batch merge in Write Operator

[yang20150702]

I'm learning the source code of goleveldb. I find an issue about batch merge in db.Write func.
I want to know the design of batch, maybe my understanding is wrong.
this picture is about test case.

in Write func: db.writeMergeC <- writeMerge{sync: sync, batch: batch}:
in writeLocked func:
this two func is synchronous. if this case executes in Write func, this logic will block.
I think the logic of batch merge should be asynchronous