ForkingClusterInvoker missing RpcContext and the attachments.

[carryxyh]

• 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: 2.7.0-SNAPSHOT
• Operating System version: mac os
• Java version: 1.8

Step to reproduce this issue

1. Export a service
2. Make cluster = "forking"
3. Set Attachment in RpcContext
4. Do rpc invoke

Expected Result

The attachment in RpcContext has element.

Actual Result

The attachment in RpcContext don't have element.

The problem

I set attachment in the consumer side like this:

Then do rpc invoke,but the attachment is missing, (AbstractInvoker#invoke):

The problem is that ForkingClusterInvoker use threapool to do rpc invoke which will miss the threadlocal:

[carryxyh]

This issue has been fixed by #2024.
Close this one.