Avoid to get manifest file size when recovering from it. (#6369)

Summary:
Right now RocksDB gets manifest file size before recovering from it. The information is available in LogReader. Use it instead to prevent one file system call.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6369

Test Plan: Run all existing tests

Differential Revision: D19714872

fbshipit-source-id: 0144be324d403c99e3da875ea2feccc8f64e883d

db/db_test.cc

@@ -2277,6 +2277,41 @@ TEST_F(DBTest, SnapshotFiles) {
     dbfull()->DisableFileDeletions();
   } while (ChangeCompactOptions());
 }
+
+TEST_F(DBTest, ReadonlyDBGetLiveManifestSize) {
+  do {
+    Options options = CurrentOptions();
+    options.level0_file_num_compaction_trigger = 2;
+    DestroyAndReopen(options);
+
+    ASSERT_OK(Put("foo", "bar"));
+    ASSERT_OK(Flush());
+    ASSERT_OK(Put("foo", "bar"));
+    ASSERT_OK(Flush());
+    ASSERT_OK(dbfull()->TEST_WaitForCompact());
+
+    Close();
+    ASSERT_OK(ReadOnlyReopen(options));
+
+    uint64_t manifest_size = 0;
+    std::vector<std::string> files;
+    dbfull()->GetLiveFiles(files, &manifest_size);
+
+    for (const std::string& f : files) {
+      uint64_t number = 0;
+      FileType type;
+      if (ParseFileName(f.substr(1), &number, &type)) {
+        if (type == kDescriptorFile) {
+          uint64_t size_on_disk;
+          env_->GetFileSize(dbname_ + "/" + f, &size_on_disk);
+          ASSERT_EQ(manifest_size, size_on_disk);
+          break;
+        }
+      }
+    }
+    Close();
+  } while (ChangeCompactOptions());
+}
 #endif
 
 TEST_F(DBTest, PurgeInfoLogs) {

db/log_reader.h

@@ -93,6 +93,10 @@ class Reader {
 
   uint64_t GetLogNumber() const { return log_number_; }
 
+  size_t GetReadOffset() const {
+    return static_cast<size_t>(end_of_buffer_offset_);
+  }
+
  protected:
   std::shared_ptr<Logger> info_log_;
   const std::unique_ptr<SequentialFileReader> file_;

db/version_set.cc

@@ -4403,12 +4403,6 @@ Status VersionSet::Recover(
         new SequentialFileReader(std::move(manifest_file), manifest_path,
                                  db_options_->log_readahead_size));
   }
-  uint64_t current_manifest_file_size;
-  s = fs_->GetFileSize(manifest_path, IOOptions(), &current_manifest_file_size,
-                       nullptr);
-  if (!s.ok()) {
-    return s;
-  }
 
   std::unordered_map<uint32_t, std::unique_ptr<BaseReferencedVersionBuilder>>
       builders;
@@ -4429,6 +4423,7 @@ Status VersionSet::Recover(
   builders.insert(
       std::make_pair(0, std::unique_ptr<BaseReferencedVersionBuilder>(
                             new BaseReferencedVersionBuilder(default_cfd))));
+  uint64_t current_manifest_file_size = 0;
   VersionEditParams version_edit_params;
   {
     VersionSet::LogReporter reporter;
@@ -4441,6 +4436,8 @@ Status VersionSet::Recover(
     s = ReadAndRecover(&reader, &read_buffer, cf_name_to_options,
                        column_families_not_found, builders,
                        &version_edit_params, db_id);
+    current_manifest_file_size = reader.GetReadOffset();
+    assert(current_manifest_file_size != 0);
   }
 
   if (s.ok()) {