While working on #2533, I knew that my proof of concept had a retain cycle. I however was very confused when it wasn't detected by this mechanism introduced first introduced in #2104.

Turns out that this had been wrong the whole time: the current implementation, as explained by the comment, was executing those blocks in LIFO order, which meant that the references were being set to `nil` before the assertions could be created.
This meant that by the time the `except { [weak purchases = self.purchases] ... }` lines were called, `self.purchases` itself was `nil`, so nothing was being checked.

This simplifies the implementation by inlining the checks, and saving a `weak` reference before clearing the singleton and all the local references.

Luckily there was only one failing test that had to do with how `Purchases` was being created, and implementation details of `expectFatalError`. To avoid this, I changed the test to use the `Purchases.init` method directly.

…gate` transaction handling

While working on #2533 I noticed that the implementation for `StoreKit2TransactionListenerDelegate` was unnecessarily complex:
```swift
@available(iOS 15.0, tvOS 15.0, macOS 12.0, watchOS 8.0, *)
extension PurchasesOrchestrator: StoreKit2TransactionListenerDelegate {
    func storeKit2TransactionListener(
        _ listener: StoreKit2TransactionListener,
        updatedTransaction transaction: StoreTransactionType
    ) async throws {
        let isRestore = self.systemInfo.observerMode

        _ = try await self.syncPurchases(receiptRefreshPolicy: .always,
                                         isRestore: isRestore,
                                         initiationSource: .queue)

        await Async.call { completion in
            self.finishTransactionIfNeeded(transaction) { @mainactor in
                completion(())
            }
        }
    }
}
```

Instead of only posting the one transaction, we were calling `syncPurchases`, which is inconsistent with how we handle SK1 transactions.
The new logic in #2533 will also make use of this single-transaction handling code, so it makes sense to unify this as well.

This only required an overload to specify the `InitiationSource`, which uncovered one incorrect `isRestore` parameter in one of our tests.
I verified that this is expected based on other tests, and the implementation of `initiationSource(for productIdentifier:restored:)`, as well as checking with @antoniobg.

While working on #2533, I knew that my proof of concept had a retain
cycle. I however was very confused when it wasn't detected by this
mechanism introduced first introduced in #2104.

Turns out that this had been wrong the whole time: the current
implementation, as explained by the comment, was executing those blocks
in LIFO order, which meant that the references were being set to `nil`
before the assertions could be created.
This meant that by the time the `except { [weak purchases =
self.purchases] ... }` lines were called, `self.purchases` itself was
`nil`, so nothing was being checked.

This simplifies the implementation by inlining the checks, and saving a
`weak` reference before clearing the singleton and all the local
references.

Luckily there was only one failing test that had to do with how
`Purchases` was being created, and implementation details of
`expectFatalError`. To avoid this, I changed the test to use the
`Purchases.init` method directly.

This improves an upcoming log from #2533:
```
DEBUG: ℹ️ Found unfinished transaction, will post receipt in lieu of fetching CustomerInfo:
<StoreTransaction:
    identifier="9CB3E6CE-A6F6-41EC-B25D-FE0CCE5783F6"
    product=""
    date="2023-05-26 21:07:55 +0000"
    quantity=1
>
```

Necessary refactor for #2533. This allows `CustomerInfoManager` to have
an instance of `TransactionPosterType` instead of the whole
`PurchasesOrchestrator`.

Essentially this becomes the hierarchy now:

![IMG_B8C56C11B7E5-1](https://github.com/RevenueCat/purchases-ios/assets/685609/36f5c30b-fc82-4590-b574-ce6d95eef13f)

### Changes:
- Created stateless `TransactionPoster`
- Extracted `handlePurchasedTransaction` into `TransactionPoster`
- The new implementation there is _mostly_ the same, with the one
difference that it's stateless. Instead of holding
`purchaseCompleteCallbacksByProductID`, that's done by
`PurchasesOrchestrator`. The methods there all take a callback, so
they're easier to use and don't require keeping a callback around.
- Created `PurchaseSource` to abstract the combo of `isRestore` and
`InitiationSource`. This is still messy using the deprecated
`allowSharingAppStoreAccount`, but at least that's abstracted out and
the new `TransactionPoster` won't deal with that deprecated property.
- I didn't mock `TransactionPoster`, which means that all existing tests
that check `PurchasesOrchestrator` continue working the same way. This
was on purpose to keep this refactor as simple as possible.
- This supersedes #2536: the change made there was now possible here
without any integration test failures, thanks to the fact that we can
process transactions independently without messing with the
`purchaseCompleteCallbacksByProductID` state now.

