[GitHub] [apisix-dashboard] liuxiran commented on pull request #1812: fix: can not configure upstream with no nodes

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liuxiran commented on pull request #1812:
URL: https://github.com/apache/apisix-dashboard/pull/1812#issuecomment-825440571


   > Hi, @liuxiran
   > In golang, you should distinguish `nil slice` and `empty slice`
   > 
   > ```
   > var x []*entity.Node
   > ```
   > 
   > is a nil slice.
   > 
   > ```
   > y := make([]*entity.Node, 0)
   > ```
   > 
   > is an empty slice. The length and value are 0.
   > If you use printf or println to print them, the result is both "[]".
   > 
   > In this situation, because the upstream.Nodes' type is interface{}.So it's 
a little more complex.
   > You can use
   > 
   > ```
   > reflect.ValueOf(upstream.Nodes).IsNil()
   > ```
   > 
   > to see the true value.
   
   @starsz Thanks for your detailed explanation, after added 
`reflect.ValueOf(upstream.Nodes).IsNil()` and debug again, I find out the 
confusion before:
   debug step:
   1. create an upstream with no nodes
   2. visit uptream page, call `GET /apisix/admin/upstreams`
   3. config the upstream ,call `GET /apisix/admin/upstream/:id`
   
   before the advice change, I got the logs:
   ![2021-04-23 
14-32-00屏幕截图](https://user-images.githubusercontent.com/2561857/115829386-853b0800-a441-11eb-8ee3-8703f220bc4e.png)
   
   after the advice change, I got the logs:
   ![2021-04-23 
14-39-50屏幕截图](https://user-images.githubusercontent.com/2561857/115829615-c7644980-a441-11eb-88c0-93964e82b626.png)
   
   The conclusion is that: 
   the original `upstream.nodes` type got from etcd is `[]interface{}`, and it 
is an empty slice, 
   it will be changed to type nodes , `[]*Node`, in FormatNodes `case 
[]interface{}` when I visit upstream page(step2), 
   and  the when I got the upstream info(step3), the data come from catch 
instead of stroge, that is to say, in this scene, the `upstream.nodes` type is 
already `[]*Node`.
   
   so the advice will solve the problem. 
   Thanks again for all your help, then I will try to add related test cases
   


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