Sparing use of "Set-Cookie: userlang=..." header

Server adds the "Set-Cookie: userlang=..." header to the response only
if the "userlang" cookie is not already present with the same value.

src/server/request_context.cpp

@@ -212,26 +212,32 @@ std::string RequestContext::get_header(const std::string& name) const {
 
 std::string RequestContext::get_user_language() const
 {
-  return userlang;
+  return userlang.lang;
 }
 
-std::string RequestContext::determine_user_language() const
+bool RequestContext::user_language_comes_from_cookie() const
+{
+  return userlang.selectedBy == UserLanguage::SelectorKind::COOKIE;
+}
+
+RequestContext::UserLanguage RequestContext::determine_user_language() const
 {
   try {
-    return get_argument("userlang");
+    return {UserLanguage::SelectorKind::QUERY_PARAM, get_argument("userlang")};
   } catch(const std::out_of_range&) {}
 
   try {
-     return cookies.at("userlang");
+     return {UserLanguage::SelectorKind::COOKIE, cookies.at("userlang")};
   } catch(const std::out_of_range&) {}
 
   try {
     const std::string acceptLanguage = get_header("Accept-Language");
     const auto userLangPrefs = parseUserLanguagePreferences(acceptLanguage);
-    return selectMostSuitableLanguage(userLangPrefs);
+    const auto lang = selectMostSuitableLanguage(userLangPrefs);
+    return {UserLanguage::SelectorKind::ACCEPT_LANGUAGE_HEADER, lang};
   } catch(const std::out_of_range&) {}
 
-  return "en";
+  return {UserLanguage::SelectorKind::DEFAULT, "en"};
 }
 
 std::string RequestContext::get_requested_format() const

src/server/request_context.h

@@ -118,6 +118,23 @@ class RequestContext {
     std::string get_user_language() const;
     std::string get_requested_format() const;
 
+    bool user_language_comes_from_cookie() const;
+
+  private: // types
+    struct UserLanguage
+    {
+      enum SelectorKind
+      {
+        QUERY_PARAM,
+        COOKIE,
+        ACCEPT_LANGUAGE_HEADER,
+        DEFAULT
+      };
+
+      SelectorKind selectedBy;
+      std::string  lang;
+    };
+
   private: // data
     std::string full_url;
     std::string url;
@@ -132,10 +149,10 @@ class RequestContext {
     std::map<std::string, std::vector<std::string>> arguments;
     std::map<std::string, std::string> cookies;
     std::string queryString;
-    std::string userlang;
+    UserLanguage userlang;
 
   private: // functions
-    std::string determine_user_language() const;
+    UserLanguage determine_user_language() const;
 
     static MHD_Result fill_header(void *, enum MHD_ValueKind, const char*, const char*);
     static MHD_Result fill_cookie(void *, enum MHD_ValueKind, const char*, const char*);

src/server/response.cpp

@@ -387,8 +387,10 @@ MHD_Result Response::send(const RequestContext& request, MHD_Connection* connect
     MHD_add_response_header(response, p.first.c_str(), p.second.c_str());
   }
 
-  const std::string cookie = "userlang=" + request.get_user_language();
-  MHD_add_response_header(response, MHD_HTTP_HEADER_SET_COOKIE, cookie.c_str());
+  if ( ! request.user_language_comes_from_cookie() ) {
+    const std::string cookie = "userlang=" + request.get_user_language();
+    MHD_add_response_header(response, MHD_HTTP_HEADER_SET_COOKIE, cookie.c_str());
+  }
 
   if (m_returnCode == MHD_HTTP_OK && m_byteRange.kind() == ByteRange::RESOLVED_PARTIAL_CONTENT)
     m_returnCode = MHD_HTTP_PARTIAL_CONTENT;

test/server.cpp

@@ -1047,7 +1047,7 @@ TEST_F(ServerTest, UserLanguageControl)
       /*url*/ "/ROOT/content/zimfile/invalid-article",
       /*Accept-Language:*/ "",
       /*Request Cookie:*/       "userlang=test",
-      /*Response Set-Cookie:*/  "userlang=test",
+      /*Response Set-Cookie:*/  NO_COOKIE,
       /* expected <h1> */ "[I18N TESTING] Content not found, but at least the server is alive"
     },
     {
@@ -1055,7 +1055,7 @@ TEST_F(ServerTest, UserLanguageControl)
       /*url*/ "/ROOT/content/zimfile/invalid-article",
       /*Accept-Language:*/ "",
       /*Request Cookie:*/       "anothercookie=123; userlang=test",
-      /*Response Set-Cookie:*/  "userlang=test",
+      /*Response Set-Cookie:*/  NO_COOKIE,
       /* expected <h1> */ "[I18N TESTING] Content not found, but at least the server is alive"
     },
     {
@@ -1063,7 +1063,7 @@ TEST_F(ServerTest, UserLanguageControl)
       /*url*/ "/ROOT/content/zimfile/invalid-article",
       /*Accept-Language:*/ "",
       /*Request Cookie:*/       "userlang=test; anothercookie=abc",
-      /*Response Set-Cookie:*/  "userlang=test",
+      /*Response Set-Cookie:*/  NO_COOKIE,
       /* expected <h1> */ "[I18N TESTING] Content not found, but at least the server is alive"
     },
     {
@@ -1071,7 +1071,7 @@ TEST_F(ServerTest, UserLanguageControl)
       /*url*/ "/ROOT/content/zimfile/invalid-article",
       /*Accept-Language:*/ "",
       /*Request Cookie:*/       "cookie1=abc; userlang=test; cookie2=xyz",
-      /*Response Set-Cookie:*/  "userlang=test",
+      /*Response Set-Cookie:*/  NO_COOKIE,
       /* expected <h1> */ "[I18N TESTING] Content not found, but at least the server is alive"
     },
     {
@@ -1079,7 +1079,7 @@ TEST_F(ServerTest, UserLanguageControl)
       /*url*/ "/ROOT/content/zimfile/invalid-article",
       /*Accept-Language:*/ "",
       /*Request Cookie:*/       "cookie1=abc; userlang=en; userlang=test; cookie2=xyz",
-      /*Response Set-Cookie:*/  "userlang=test",
+      /*Response Set-Cookie:*/  NO_COOKIE,
       /* expected <h1> */ "[I18N TESTING] Content not found, but at least the server is alive"
     },
     {
@@ -1103,7 +1103,7 @@ TEST_F(ServerTest, UserLanguageControl)
       /*url*/ "/ROOT/content/zimfile/invalid-article",
       /*Accept-Language:*/ "test",
       /*Request Cookie:*/       "userlang=en",
-      /*Response Set-Cookie:*/  "userlang=en",
+      /*Response Set-Cookie:*/  NO_COOKIE,
       /* expected <h1> */ "Not Found"
     },
     {