Delete C++ Firestore objects when C# has no more references to them.

This was achieved by using the CppInstanceManager, which provides this functionality elsewhere in the Unity SDK. When running UIHandlerAutomated, this eliminates all 11 leaks from Firestore::GetFirestore().

PiperOrigin-RevId: 320609048

Author: Googler

firestore/src/include/firebase/csharp/firestore_instance_management.cc

@@ -0,0 +1,39 @@
+#include "firebase/csharp/firestore_instance_management.h"
+
+#include "firebase/app/client/unity/src/cpp_instance_manager.h"
+#include "firebase/firestore.h"
+
+namespace firebase {
+namespace firestore {
+namespace csharp {
+
+namespace {
+
+CppInstanceManager<Firestore>& GetFirestoreInstanceManager() {
+  // Allocate the CppInstanceManager on the heap to prevent its destructor
+  // from executing (go/totw/110#the-fix-safe-initialization-no-destruction).
+  static CppInstanceManager<Firestore>* firestore_instance_manager =
+      new CppInstanceManager<Firestore>();
+  return *firestore_instance_manager;
+}
+
+}  // namespace
+
+Firestore* GetFirestoreInstance(App* app) {
+  auto& firestore_instance_manager = GetFirestoreInstanceManager();
+  // Acquire the lock used internally by CppInstanceManager::ReleaseReference()
+  // to avoid racing with deletion of Firestore instances.
+  MutexLock lock(firestore_instance_manager.mutex());
+  Firestore* instance =
+      Firestore::GetInstance(app, /*init_result_out=*/nullptr);
+  firestore_instance_manager.AddReference(instance);
+  return instance;
+}
+
+void ReleaseFirestoreInstance(Firestore* firestore) {
+  GetFirestoreInstanceManager().ReleaseReference(firestore);
+}
+
+}  // namespace csharp
+}  // namespace firestore
+}  // namespace firebase

firestore/src/include/firebase/csharp/firestore_instance_management.h

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+#ifndef FIREBASE_FIRESTORE_CLIENT_CPP_SRC_INCLUDE_FIREBASE_CSHARP_FIRESTORE_INSTANCE_MANAGEMENT_H_
+#define FIREBASE_FIRESTORE_CLIENT_CPP_SRC_INCLUDE_FIREBASE_CSHARP_FIRESTORE_INSTANCE_MANAGEMENT_H_
+
+namespace firebase {
+
+class App;
+
+namespace firestore {
+
+class Firestore;
+
+namespace csharp {
+
+/**
+ * Returns the Firestore instance for the given App, creating it if necessary.
+ * This method is merely a wrapper around Firestore::GetInstance() that
+ * increments a reference count each time a given Firestore pointer is returned.
+ * The caller must call ReleaseFirestoreInstance() with the returned pointer
+ * once it is no longer referenced to ensure proper garbage collection.
+ */
+Firestore* GetFirestoreInstance(App* app);
+
+/**
+ * Decrements the reference count of the given Firestore, deleting it if the
+ * reference count becomes zero. The given Firestore pointer must have been
+ * returned by a previous invocation of GetFirestoreInstance().
+ */
+void ReleaseFirestoreInstance(Firestore* firestore);
+
+}  // namespace csharp
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIREBASE_FIRESTORE_CLIENT_CPP_SRC_INCLUDE_FIREBASE_CSHARP_FIRESTORE_INSTANCE_MANAGEMENT_H_