CXXCBC-336: Do not fallback to 8.8.8.8, if we cannot obtain system DNS server

.gitignore

@@ -4,6 +4,7 @@
 /cmake-build-report.tar.gz
 /.idea/vcs.xml
 /.idea/workspace.xml
+/.idea/editor.xml
 /.idea/sonarlint
 /.cache
 /.vs

core/io/dns_client.cxx

@@ -285,6 +285,10 @@ dns_client::query_srv(const std::string& name,
                       const dns_config& config,
                       utils::movable_function<void(dns_srv_response&&)>&& handler)
 {
+    if (config.nameserver().empty()) {
+        return handler({ {} });
+    }
+
     std::error_code ec;
     auto address = asio::ip::make_address(config.nameserver(), ec);
     if (ec) {

core/io/dns_config.cxx

@@ -53,6 +53,7 @@ load_resolv_conf()
     fixed_info = (FIXED_INFO*)malloc(sizeof(FIXED_INFO));
     if (fixed_info == NULL) {
         CB_LOG_WARNING("Error allocating memory needed to call GetNetworkParams");
+        return {};
     }
     buf = sizeof(FIXED_INFO);
 
@@ -63,6 +64,7 @@ load_resolv_conf()
         fixed_info = (FIXED_INFO*)malloc(buf);
         if (fixed_info == NULL) {
             CB_LOG_WARNING("Error allocating memory needed to call GetNetworkParams");
+            return {};
         }
     }
 
@@ -86,10 +88,12 @@ load_resolv_conf()
         }
     } else {
         CB_LOG_WARNING("GetNetworkParams failed with error: {}", ret);
+        return {};
     }
 
-    if (fixed_info)
+    if (fixed_info) {
         free(fixed_info);
+    }
 
     if (dns_servers.size() > 0) {
         CB_LOG_DEBUG(
@@ -153,10 +157,17 @@ dns_config::system_config()
         std::error_code ec;
         asio::ip::make_address(nameserver, ec);
         if (ec) {
-            CB_LOG_DEBUG("Unable to parse \"{}\" as a network address, fall back to \"{}\"", nameserver, default_nameserver);
-            nameserver = default_nameserver;
+            std::string extra_info{};
+#ifndef _WIN32
+            extra_info = fmt::format(" in \"{}\"", default_resolv_conf_path);
+#endif
+            CB_LOG_WARNING("System DNS detection failed: unable to parse \"{}\" as a network address{}. DNS-SRV will not work "
+                           "unless nameserver is specified explicitly in the options.",
+                           nameserver,
+                           extra_info);
+        } else {
+            instance.nameserver_ = nameserver;
         }
-        instance.nameserver_ = nameserver;
     });
 
     return instance;

core/io/dns_config.hxx

@@ -40,7 +40,7 @@ class dns_config
     [[nodiscard]] std::chrono::milliseconds timeout() const;
 
   private:
-    std::string nameserver_{ default_nameserver };
+    std::string nameserver_{};
     std::uint16_t port_{ default_port };
     std::chrono::milliseconds timeout_{ timeout_defaults::dns_srv_timeout };
 };