add a mutex around logging to db

src/zm_logger.cpp

@@ -538,6 +538,7 @@ void Logger::logPrint( bool hex, const char * const filepath, const int line, co
     char sql[ZM_SQL_MED_BUFSIZ];
     char escapedString[(strlen(syslogStart)*2)+1];
 
+    db_mutex.lock();
     mysql_real_escape_string( &dbconn, escapedString, syslogStart, strlen(syslogStart) );
 
     snprintf( sql, sizeof(sql), "insert into Logs ( TimeKey, Component, ServerId, Pid, Level, Code, Message, File, Line ) values ( %ld.%06ld, '%s', %d, %d, %d, '%s', '%s', '%s', %d )", timeVal.tv_sec, timeVal.tv_usec, mId.c_str(), staticConfig.SERVER_ID, tid, level, classString, escapedString, file, line );
@@ -547,6 +548,7 @@ void Logger::logPrint( bool hex, const char * const filepath, const int line, co
       Error( "Can't insert log entry: sql(%s) error(%s)", sql,  mysql_error( &dbconn ) );
       databaseLevel(tempDatabaseLevel);
     }
+    db_mutex.unlock();
   }
   if ( level <= mSyslogLevel ) {
     int priority = smSyslogPriorities[level];