diag: atomic stderr write

Author: mdashti

src/distributed_planner/distribute_plan.rs

@@ -146,10 +146,14 @@ fn _distribute_plan(
         // This is a CollectLeft HashJoinExec with the build side marked as being broadcast. we
         // need to insert a NetworkBroadcastExec and scale up the BroadcastExec consumer_tasks.
         PlanOrNetworkBoundary::Broadcast => {
-            eprintln!(
-                "DISTPLAN.Broadcast: task_count={task_count} max_child_task_count={max_child_task_count:?} task_count_annot={:?}",
+            let line = format!(
+                "DISTPLAN_BCAST pid={} task_count={task_count} max_child={max_child_task_count:?} annot={:?}\n",
+                std::process::id(),
                 annotated_plan.task_count
             );
+            // Single write so output isn't interleaved.
+            use std::io::Write;
+            let _ = std::io::stderr().write_all(line.as_bytes());
             // It would need a network boundary, but on both sides of the boundary there is just 1 task,
             // so we are fine with not introducing any network boundary.
             if task_count == 1 && max_child_task_count == Some(1) {