Unstable workflow test

[tonyyang-svail]

The following workflow test occasionally fails without any signal.

sqlflow/scripts/test/workflow.sh

Line 31 in 67fad86

function test_couler() {

Reference build: https://travis-ci.com/sql-machine-learning/sqlflow/jobs/277839812

[wangkuiyi]

I have no idea why. Still looking into it.

A minor suggestion is to add an echo message before

sqlflow/scripts/test/workflow.sh

Line 57 in 67fad86

return 1