Add new Tablet API to expose optionalSections metadata

Summary: This new API will unlock use cases such as enumerating all optional sections in the Nimble file. It will also empower `nimble_dump` to be able to provide relevant information about the optional sections.

Differential Revision: D67949242

dwio/nimble/tablet/TabletReader.h

@@ -251,6 +251,13 @@ class TabletReader {
       const StripeIdentifier& stripe,
       std::span<const uint32_t> streamIdentifiers) const;
 
+  std::unordered_map<
+      std::string,
+      std::tuple<uint64_t, uint32_t, CompressionType>>
+  optionalSections() const {
+    return optionalSections_;
+  }
+
   std::optional<Section> loadOptionalSection(
       const std::string& name,
       bool keepCache = false) const;

dwio/nimble/tablet/tests/TabletTests.cpp

@@ -561,6 +561,11 @@ TEST(TabletTests, OptionalSections) {
         file, useChaniedBuffers);
     nimble::TabletReader tablet{*pool, &readFile};
 
+    ASSERT_EQ(tablet.optionalSections().size(), 3);
+    ASSERT_TRUE(tablet.optionalSections().contains("section1"));
+    ASSERT_TRUE(tablet.optionalSections().contains("section2"));
+    ASSERT_TRUE(tablet.optionalSections().contains("section3"));
+
     auto check1 = [&]() {
       auto section = tablet.loadOptionalSection("section1");
       ASSERT_TRUE(section.has_value());
@@ -607,6 +612,8 @@ TEST(TabletTests, OptionalSectionsEmpty) {
         file, useChaniedBuffers);
     nimble::TabletReader tablet{*pool, &readFile};
 
+    ASSERT_TRUE(tablet.optionalSections().empty());
+
     auto section = tablet.loadOptionalSection("section1");
     ASSERT_FALSE(section.has_value());
   }