chore: Add read/write mode support for persistent store. (#206)

Adds support for controlling the read/write mode of the persistent
store.
Added an exception for failure to write to the persistent store.

Adds a test suite for the WriteThroughStore.
Almost the entire PR size is from tests.


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds persistence mode to `WriteThroughStore` with conditional
writes/init and improved apply error handling; wires mode in
`FDv2DataSystem`; introduces a comprehensive test suite.
> 
> - **Data system**:
> - `FDv2DataSystem`: Pass
`configuration.DataSystem.PersistentDataStoreMode` to
`WriteThroughStore`.
> - **Data store**:
>   - `WriteThroughStore`:
> - Add `_persistenceMode` and ctor param to control persistent store
behavior.
> - Init/Upsert/Apply write to persistent store only in `ReadWrite`
mode; `ReadOnly` skips writes.
> - Switch active read store to memory on first initializing payload;
`Initialized()` uses `_activeReadStore`.
> - In legacy persistence path, throw on failed apply (partial upserts)
with clear exception message.
> - **Tests**:
> - New `WriteThroughStoreTest`: covers constructor behavior,
init/get/getAll, upsert, apply (transactional vs legacy), store
switching, initialized/selector/status monitoring, dispose, and error
handling (including switch-to-memory on persistence failures).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0e23440. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: kinyoklion

pkgs/sdk/server/src/Internal/DataSystem/FDv2DataSystem.cs

@@ -60,7 +60,8 @@ public static FDv2DataSystem Create(Logger logger, Configuration configuration,
             var persistentStore =
                 dataSystemConfiguration.PersistentStore?.Build(clientContext.WithDataStoreUpdates(dataStoreUpdates));
 
-            var writeThroughStore = new WriteThroughStore(memoryStore, persistentStore);
+            var writeThroughStore = new WriteThroughStore(memoryStore, persistentStore,
+                dataSystemConfiguration.PersistentDataStoreMode);
 
             // TODO: When a persistent store is available we monitor it, is this a consistent choice.
             // TODO: Update the responses data store monitoring?

pkgs/sdk/server/src/Internal/DataSystem/WriteThroughStore.cs

@@ -26,15 +26,18 @@ internal class WriteThroughStore : IDataStore, ITransactionalDataStore
         /// </para>
         /// </summary>
         private readonly AtomicBoolean _hasReceivedAnInitializingPayload = new AtomicBoolean(false);
+        
+        private readonly DataSystemConfiguration.DataStoreMode _persistenceMode;
 
-        public WriteThroughStore(IDataStore memoryStore, IDataStore persistentStore)
+        public WriteThroughStore(IDataStore memoryStore, IDataStore persistentStore, DataSystemConfiguration.DataStoreMode persistenceMode)
         {
             _memoryStore = memoryStore;
             _txMemoryStore = (ITransactionalDataStore)_memoryStore;
             _persistentStore = persistentStore;
             _hasPersistence = persistentStore != null;
             // During initializations read will happen from the persistent store.
             _activeReadStore = _hasPersistence ? _persistentStore : _memoryStore;
+            _persistenceMode = persistenceMode;
         }
 
         public void Dispose()
@@ -56,8 +59,12 @@ private void Dispose(bool disposing)
         public void Init(DataStoreTypes.FullDataSet<DataStoreTypes.ItemDescriptor> allData)
         {
             _memoryStore.Init(allData);
-            _persistentStore?.Init(allData);
             MaybeSwitchStore();
+
+            if (_persistenceMode == DataSystemConfiguration.DataStoreMode.ReadWrite)
+            {
+                _persistentStore?.Init(allData);
+            }
         }
 
         public DataStoreTypes.ItemDescriptor? Get(DataStoreTypes.DataKind kind, string key)
@@ -73,7 +80,7 @@ public void Init(DataStoreTypes.FullDataSet<DataStoreTypes.ItemDescriptor> allDa
         public bool Upsert(DataStoreTypes.DataKind kind, string key, DataStoreTypes.ItemDescriptor item)
         {
             var result = _memoryStore.Upsert(kind, key, item);
-            if (_hasPersistence)
+            if (_hasPersistence && _persistenceMode == DataSystemConfiguration.DataStoreMode.ReadWrite)
             {
                 result &= _persistentStore.Upsert(kind, key, item);
             }
@@ -86,29 +93,30 @@ public bool Upsert(DataStoreTypes.DataKind kind, string key, DataStoreTypes.Item
 
         public bool Initialized()
         {
-            return _hasPersistence ? _persistentStore.Initialized() : _memoryStore.Initialized();
+            return _activeReadStore.Initialized();
         }
 
         public void Apply(DataStoreTypes.ChangeSet<DataStoreTypes.ItemDescriptor> changeSet)
         {
             _txMemoryStore.Apply(changeSet);
+            MaybeSwitchStore();
 
-            if (_hasPersistence)
+            if (!_hasPersistence || _persistenceMode != DataSystemConfiguration.DataStoreMode.ReadWrite) return;
+
+            if (_persistentStore is ITransactionalDataStore txPersistentStore)
             {
-                if (_persistentStore is ITransactionalDataStore txPersistentStore)
-                {
-                    txPersistentStore.Apply(changeSet);
-                }
-                else
+                txPersistentStore.Apply(changeSet);
+            }
+            else
+            {
+                // If an apply fails at init, that will throw on its own, but if it fails via an upsert, then
+                // we need to throw something to work with the current data source updates implementation.
+                if (!ApplyToLegacyPersistence(changeSet))
                 {
-                    if (!ApplyToLegacyPersistence(changeSet))
-                    {
-                        // TODO: Probably throw?
-                    }
+                    // The exception type doesn't matter here, as it will be converted to data store status.
+                    throw new Exception("Failure to apply data set to persistent store.");
                 }
             }
-
-            MaybeSwitchStore();
         }
 
         public Selector Selector => _txMemoryStore.Selector;