Optimized implementation of the worker task to initialize the FQDN once

This change increase stability of the worker for potential hiccups
of the DNS services during high frequency query processing.

src/wbase/Task.cc

@@ -85,6 +85,8 @@ size_t const MB_SIZE_BYTES = 1024 * 1024;
 
 namespace lsst::qserv::wbase {
 
+string const Task::_fqdn = util::get_current_host_fqdn();
+
 // Task::ChunkEqual functor
 bool Task::ChunkEqual::operator()(Task::Ptr const& x, Task::Ptr const& y) {
     if (!x || !y) {
@@ -142,14 +144,13 @@ Task::Task(TaskMsgPtr const& t, int fragmentNumber, shared_ptr<UserQueryInfo> co
     auto const resultDeliveryProtocol = workerConfig->resultDeliveryProtocol();
     _resultFileName = ::buildResultFileName(t);
     _resultFileAbsPath = ::buildResultFilePath(_resultFileName, workerConfig->resultsDirname());
-    auto const fqdn = util::get_current_host_fqdn();
     if (resultDeliveryProtocol == wconfig::ConfigValResultDeliveryProtocol::XROOT) {
         // NOTE: one extra '/' after the <host>[:<port>] spec is required to make
         // a "valid" XROOTD url.
-        _resultFileXrootUrl = "xroot://" + fqdn + ":" + to_string(workerConfig->resultsXrootdPort()) + "/" +
+        _resultFileXrootUrl = "xroot://" + _fqdn + ":" + to_string(workerConfig->resultsXrootdPort()) + "/" +
                               _resultFileAbsPath;
     } else if (resultDeliveryProtocol == wconfig::ConfigValResultDeliveryProtocol::HTTP) {
-        _resultFileHttpUrl = "http://" + fqdn + ":" + to_string(resultsHttpPort) + "/" + _resultFileName;
+        _resultFileHttpUrl = "http://" + _fqdn + ":" + to_string(resultsHttpPort) + "/" + _resultFileName;
     } else {
         throw runtime_error("wbase::Task::Task: unsupported results delivery protocol: " +
                             wconfig::ConfigValResultDeliveryProtocol::toString(resultDeliveryProtocol));

src/wbase/Task.h

@@ -381,6 +381,8 @@ class Task : public util::CommandForThreadPool {
     TIMEPOINT _bootedTime;
 
     bool _unitTest = false;  ///<
+
+    static std::string const _fqdn;  ///< Fully qualified domain name of the host. Acquired once at startup.
 };
 
 }  // namespace lsst::qserv::wbase