FIX: Use the same mode for chinese search when indexing and querying. (#14780)

The `白名单` term becomes `名单 白名单` after it is processed by
cppjieba in :query mode. However, `白名单` is not tokenized as such by cppjieba when it
appears in a string of text. Therefore, this may lead to failed matches as
the search data generated while indexing may not contain all of the
terms generated by :query mode. We've decided to maintain parity for now
such that both indexing and querying uses the same :mix mode. This may
lead to less accurate search but our plan is to properly support CJK
search in the future.

lib/search.rb

@@ -69,19 +69,17 @@ class Search
       SiteSetting.search_tokenize_chinese_japanese_korean
   end
 
-  def self.prepare_data(search_data, purpose = :query)
-    purpose ||= :query
-
+  def self.prepare_data(search_data, purpose = nil)
     data = search_data.dup
     data.force_encoding("UTF-8")
+
     if purpose != :topic
       # TODO cppjieba_rb is designed for chinese, we need something else for Japanese
       # Korean appears to be safe cause words are already space separated
       # For Japanese we should investigate using kakasi
       if segment_cjk?
         require 'cppjieba_rb' unless defined? CppjiebaRb
-        mode = (purpose == :query ? :query : :mix)
-        data = CppjiebaRb.segment(search_data, mode: mode)
+        data = CppjiebaRb.segment(search_data, mode: :mix)
 
         # TODO: we still want to tokenize here but the current stopword list is too wide
         # in cppjieba leading to words such as volume to be skipped. PG already has an English

spec/components/search_spec.rb

@@ -1107,7 +1107,7 @@ describe Search do
     it 'splits English / Chinese and filter out stop words' do
       SiteSetting.default_locale = 'zh_CN'
       data = Search.prepare_data(sentence).split(' ')
-      expect(data).to eq(["Discourse", "中国", "基础", "设施", "基础设施", "网络", "正在", "组装"])
+      expect(data).to eq(["Discourse", "中国", "基础设施", "网络", "正在", "组装"])
     end
 
     it 'splits for indexing and filter out stop words' do
@@ -1119,12 +1119,6 @@ describe Search do
     it 'splits English / Traditional Chinese and filter out stop words' do
       SiteSetting.default_locale = 'zh_TW'
       data = Search.prepare_data(sentence_t).split(' ')
-      expect(data).to eq(["Discourse", "太平", "平山", "太平山", "森林", "遊樂區"])
-    end
-
-    it 'splits for indexing and filter out stop words' do
-      SiteSetting.default_locale = 'zh_TW'
-      data = Search.prepare_data(sentence_t, :index).split(' ')
       expect(data).to eq(["Discourse", "太平山", "森林", "遊樂區"])
     end