Preliminary support for Accept-Language: header

In the absence of the "userlang" query parameter in the URL, the value
of the "Accept-Language" header is used. However, it is assumed that
"Accept-Language" specifies a single language (rather than a comma
separated list of languages possibly weighted with quality values).

Example:

Accept-Language: fr
// should work

Accept-Language: fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5
// The requested language will be considered to be
// "fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5".
// The i18n code will fail to find resources for such a language
// and will use the default "en" instead.

src/server/request_context.cpp

@@ -195,7 +195,15 @@ std::string RequestContext::get_query() const {
 
 std::string RequestContext::get_user_language() const
 {
-  return get_optional_param<std::string>("userlang", "en");
+  try {
+    return get_argument("userlang");
+  } catch(const std::out_of_range&) {}
+
+  try {
+    return get_header("Accept-Language");
+  } catch(const std::out_of_range&) {}
+
+  return "en";
 }
 
 }

test/server.cpp

@@ -57,7 +57,6 @@ std::string removeEOLWhitespaceMarkers(const std::string& s)
   return std::regex_replace(s, pattern, "");
 }
 
-
 class ZimFileServer
 {
 public: // types
@@ -879,6 +878,79 @@ TEST_F(ServerTest, 500)
   EXPECT_EQ(r->body, expectedBody);
 }
 
+TEST_F(ServerTest, UserLanguageControl)
+{
+  struct TestData
+  {
+    const std::string url;
+    const std::string acceptLanguageHeader;
+    const std::string expectedH1;
+
+    operator TestContext() const
+    {
+      return TestContext{
+          {"url", url},
+          {"acceptLanguageHeader", acceptLanguageHeader},
+      };
+    }
+  };
+
+  const TestData testData[] = {
+    {
+      /*url*/ "/ROOT/zimfile/invalid-article",
+      /*Accept-Language:*/ "",
+      /* expected <h1> */ "Not Found"
+    },
+    {
+      /*url*/ "/ROOT/zimfile/invalid-article?userlang=en",
+      /*Accept-Language:*/ "",
+      /* expected <h1> */ "Not Found"
+    },
+    {
+      /*url*/ "/ROOT/zimfile/invalid-article?userlang=hy",
+      /*Accept-Language:*/ "",
+      /* expected <h1> */ "Սխալ հասցե"
+    },
+    {
+      /*url*/ "/ROOT/zimfile/invalid-article",
+      /*Accept-Language:*/ "*",
+      /* expected <h1> */ "Not Found"
+    },
+    {
+      /*url*/ "/ROOT/zimfile/invalid-article",
+      /*Accept-Language:*/ "hy",
+      /* expected <h1> */ "Սխալ հասցե"
+    },
+    {
+      // userlang query parameter takes precedence over Accept-Language
+      /*url*/ "/ROOT/zimfile/invalid-article?userlang=en",
+      /*Accept-Language:*/ "hy",
+      /* expected <h1> */ "Not Found"
+    },
+    {
+      // The value of the Accept-Language header is not currently parsed.
+      // In case of a comma separated list of languages (optionally weighted
+      // with quality values) the default (en) language is used instead.
+      /*url*/ "/ROOT/zimfile/invalid-article",
+      /*Accept-Language:*/ "hy;q=0.9, en;q=0.2",
+      /* expected <h1> */ "Not Found"
+    },
+  };
+
+  const std::regex h1Regex("<h1>(.+)</h1>");
+  for ( const auto& t : testData ) {
+    std::smatch h1Match;
+    Headers headers;
+    if ( !t.acceptLanguageHeader.empty() ) {
+      headers.insert({"Accept-Language", t.acceptLanguageHeader});
+    }
+    const auto r = zfs1_->GET(t.url.c_str(), headers);
+    std::regex_search(r->body, h1Match, h1Regex);
+    const std::string h1(h1Match[1]);
+    EXPECT_EQ(h1, t.expectedH1) << t;
+  }
+}
+
 TEST_F(ServerTest, RandomPageRedirectsToAnExistingArticle)
 {
   auto g = zfs1_->GET("/ROOT/random?content=zimfile");