### Future improvements
This is just a start, there's a lot more than can be done to simplify
the `PurchasesOrchestrator` monster. The main thing would be to remove
all the custom code for `restorePurchases`, which is largely the same as
what's in `TransactionPoster` now.

### Notes:
Just posting this here for posteriority. I had to trace the call
hierarchy for both SK1 and SK2. This "block" is what's now in
`TransactionPoster`:

![IMG_C60274393F7F-1](https://github.com/RevenueCat/purchases-ios/assets/685609/5a7377f0-8301-474f-a774-a90b6e232bc3)

All these tests were calling the method without waiting for `completion`. This breaks things in an upcoming PR (#2533) because it's no longer synchronous, but also this avoids leaving things running behind when the tests are done.

… to finish

All these tests were calling the method without waiting for `completion`. This breaks things in an upcoming PR (#2533) because it's no longer synchronous, but also this avoids leaving things running behind when the tests are done.

… parameters (#2542)

Follow up to #2540.

This was necessary to ensure that we can implement #2533 without
reference cycles.
This was the hierarchy before this change:

![IMG_B25ECE86C2F9-1](https://github.com/RevenueCat/purchases-ios/assets/685609/70e1d939-8fa5-424c-8673-bb519f386118)

The red arrows are what this PR removes.

## Changes:
- `TransactionPoster` no longer has references to `IdentityManager` and
`Attribution`
- `markSyncedIfNeeded` is no longer in `TransactionPoster`, moved back
to `PurchasesOrchestrator`
- Changed `TransactionPoster`'s callback to return `Result<CustomerInfo,
BackendError>`, providing greater flexibility for users to be able to
read the specific errors.
- Extracted `PurchasedTransactionData` to be reduce the number of
parameters passed around. This removed the need for 2
`swiftlint:disable:next function_parameter_count`. This is the majority
of this diff. It was helpful to combine in this PR because I needed to
add another parameter to the flow: `unsyncedAttributes`, since
`TransactionPoster` could no longer read these directly.

**This is an automatic release.**

### New Features
* `Offline Entitlements`: use offline-computed `CustomerInfo` when
server is down (#2368) via NachoSoto (@NachoSoto)

