fix JobGraph::run to not busy-wait

crates/fayalite/src/build/graph.rs

@@ -703,8 +703,12 @@ impl JobGraph {
             }
             let mut running_jobs = HashMap::default();
             let (finished_jobs_sender, finished_jobs_receiver) = mpsc::channel();
+            let mut next_finished_job = None;
             loop {
-                while let Some(finished_job) = finished_jobs_receiver.try_recv().ok() {
+                if let Some(finished_job) = next_finished_job
+                    .take()
+                    .or_else(|| finished_jobs_receiver.try_recv().ok())
+                {
                     let Some(RunningJob { job, thread }) = running_jobs.remove(&finished_job)
                     else {
                         unreachable!();
@@ -736,6 +740,7 @@ impl JobGraph {
                             }
                         }
                     }
+                    continue;
                 }
                 if let Some(WaitingJobState {
                     job_node_id,
@@ -791,12 +796,15 @@ impl JobGraph {
                                 .expect("failed to spawn thread for job"),
                         },
                     );
+                    continue;
                 }
                 if running_jobs.is_empty() {
                     assert!(item_name_to_waiting_jobs_map.is_empty());
                     assert!(ready_jobs.is_empty());
                     return Ok(());
                 }
+                // nothing to do yet, block to avoid busy waiting
+                next_finished_job = finished_jobs_receiver.recv().ok();
             }
         })
     }