cancel step on semaphore context deadline exceeded

pipeline/runtime/execer.go

@@ -145,10 +145,24 @@ func (e *Execer) exec(ctx context.Context, state *pipeline.State, spec Spec, ste
 		// the semaphore limits the number of steps that can run
 		// concurrently. acquire the semaphore and release when
 		// the pipeline completes.
-		if err := e.sem.Acquire(ctx, 1); err != nil {
+		err := e.sem.Acquire(ctx, 1)
+
+		// if acquiring the semaphore failed because the context
+		// deadline exceeded (e.g. the pipeline timed out) the
+		// state should be canceled.
+		switch ctx.Err() {
+		case context.Canceled, context.DeadlineExceeded:
+			state.Cancel()
 			return nil
 		}
 
+		// if acquiring the semaphore failed for unexpected reasons
+		// the pipeline should error.
+		if err != nil {
+			log.WithError(err).Errorln("failed to acquire semaphore.")
+			return err
+		}
+
 		defer func() {
 			// recover from a panic to ensure the semaphore is
 			// released to prevent deadlock. we do not expect a