Fix #271

Silly mistake: when a pipelined question failed to send, we were
rejecting the promise for the *original* question, not the new one we
just failed to send -- so when the return message for the original came
back, we got a panic. Solution is to reject q2, not q.

Note that the tests also needed tweaking, because we don't expect no
error, we just don't want a panic -- the error should reflect the
canceled context.

rpc/level0_test.go

@@ -1798,7 +1798,7 @@ func TestHandleReturn_regression(t *testing.T) {
 		defer release()
 
 		_, err := f.Struct()
-		assert.NoError(t, err)
+		assert.ErrorIs(t, err, ctx.Err())
 	})
 
 	t.Run("BootstrapWithExpiredContext", func(t *testing.T) {
@@ -1817,7 +1817,7 @@ func TestHandleReturn_regression(t *testing.T) {
 		defer release()
 
 		_, err := f.Struct()
-		assert.NoError(t, err)
+		assert.ErrorIs(t, err, ctx.Err())
 	})
 }
 

rpc/question.go

@@ -161,7 +161,7 @@ func (q *question) PipelineSend(ctx context.Context, transform []capnp.PipelineO
 					q.c.questions[q2.id] = nil
 					q.c.questionID.remove(uint32(q2.id))
 				})
-				q.p.Reject(rpcerr.Failedf("send message: %w", err))
+				q2.p.Reject(rpcerr.Failedf("send message: %w", err))
 				return
 			}