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76ed3511b5
This implements a value-based lock in the crypto stores. The intent is to use that for multiple processes to be able to make writes into the store concurrently, while still cooperating on who does them. In particular, we need this for #1928, since we may have up to two different processes trying to write into the crypto store at the same time. ## New methods in the `CryptoStore` trait The idea is to introduce two new methods touching **custom values** in the crypto store: - one to atomically insert a value, only if it was missing (so, not following the semantics of `upsert` used in the `set_custom_value`) - one to atomically remove a custom value Those two operations match the semantics we want: - take the lock only if it ain't taken already == insert an entry only if it was missing - release the lock = remove the entry By looking at the number of lines affected by the query, we can infer whether the insert/remove happened or not, that is, if we managed to take the lock or not. ## High-level APIs I've also added an high-level API, `CryptoStoreLock`, that helps managing such a lock, and adds some niceties on top of that: - exponential backoff to retry attempts at acquiring the lock, when it was already taken - attempt to gracefully recover when the lock has been taken by an app that's been killed by the environment - full configuration of the key / value / backoff parameters While it'd be nice to have something like a `CryptoStoreLockGuard`, it's hard to implement without being racy, because of the `async` statements that would happen in the `Drop` method (and async drop isn't stable yet). ## Test program There's also a test program in which I shamelessly show my rudimentary unix skills; I've put it in the `labs/` directory but this could as well be a large integration test. A parent program initially fills a custom crypto store, then creates a `pipe()` for 1-way communication with a child created with `fork()`; then the parent sends commands to the child. These commands consist in reading and writing into the crypto store, using a lock. And while the child attempts to perform these operations, the parent tries hard to get the lock at the same time. This helps figuring out a few issues and making sure that cross-process locking would work as intended.