tree 0f253c3cd6881569da8c9ccb27cf77d249ad165e
parent 8806403b3a929e55c1a9edb2695836bcda1fffb9
author Kun Zhang <zhangkun83@users.noreply.github.com> 1551495933 -0800
committer GitHub <noreply@github.com> 1551495933 -0800
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core: refactor load-balancing config handling (#5397)

The LoadBalancingConfig message, which looks like
```json
{
  "policy_name" : {
    "config_key1" : "config_value1",
    "config_key2" : "config_value2"
   }
}
```
appears multiple times. It gets super tedious and confusing to handle, because both the whole config and the value (in the above example is `{ "config_key1" : "config_value1" }`) are just `Map<String, Object>`, and each user needs to do the following validation:
 1. The whole config must have exactly one key
 2. The value must be a map

Here I define `LbConfig` that holds the policy name and the config value, and a method in `ServiceConfigUtil` that converts the parsed JSON format into `LbConfig`.

There is also multiple cases where you need to handle a list of configs (top-level balancing policy, child and fallback policies in xds, grpclb child policies). I also made another helper method in `ServiceConfigUtil` to convert them into `List<LbConfig>`.

Found and fixed a bug in the xds code, where the top-level balancer should pass the config value (excluding the policy name), not the whole config to the child balancers. Search for "supported_1_option" in the diff to see it in the tests.