GH-39339: [C++] Add ForceCachedHierarchicalNamespaceSupport to help with testing

cpp/src/arrow/filesystem/azurefs.cc

@@ -934,14 +934,38 @@ class AzureFileSystem::Impl {
         break;
     }
     ARROW_ASSIGN_OR_RAISE(
-        cached_hns_support_,
+        auto hns_support,
         internal::CheckIfHierarchicalNamespaceIsEnabled(adlfs_client, options_));
-    DCHECK_NE(cached_hns_support_, HNSSupport::kUnknown);
-    // Caller should handle kContainerNotFound case appropriately.
-    return cached_hns_support_;
+    DCHECK_NE(hns_support, HNSSupport::kUnknown);
+    if (hns_support == HNSSupport::kContainerNotFound) {
+      // Caller should handle kContainerNotFound case appropriately as it knows the
+      // container this refers to, but the cached value in that case should remain
+      // kUnknown before we get a CheckIfHierarchicalNamespaceIsEnabled result that
+      // is not kContainerNotFound.
+      cached_hns_support_ = HNSSupport::kUnknown;
+    } else {
+      cached_hns_support_ = hns_support;
+    }
+    return hns_support;
   }
 
  public:
+  /// This is used from unit tests to ensure we perform operations on all the
+  /// possible states of cached_hns_support_.
+  void ForceCachedHierarchicalNamespaceSupport(int support) {
+    auto hns_support = static_cast<HNSSupport>(support);
+    switch (hns_support) {
+      case HNSSupport::kUnknown:
+      case HNSSupport::kContainerNotFound:
+      case HNSSupport::kDisabled:
+      case HNSSupport::kEnabled:
+        cached_hns_support_ = hns_support;
+        return;
+    }
+    // This is reachable if an invalid int is cast to enum class HNSSupport.
+    DCHECK(false) << "Invalid enum HierarchicalNamespaceSupport value.";
+  }
+
   Result<FileInfo> GetFileInfo(const AzureLocation& location) {
     if (location.container.empty()) {
       DCHECK(location.path.empty());
@@ -1553,6 +1577,12 @@ AzureFileSystem::AzureFileSystem(std::unique_ptr<Impl>&& impl)
   default_async_is_sync_ = false;
 }
 
+AzureFileSystem* AzureFileSystem::ForceCachedHierarchicalNamespaceSupport(
+    int hns_support) {
+  impl_->ForceCachedHierarchicalNamespaceSupport(hns_support);
+  return this;
+}
+
 Result<std::shared_ptr<AzureFileSystem>> AzureFileSystem::Make(
     const AzureOptions& options, const io::IOContext& io_context) {
   ARROW_ASSIGN_OR_RAISE(auto impl, AzureFileSystem::Impl::Make(options, io_context));

cpp/src/arrow/filesystem/azurefs.h

@@ -154,6 +154,9 @@ class ARROW_EXPORT AzureFileSystem : public FileSystem {
 
   explicit AzureFileSystem(std::unique_ptr<Impl>&& impl);
 
+ protected:
+  AzureFileSystem* ForceCachedHierarchicalNamespaceSupport(int hns_support);
+
  public:
   ~AzureFileSystem() override = default;