This adds a new `StoreTransaction` type, that wraps a `StoreCache` and a `Store`. The idea is that it will allow write access to the `Store` (and maintains the cache at the same time), while the `Store::cache` method will only give read-only access to the store's content.
Another new data type is introduced, `PendingChanges`, that reflects `Changes` but for fields that are properly maintained in the `StoreCache` and that one can write in the `StoreTransaction`. In the future, it wouldn't be possible to use `save_pending_changes` from the outside of a `StoreTransaction` context.
The layering is the following:
- `Store` wraps the `DynCryptoStore`, contains a reference to a `StoreCache`.
- When read-only access is sufficient, one can get a handle to the cache with `Store::cache()`.
- When a write happens, then one can create a `StoreTransaction` (later, only one at a time will be allowed, by putting the `StoreCache` behind a `RwLock`; this has been deferred to not make the PR grow too much).
- Any field in the `StoreCache` will get a method to get a reference to the cached thing: it will either load from the DB if not cached, or return the previously cached value.
- Any field that can be written to will get a method to get a mutable reference in the `StoreTransaction`: it will either load from the cache into a `PendingChanges` scratch pad, or return the scratchpad temporary value.
- When a `StoreTransaction::commit()` happens, fields are backpropagated into the DB *and* the cache.
Then, this `StoreTransaction` is used to update a `ReadOnlyAccount` in multiple places (and usage of `ReadOnlyAccount` is minimized so as not to require a transaction or cache function call as much as possible). With this, the read-only account only exists transiently, and it's only stored long-term in the cache.
Followup PRs include:
- making the `ReadOnlyAccount` not cloneable
- remove inner mutability from the `ReadOnlyAccount`
- add a `RwLock` on the `StoreTransaction`
Part of https://github.com/matrix-org/matrix-rust-sdk/issues/2624 + https://github.com/matrix-org/matrix-rust-sdk/issues/2000.
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* crypto: Replace some uses of `ReadOnlyAccount` with `StaticAccountData` and identify tests
* crypto: introduce `StoreTransaction` to modify a `ReadOnlyAccount`
* crypto: introduce `save_pending_changes`, aka `save_changes` v2
* crypto: Start using `StoreTransaction`s to save the account, get rid of `Store::save_account` + `Account::save`
* crypto: use `StoreTransaction` to save an account in `keys_for_upload`
* crypto: use `StoreTransaction` and the cache in more places
* crypto: remove `Account` from the `Changes` \o/
* crypto: remove last (test-only) callers of `Store::account()`
* crypto: move `ReadOnlyAccount` inside the cache only
* crypto: use `ReadOnlyAccount` and `Account` in fewer places
whenever we can use `StaticAccountData` in place.
* crypto: make tests rely less on OlmMachine
* crypto: Don't put the `ReadOnlyAccount` behind a RwLock just yet
+ clippy
Damir Jelić