ReferenceConfigCache doesn't work well when no provider

[YoungHu]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.6.2 and lower
• Operating System version: all version
• Java version: all version

Step to reproduce this issue

1. shutdown provider
2. consumer side use ReferenceConfigCache.get() to invoke provider
3. invoke provider one time
4. startup provider
5. invoker provider again

Pls. provide [GitHub address] to reproduce this issue.

Expected Result

when provider startup, the second time invoke should success.

Actual Result

second time invoke failed

What cause it

When no provider, config.get() return null, at this time we shouldn't store this reference into cache.

public <T> T get(ReferenceConfig<T> referenceConfig) {
        String key = generator.generateKey(referenceConfig);

        ReferenceConfig<?> config = cache.get(key);
        if (config != null) {
            return (T) config.get();
        }

        cache.putIfAbsent(key, referenceConfig);
        config = cache.get(key);
        return (T) config.get();
    }

[diecui1202]

@YoungHu Could u please send a PR to solve this problem ?

[YoungHu]

PR already done

[diecui1202]

The same issue: #1765 . But it's not ReferenceConfigCache's problem.

I think we need to fix it like #1999

[diecui1202]

@YoungHu Could u refer to #1999 and fix it on dubbo-2.6.x ?

[YoungHu]

will check it tomorrow by using the master branch code

[YoungHu]

confirmed this issue fix at master branch with #1999 ,close this one