On Sun, 26 Jul 2020 at 19:11, Vinay Sharma via Python-ideas email@example.com wrote:
Problem: Currently, let’s say I create a shared_memory segment using mulitprocessing.shared_memory.SharedMemory in Process 1 and open the same in Process 2. Then, I try to write some data to the shared memory segment using both the processes, so for me to prevent any race condition (data corruption), either these operations must be atomic, or I should be able to lock / unlock shared memory segment, which I cannot at the moment.
I earlier posted a solution to this problem, which received positive response, but there weren’t many responses to it, despite the fact this problem makes shared_memory practically unusable if there are simultaneous writes. So, the purpose of this post is to have discussion about the solution of the same.
One thing that is worth thinking about is the safety of the API that is put together. A memory segment plus a separate detached semaphore or mutex can be used to build a safe API, but is not itself a safe API.
A safe API shouldn't allow writes to the memory segment while the mutex is unlocked, rather than allowing one to build a safe API from the various pieces. (There may / will be lower level primitives that are unsafe).
We can look at a lot of the APIs in the Rust community for examples of this sort of thing.
Python doesn't have the borrow checker to enforce usage, but we could still work from the same basic principle - given there are multiple processes involved that make it easier to have safe outcomes.
For instance, we could have an object representing a memory range that doesn't offer read/write at all, but allows: - either one process write access over the range - or any number of readers read access over the range - allows subdividing the range (so that you can e.g. end one write lock and keep another)
For instance, https://doc.rust-lang.org/std/vec/struct.Vec.html#method.split_at_mut is an in-process API that is very similar.