labeling MVar and STM data structures

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

Hello,

I'm seeking of the source of the bug that my server based on Warp gets
not-responding. "listThreads" shows that IOManager is blocked on a
MVar permanently:

2 IOManager on cap 0: ThreadBlocked BlockedOnForeignCall
3 IOManager on cap 1: ThreadBlocked BlockedOnMVar
4 TimerManager: ThreadBlocked BlockedOnForeignCall

IOManager should have the BlockedOnForeignCall reason since it's
calling epoll_wait().

It's quit hard to debug this bug since I don't understand which MVar blocks
IOManager of 3. Users can register MVar operations to IOManager while
IOManager itself is using MVars internally.

To make this kind of debugging easier, I would like ask GHC developers
to extend MVar to hold its label and provide "labelMVar" like
"labelThread". The same mechanism should be provided for STM as well.
"BlockReason" should contain the target label.

What do you think?

Background: https://github.com/haskell/network/issues/590

--Kazu