[Dubbo-3169]Check future status before get(), return default value if not completed yet.

[chickenlj]

What is the purpose of the change

• Check future status before get(), return the default value if not completed yet.
• Add comment to announce AsyncRpcResult as unstable and should only be used internally.

Brief changelog

• AsyncRpcResult

Verifying this change

run dubbo-samples-async

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[codecov-io]

Codecov Report

Merging #3185 into 2.7.0-release will decrease coverage by 0.04%.
The diff coverage is 0%.

@@                 Coverage Diff                 @@
##             2.7.0-release    #3185      +/-   ##
===================================================
- Coverage            63.83%   63.78%   -0.05%     
  Complexity              75       75              
===================================================
  Files                  651      651              
  Lines                28317    28317              
  Branches              4793     4793              
===================================================
- Hits                 18076    18063      -13     
- Misses                7973     7982       +9     
- Partials              2268     2272       +4

Impacted Files	Coverage Δ	Complexity Δ
...ava/org/apache/dubbo/rpc/SimpleAsyncRpcResult.java	60% <ø> (ø)	0 <0> (ø)	⬇️
...main/java/org/apache/dubbo/rpc/AsyncRpcResult.java	32.75% <0%> (ø)	0 <0> (ø)	⬇️
.../remoting/transport/netty4/NettyClientHandler.java	75% <0%> (-11.12%)	0% <0%> (ø)
.../dubbo/remoting/transport/netty4/NettyChannel.java	61.17% <0%> (-4.71%)	0% <0%> (ø)
...e/dubbo/remoting/transport/netty/NettyChannel.java	57.64% <0%> (-4.71%)	0% <0%> (ø)
.../apache/dubbo/remoting/transport/AbstractPeer.java	58.69% <0%> (-4.35%)	0% <0%> (ø)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	0% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	63.79% <0%> (-1.73%)	0% <0%> (ø)
.../exchange/support/header/HeaderExchangeServer.java	58.11% <0%> (-1.71%)	0% <0%> (ø)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	72.5% <0%> (-1.67%)	0% <0%> (ø)
... and 4 more

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[khanimteyaz]

@chickenlj
Other than one minor comment which I have given, it looks fine to me.

[carryxyh]

@chickenlj
I found that the pr on this issue may cause an additional problem with CacheFilter:

String key = StringUtils.toArgumentString(invocation.getArguments());
Object value = cache.get(key);
if (value != null) {
    if (value instanceof ValueWrapper) {
        return new RpcResult(((ValueWrapper)value).get());
    } else {
        return new RpcResult(value);
    }
}
// In an asynchronous scenario, result.getValue() will return an empty RpcResult
Result result = invoker.invoke(invocation);
if (!result.hasException()) {
    // since the rpcResult#hasException will return false, this empty result will be cached.
    cache.put(key, new ValueWrapper(result.getValue()));
}
return result; 

In this scenario, CacheFilter will cache an empty RpcResult, which is incorrect.