add trace logging for semaphores

pipeline/runtime/execer.go

@@ -142,6 +142,8 @@ func (e *Execer) exec(ctx context.Context, state *pipeline.State, spec Spec, ste
 	ctx = logger.WithContext(ctx, log)
 
 	if e.sem != nil {
+		log.Trace("acquiring semaphore")
+
 		// the semaphore limits the number of steps that can run
 		// concurrently. acquire the semaphore and release when
 		// the pipeline completes.
@@ -152,6 +154,7 @@ func (e *Execer) exec(ctx context.Context, state *pipeline.State, spec Spec, ste
 		// state should be canceled.
 		switch ctx.Err() {
 		case context.Canceled, context.DeadlineExceeded:
+			log.Trace("acquiring semaphore canceled")
 			state.Cancel()
 			return nil
 		}
@@ -172,6 +175,7 @@ func (e *Execer) exec(ctx context.Context, state *pipeline.State, spec Spec, ste
 			}
 			// release the semaphore
 			e.sem.Release(1)
+			log.Trace("semaphore released")
 		}()
 	}