chore: prevent Dispatch fiber to be launched during migration

[romange]

No description provided.

[kostasrim]

This should break SendAsync

[romange]

in what sense?

[kostasrim]

What I had in mind is that because we don't wake the dispatch fiber it would miss the evc.notify and when the fiber was launched again it would not consume the items that were pushed back on the dispatch queue. I am wrong, I double checked the flow and we guard against this case in the dispatch fiber's main loop:

  1340     evc_.await(
  1341         [this] { return cc_->conn_closing || (!dispatch_q_.empty() && !cc_->sync_dispatch); });
  1342     if (cc_->conn_closing)                                                                        
  1343       break;

Also a corner case that came into my mind, if after a migration the connection for some reason is closing are there any cases where we need to drain the dispatch queue ?

[romange]

ClearPipelinedMessages is called at the end and it drains the queue

[adiholden]

We still fail in replication_test.py::test_replicaof_reject_on_load
I20240604 12:10:57.323175 2092379 accept_server.cc:202] AcceptServer::BreakListeners finished
I20240604 12:10:57.323591 2092382 listener_interface.cc:339] Migrating 0x342394b0a00 from 2092382(2) 0x7f530bf77e40 to 2092401
I20240604 12:10:57.323897 2092401 listener_interface.cc:372] Migrated 0x342394b0a00 from 2 to 7 0x7f530bdb9e40
F20240604 12:10:57.323930 2092401 dragonfly_connection.cc:576] Check failed: !dispatch_fb_.IsJoinable()