[GitHub] [incubator-dubbo] JerryChin opened pull request #3649: [Dubbo-3629] Fix ServiceConfig duplicate export produces misleading exception

["JerryChin (GitHub)" <gi...@apache.org>]

## What is the purpose of the change

fix #3629 

First, the existing implementation by using two variables for controlling `ServiceConfig` exporting state is not only awkward to code, but also can theoretically lead invalid state transitions (`exported` and `unexported` both simultaneously true).

Secondly, the `exported` is set in `doExport()` method which could be called asynchronously, this makes it hard to refactor to prevent duplication exporting. 

## Brief changelog

the following changes are introduced:

1. refactor the two variables `exported` and `unexported` into one `AtomicInteger` variable `exportingStage`.
2. `compare and set` exporting state in `export()` and `unexport()` methods **before doing anything**.
3. remove the `synchronized` from the two methods.
4. add unit-test for this fix to verify everything works as expected.


The existing exporting state transition flow is as follows, remains unchanged after this PR :
```
        +----------------+
        | INITIAL_STAGE  |
        +----------------+
              |     ^
              |     |
              V     |
        +-----------------+
        | EXPORTING_STAGE |
        +-----------------+
                 |
                 |
                 V
        +-----------------+
        | EXPORTED_STAGE |
        +-----------------+
                 |
                 |
                 V
        +-----------------+
        | UNEXPORTED_STAGE|
        +-----------------+
```

## Verifying this change

Necessary unit-test is added, see `ServiceConfigTest`

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[ Full content available at: https://github.com/apache/incubator-dubbo/pull/3649 ]
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