Avoid reading from files in construction time of the protobuf domain.

PiperOrigin-RevId: 792721029

fuzztest/internal/domains/protobuf_domain_impl.h

diff --git a/fuzztest/internal/domains/protobuf_domain_impl.h b/fuzztest/internal/domains/protobuf_domain_impl.h
index 3086e5f..159e436 100644
--- a/fuzztest/internal/domains/protobuf_domain_impl.h
+++ b/fuzztest/internal/domains/protobuf_domain_impl.h
@@ -344,6 +344,31 @@
     return caches_->SetFields(descriptor, GetProtobufFields(descriptor));
   }
 
+  static const std::vector<const FieldDescriptor*>& GetProtobufFields(
+      const ProtoDescriptor* descriptor) {
+    ABSL_CONST_INIT static absl::Mutex mutex(absl::kConstInit);
+    static absl::NoDestructor<absl::flat_hash_map<
+        const ProtoDescriptor*, std::vector<const FieldDescriptor*>>>
+        descriptor_to_fields ABSL_GUARDED_BY(mutex);
+    {
+      absl::MutexLock l(&mutex);
+      auto it = descriptor_to_fields->find(descriptor);
+      if (it != descriptor_to_fields->end()) return it->second;
+    }
+    std::vector<const FieldDescriptor*> fields;
+    fields.reserve(descriptor->field_count());
+    for (int i = 0; i < descriptor->field_count(); ++i) {
+      fields.push_back(descriptor->field(i));
+    }
+    absl::MutexLock l(&mutex);
+    if (ShouldEnumerateExtensions(descriptor)) {
+      descriptor->file()->pool()->FindAllExtensions(descriptor, &fields);
+    }
+    auto [it, _] =
+        descriptor_to_fields->insert({descriptor, std::move(fields)});
+    return it->second;
+  }
+
  private:
   static bool IsMessageSetFuzzingEnabled() {
     // TODO(b/413402115): Create protobuf domain API enabling MessageSet fuzzing
@@ -377,31 +402,6 @@
     return !IsMessageSet(descriptor);
   }
 
-  static const std::vector<const FieldDescriptor*>& GetProtobufFields(
-      const ProtoDescriptor* descriptor) {
-    ABSL_CONST_INIT static absl::Mutex mutex(absl::kConstInit);
-    static absl::NoDestructor<absl::flat_hash_map<
-        const ProtoDescriptor*, std::vector<const FieldDescriptor*>>>
-        descriptor_to_fields ABSL_GUARDED_BY(mutex);
-    {
-      absl::MutexLock l(&mutex);
-      auto it = descriptor_to_fields->find(descriptor);
-      if (it != descriptor_to_fields->end()) return it->second;
-    }
-    std::vector<const FieldDescriptor*> fields;
-    fields.reserve(descriptor->field_count());
-    for (int i = 0; i < descriptor->field_count(); ++i) {
-      fields.push_back(descriptor->field(i));
-    }
-    absl::MutexLock l(&mutex);
-    if (ShouldEnumerateExtensions(descriptor)) {
-      descriptor->file()->pool()->FindAllExtensions(descriptor, &fields);
-    }
-    auto [it, _] =
-        descriptor_to_fields->insert({descriptor, std::move(fields)});
-    return it->second;
-  }
-
   // All caches for the policy that contain cached information about subfields
   // and can be passed down to the subfield policies recursively.
   class RecursiveFieldsCaches {
@@ -595,7 +595,7 @@
         always_set_oneofs_(),
         uncustomizable_oneofs_(),
         unset_oneof_fields_(),
-        fields_cache_(policy_.GetFields(prototype_.Get()->GetDescriptor())) {}
+        fields_cache_() {}
 
   ProtobufDomainUntypedImpl(const ProtobufDomainUntypedImpl& other)
       : prototype_(other.prototype_),
@@ -1490,9 +1490,14 @@
     return field;
   }
 
-  auto GetFieldCount() const { return fields_cache_.size(); }
+  auto GetFieldCount() const { return GetProtobufFields().size(); }
 
   const std::vector<const FieldDescriptor*>& GetProtobufFields() const {
+    if (fields_cache_.empty()) {
+      absl::MutexLock l(&mutex_);
+      fields_cache_ = ProtoPolicy<Message>::GetProtobufFields(
+          prototype_.Get()->GetDescriptor());
+    }
     return fields_cache_;
   }
 
@@ -2024,7 +2029,7 @@
   absl::flat_hash_set<int> always_set_oneofs_;
   absl::flat_hash_set<int> uncustomizable_oneofs_;
   absl::flat_hash_set<int> unset_oneof_fields_;
-  std::vector<const FieldDescriptor*> fields_cache_;
+  mutable std::vector<const FieldDescriptor*> fields_cache_;
 };
 
 // Domain for `T` where `T` is a Protobuf message type.