### Bugfixes
* `AppleReceipt.debugDescription`: don't pretty-print JSON (#2564) via
NachoSoto (@NachoSoto)
* `SK2StoreProduct`: fix crash on iOS 12 (#2565) via NachoSoto
(@NachoSoto)
* `GetCustomerInfo` posts receipts if there are pending transactions
(#2533) via NachoSoto (@NachoSoto)
### Performance Improvements
* `PurchasedProductsFetcher`: cache current entitlements (#2507) via
NachoSoto (@NachoSoto)
* Performance: new check to ensure serialization / deserialization is
done from background thread (#2496) via NachoSoto (@NachoSoto)
### Dependency Updates
* Bump fastlane from 2.212.2 to 2.213.0 (#2544) via dependabot[bot]
(@dependabot[bot])
### Other Changes
* `CustomerInfoManager`: post all unfinished transactions (#2563) via
NachoSoto (@NachoSoto)
* `PostReceiptOperation`: added ability to also post `AdServices` token
(#2566) via NachoSoto (@NachoSoto)
* `Offline Entitlements`: improved computation log (#2562) via NachoSoto
(@NachoSoto)
* Added `TransactionPoster` tests (#2557) via NachoSoto (@NachoSoto)
* Refactored `TransactionPoster`: removed 2 dependencies and abstracted
parameters (#2542) via NachoSoto (@NachoSoto)
* `CustomerInfoManagerTests`: wait for `getAndCacheCustomerInfo` to
finish (#2555) via NachoSoto (@NachoSoto)
* `StoreTransaction`: implemented `description` (#2556) via NachoSoto
(@NachoSoto)
* `Backend.ResponseHandler` is now `@Sendable` (#2541) via NachoSoto
(@NachoSoto)
* Extracted `TransactionPoster` from `PurchasesOrchestrator` (#2540) via
NachoSoto (@NachoSoto)
* `enableAdServicesAttributionTokenCollection`: added integration test
(#2551) via NachoSoto (@NachoSoto)
* `AttributionPoster`: replaced hardcoded strings with constants (#2548)
via NachoSoto (@NachoSoto)
* `DefaultDecodable`: moved to `Misc/Codable/DefaultDecodable.swift`
(#2528) via NachoSoto (@NachoSoto)
* `CircleCI`: specify device to run `backend_integration_tests` (#2547)
via NachoSoto (@NachoSoto)
* Created `StoreKit2TransactionFetcher` (#2539) via NachoSoto
(@NachoSoto)
* Fix load shedder integration tests (#2546) via Josh Holtz
(@joshdholtz)
* Fix doc on `Offering.getMetadataValue` (#2545) via Josh Holtz
(@joshdholtz)
* Extracted and tested `AsyncSequence.extractValues` (#2538) via
NachoSoto (@NachoSoto)
* `Offline Entitlements`: don't compute offline `CustomerInfo` when
purchasing a consumable products (#2522) via NachoSoto (@NachoSoto)
* `OfflineEntitlementsManager`: disable offline `CustomerInfo` in
observer mode (#2520) via NachoSoto (@NachoSoto)
* `BasePurchasesTests`: fixed leak detection (#2534) via NachoSoto
(@NachoSoto)
* `PurchaseTesterSwiftUI`: added `ProxyView` to `iOS` (#2531) via
NachoSoto (@NachoSoto)
* `PurchasedProductsFetcher`: removed `AppStore.sync` call (#2521) via
NachoSoto (@NachoSoto)
* `PurchaseTesterSwiftUI`: added new window on Mac to manage proxy
(#2518) via NachoSoto (@NachoSoto)
* `PurchasedProductsFetcher`: added log if fetching purchased products
is slow (#2515) via NachoSoto (@NachoSoto)
* `Offline Entitlements`: disable for custom entitlements mode (#2509)
via NachoSoto (@NachoSoto)
* `Offline Entitlements`: fixed iOS 12 tests (#2514) via NachoSoto
(@NachoSoto)
* `PurchasedProductsFetcher`: don't throw errors if purchased products
were found (#2506) via NachoSoto (@NachoSoto)
* `Offline Entitlements`: allow creating offline `CustomerInfo` with
empty `ProductEntitlementMapping` (#2504) via NachoSoto (@NachoSoto)
* `Offline Entitlements`: integration tests (#2501) via NachoSoto
(@NachoSoto)
* `CustomerInfoManager`: don't cache offline `CustomerInfo` (#2378) via
NachoSoto (@NachoSoto)
* `DangerousSettings`: debug-only `forceServerErrors` (#2486) via
NachoSoto (@NachoSoto)
* `CocoapodsInstallation`: fixed `Xcode 14.3.0` issue (#2489) via
NachoSoto (@NachoSoto)
* `CarthageInstallation`: removed workaround (#2488) via NachoSoto
(@NachoSoto)

---------

Co-authored-by: NachoSoto <ignaciosoto90@gmail.com>

Similar to #2786 and #2777.
This test was verifying `getCustomerInfoCallCount`, but that gets updated asynchronously since #2533.

The correct version waits for it to be updated, then ensures that it doesn't get updated again after logging out.

Similar to #2786 and #2777.
This test was verifying `getCustomerInfoCallCount`, but that gets
updated asynchronously since #2533.

The correct version waits for it to be updated, then ensures that it
doesn't get updated again after logging out.

Fixes
https://app.circleci.com/pipelines/github/RevenueCat/purchases-ios/12762/workflows/a7e8c4ac-cecb-4c25-90ff-4390ed226bc4/jobs/88794