Why mutex is used in ServerCalls?

[bagjinok]

Hi 😃 grpc-kotlin team!
I'm a very newbie on Grpc and Kotlin!
I just wondering to know that why mutex is used in ServerCalls.
If it’s not too much to ask, could you please explain why mutex is used?
https://github.com/grpc/grpc-kotlin/blob/master/stub/src/main/java/io/grpc/kotlin/ServerCalls.kt#L243

[lowasser]

If you look in the revision history, you can see the commit linked to the bug that describes a race that could have happened otherwise. ef06b07#diff-8f7b388f007f7822b342a47efedb08d735860ccdc4aeaec73a0d09406d96f54f