A";
+        let err = "foo\nbar\nbuzz";
+
+        // old method
+        let old_error = {
+            #[allow(clippy::format_collect)]
+            let err: String = err.lines().map(|line| format!("    {line}\n")).collect();
+            format!("- machine `{machine}`:\n{err}\n")
+        };
+        let new_error = super::format_error(machine, err);
+        assert_eq!(old_error, new_error)
+    }
+}

binaries/daemon/src/spawn.rs

@@ -405,7 +405,12 @@ pub async fn spawn_node(
                 .write_all(message.as_bytes())
                 .await
                 .map_err(|err| error!("Could not log {message} to file due to {err}"));
-            let formatted: String = message.lines().map(|l| format!("      {l}\n")).collect();
+            let formatted = message.lines().fold(String::default(), |mut output, line| {
+                output.push_str("      ");
+                output.push_str(line);
+                output.push('\n');
+                output
+            });
             debug!("{dataflow_id}/{} logged:\n{formatted}", node.id.clone());
             // Make sure that all data has been synced to disk.
             let _ = file