Use ReadFileToString() to get content from IDENTITY file (#6365)

Summary:
Right now when reading IDENTITY file, we use a very similar logic as ReadFileToString() while it does an extra file size check, which may be expensive in some file systems. There is no reason to duplicate the logic. Use ReadFileToString() instead.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6365

Test Plan: RUn all existing tests.

Differential Revision: D19709399

fbshipit-source-id: 3bac31f3b2471f98a0d2694278b41e9cd34040fe

db/db_impl/db_impl.cc

@@ -3354,31 +3354,12 @@ Status DBImpl::GetDbIdentity(std::string& identity) const {
 Status DBImpl::GetDbIdentityFromIdentityFile(std::string* identity) const {
   std::string idfilename = IdentityFileName(dbname_);
   const FileOptions soptions;
-  std::unique_ptr<SequentialFileReader> id_file_reader;
-  Status s;
-  {
-    std::unique_ptr<FSSequentialFile> idfile;
-    s = fs_->NewSequentialFile(idfilename, soptions, &idfile, nullptr);
-    if (!s.ok()) {
-      return s;
-    }
-    id_file_reader.reset(
-        new SequentialFileReader(std::move(idfile), idfilename));
-  }
 
-  uint64_t file_size;
-  s = fs_->GetFileSize(idfilename, IOOptions(), &file_size, nullptr);
+  Status s = ReadFileToString(fs_.get(), idfilename, identity);
   if (!s.ok()) {
     return s;
   }
-  char* buffer =
-      reinterpret_cast<char*>(alloca(static_cast<size_t>(file_size)));
-  Slice id;
-  s = id_file_reader->Read(static_cast<size_t>(file_size), &id, buffer);
-  if (!s.ok()) {
-    return s;
-  }
-  identity->assign(id.ToString());
+
   // If last character is '\n' remove it from identity
   if (identity->size() > 0 && identity->back() == '\n') {
     identity->pop_back();