Optimize memory cache's singleflight performance

[2547409617]

What is the problem your feature solves, or the need it fulfills?

Acquiring write locks is not that easy in a high-concurrency environment,
imagine there are many read lock requests, the write lock will be acquired later.
After write lock acquired perhaps another one has already completed the lookup and removed the lock from locker container.

Describe the solution you'd like

After write lock acquired, we should first check whether data already exists at the current point.

Describe alternatives you've considered

What other solutions, features, or workarounds have you considered that might also solve the issue?
What are the tradeoffs for these alternatives compared to what you're proposing?

Additional context

This could include references to documentation or papers, prior art, screenshots, or benchmark
results.

[2547409617]

suppose there are two threads(coroutine) t1 t2
t1 get data -> None
t1 get container read lock, get CacheLock -> None
t1 get container write lock, insert CacheLock
t1 lookup data
t2 get data -> None
t2 pause for some reason such as thread switching
t1 put lookuped data
t1 get container write lock, remove CacheLock
t2 resume and get container read lock, get CacheLock -> None
t2 get container write lock, insert another CacheLock
t2 lookup data

So it is waste that t2 lookup data in this situation.
Double check should be done when get write lock like thread-safe lazy initialization.