Capture MySQL thread identifiers for worker queries

The identifiers are captured for the worker task monitoring
purposes. They are used by the Web Dashboard for associating task
activities with the corresponding MySQL queries issued by the tasks.
The task monitoring page of the Dashboard has been extended to display
a value of the identifier (if it was set for the task).

src/mysql/MySqlConnection.cc

@@ -236,6 +236,7 @@ MYSQL* MySqlConnection::_connectHelper() {
         mysql_close(m);
         return c;
     }
+    _threadId = mysql_thread_id(m);
     return m;
 }
 

src/mysql/MySqlConnection.h

@@ -65,6 +65,8 @@ class MySqlConnection : boost::noncopyable {
     static bool checkConnection(mysql::MySqlConfig const& mysqlconfig);
 
     bool connected() const { return _isConnected; }
+    unsigned long threadId() const { return _threadId; }
+
     // instance destruction invalidates this return value
     MYSQL* getMySql() { return _mysql; }
     MySqlConfig const& getMySqlConfig() const { return *_sqlConfig; }
@@ -104,6 +106,7 @@ class MySqlConnection : boost::noncopyable {
     MYSQL* _mysql;
     MYSQL_RES* _mysql_res;
     bool _isConnected;
+    unsigned long _threadId = 0;  ///< 0 if not connected
     std::shared_ptr<MySqlConfig> _sqlConfig;
     bool _isExecuting;  ///< true during mysql_real_query and mysql_use_result
     bool _interrupted;  ///< true if cancellation requested

src/wbase/Task.cc

@@ -486,6 +486,7 @@ nlohmann::json Task::getJson() const {
     js["queryTime_msec"] = util::TimeUtils::tp2ms(_queryTime);
     js["finishTime_msec"] = util::TimeUtils::tp2ms(_finishTime);
     js["sizeSoFar"] = _totalSize;
+    js["mysqlThreadId"] = _mysqlThreadId.load();
     return js;
 }
 

src/wbase/Task.h

@@ -282,6 +282,11 @@ class Task : public util::CommandForThreadPool {
     /// Return a reference to the list of subchunk ids.
     const IntVector& getSubchunksVect() const { return _dbTblsAndSubchunks->subchunksVect; }
 
+    /// Set MySQL thread associated with a MySQL connection open before executing
+    /// task's queries. The identifier is sampled by the worker tasks monitoring
+    /// system in order to see what MySQL queries are being executed by tasks.
+    void setMySqlThreadId(unsigned long id) { _mysqlThreadId.store(id); }
+
 private:
     std::shared_ptr<UserQueryInfo> _userQueryInfo;  ///< Details common to Tasks in this UserQuery.
     std::shared_ptr<ChannelShared> _sendChannel;    ///< Send channel.
@@ -339,6 +344,8 @@ class Task : public util::CommandForThreadPool {
 
     /// Stores information on the query's resource usage.
     std::weak_ptr<wpublish::QueryStatistics> _queryStats;
+
+    std::atomic<unsigned long> _mysqlThreadId{0};  ///< 0 if not connected to MySQL
 };
 
 }  // namespace lsst::qserv::wbase

src/wdb/QueryRunner.cc

@@ -121,6 +121,7 @@ bool QueryRunner::_initConnection() {
         _multiError.push_back(error);
         return false;
     }
+    _task->setMySqlThreadId(_mysqlConn->threadId());
     return true;
 }
 

src/www/qserv/js/QservWorkerTasks.js

@@ -136,6 +136,7 @@ function(CSSLoader,
           <th class="sticky" style="text-align:right;">queried</th>
           <th class="sticky right-aligned"><elem style="color:red;">&rarr;</elem></th>
           <th class="sticky" style="text-align:right;">finished</th>
+          <th class="sticky" style="text-align:right;">MySQL thrd</th>
         </tr>    
       </thead>
       <caption class="updating">Loading...</caption>
@@ -331,6 +332,7 @@ function(CSSLoader,
   <td style="text-align:right;"><pre>${task.queryTime_msec ? QservWorkerTasks._timestamp2hhmmss(task.queryTime_msec) : ""}</pre></td>
   <th style="text-align:right;"><pre>${QservWorkerTasks._timestamps_diff2str(task.queryTime_msec, task.finishTime_msec, snapshotTime_msec)}</pre></th>
   <td style="text-align:right;"><pre>${task.finishTime_msec ? QservWorkerTasks._timestamp2hhmmss(task.finishTime_msec) : ""}</pre></td>
+  <td style="text-align:right;"><pre>${task.mysqlThreadId || "&nbsp;"}</pre></td>
 </tr>`;
                             rowspan++;
                             numTasksDisplayed++;