bugfix: failed to get host in health check configuration.

[shuaijinchao]

What this PR does / why we need it:

Pre-submission checklist:

FIX #1869

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible?

[shuaijinchao]

@membphis the test case cannot get the warn log thrown by the health check, please help to check.

[membphis]

I check the source code, and I found two points.

the healthcheck was ignored

1. if the count of upstream node is 1, it will return the upstream node directly, the healthcheck was ignored.

https://github.com/apache/incubator-apisix/blob/master/apisix/balancer.lua#L180-L185

I think we can create a new PR to fix this bug first.

The health check prints the log only when the status of the upstream node is changed

The default status is SUCCESS, if the status is SUCCESS always, we never can get a log like healthy SUCCESS.

[shuaijinchao]

I check the source code, and I found two points.

the healthcheck was ignored

1. if the count of upstream node is 1, it will return the upstream node directly, the healthcheck was ignored.

https://github.com/apache/incubator-apisix/blob/master/apisix/balancer.lua#L180-L185

I think we can create a new PR to fix this bug first.

The health check prints the log only when the status of the upstream node is changed

The default status is SUCCESS, if the status is SUCCESS always, we never can get a log like healthy SUCCESS.

1. If the number of nodes is 1 and the node is a faulty node, the health check is of little significance at this time, and APISIX will also have a retry mechanism.
2. adding a faulty node to trigger the status change of the health check, the verification of the test case is completed.

Thanks @membphis for your help.

[membphis]

@shuaijinchao

Is this PR backward compatible?

I think this PR is not backward compatible. please confirm this.

[shuaijinchao]

@shuaijinchao

Is this PR backward compatible?

I think this PR is not backward compatible. please confirm this.

@membphis this is a bug fix, using the previous data structure, so it is backward compatible.

[membphis]

@membphis this is a bug fix, using the previous data structure, so it is backward compatible.

ok, got it. let us merge this PR.

[membphis]

@shuaijinchao merged, many thx