Remove header set for uninitialized response.

[shinrich]

One of our groups was seeing the following stack track

[ 00 ] libc-2.12.so        __GI___waitpid                                                               
[ 01 ] traffic_server      crash_logger_invoke                                                          ( Crash.cc:165 )
[ 02 ] libc-2.12.so                                                                                     
[ 03 ] traffic_server      mime_hdr_field_find                                                          ( MIME.cc:1319 )
[ 04 ] traffic_server      mime_hdr_prepare_for_value_set                                               ( MIME.cc:1692 )
[ 05 ] traffic_server      MIMEHdr::value_set(char const*, int, char const*, int)                       ( MIME.h:1419 )
[ 06 ] traffic_server      HttpSM::do_drain_request_body()                                              ( HttpSM.cc:5755 )
[ 07 ] traffic_server      HttpSM::set_next_state()                                                     ( HttpSM.cc:7601 )
[ 08 ] traffic_server      HttpSM::call_transact_and_set_next_state(void (*)(HttpTransact::State*))     ( HttpSM.cc:7335 )
[ 09 ] traffic_server      HttpSM::handle_api_return()                                                  ( HttpSM.cc:1661 )
[ 10 ] traffic_server      HttpSM::state_api_callout(int, void*)                                        ( HttpSM.cc:1596 )
[ 11 ] traffic_server      HttpSM::state_api_callback(int, void*)                                       ( HttpSM.cc:1363 )
[ 12 ] traffic_server      TSHttpTxnReenable                                                            ( InkAPI.cc:5921 )
[ 13 ] esi.so              globalHookHandler                                                            ( esi.cc:1659 )
[ 14 ] traffic_server      INKContInternal::handle_event(int, void*)                                    ( InkAPI.cc:1064 )
[ 15 ] traffic_server      Continuation::handleEvent(int, void*)                                        ( I_Continuation.h:153 )
[ 16 ] traffic_server      APIHook::invoke(int, void*)                                                  ( InkAPI.cc:1284 )
[ 17 ] traffic_server      HttpSM::state_api_callout(int, void*)                                        ( HttpSM.cc:1523 )
[ 18 ] traffic_server      HttpSM::do_api_callout_internal()                                            ( HttpSM.cc:5270 )
[ 19 ] traffic_server      HttpSM::do_api_callout()                                                     ( HttpSM.cc:438 )
[ 20 ] traffic_server      HttpSM::set_next_state()                                                     ( HttpSM.cc:7368 )
[ 21 ] traffic_server      HttpSM::call_transact_and_set_next_state(void (*)(HttpTransact::State*))     ( HttpSM.cc:7335 )
[ 22 ] traffic_server      HttpSM::handle_api_return()                                                  ( HttpSM.cc:1661 )
[ 23 ] traffic_server      HttpSM::state_api_callout(int, void*)                                        ( HttpSM.cc:1596 )
[ 24 ] traffic_server      HttpSM::state_api_callback(int, void*)                                       ( HttpSM.cc:1363 )
[ 25 ] traffic_server      TSHttpTxnReenable                                                            ( InkAPI.cc:5921 )
[ 26 ] libatscppapi.so.7   (anonymous namespace)::handleTransactionEvents(tsapi_cont*, TSEvent, void*)  ( utils_internal.cc:93 )
[ 27 ] traffic_server      INKContInternal::handle_event(int, void*)                                    ( InkAPI.cc:1064 )
[ 28 ] traffic_server      Continuation::handleEvent(int, void*)                                        ( I_Continuation.h:153 )
[ 29 ] traffic_server      APIHook::invoke(int, void*)                                                  ( InkAPI.cc:1284 )
[ 30 ] traffic_server      HttpSM::state_api_callout(int, void*)                                        ( HttpSM.cc:1523 )
[ 31 ] traffic_server      HttpSM::do_api_callout_internal()                                            ( HttpSM.cc:5270 )
[ 32 ] traffic_server      HttpSM::do_api_callout()                                                     ( HttpSM.cc:438 )
[ 33 ] traffic_server      HttpSM::set_next_state()                                                     ( HttpSM.cc:7368 )
[ 34 ] traffic_server      HttpSM::call_transact_and_set_next_state(void (*)(HttpTransact::State*))     ( HttpSM.cc:7335 )
[ 35 ] traffic_server      HttpSM::state_cache_open_read(int, void*)                                    ( HttpSM.cc:2671 )
[ 36 ] traffic_server      HttpSM::main_handler(int, void*)                                             ( HttpSM.cc:2733 )
[ 37 ] traffic_server      Continuation::handleEvent(int, void*)                                        ( I_Continuation.h:153 )
[ 38 ] traffic_server      HttpCacheSM::state_cache_open_read(int, void*)                               ( HttpCacheSM.cc:132 )
[ 39 ] traffic_server      Continuation::handleEvent(int, void*)                                        ( I_Continuation.h:153 )
[ 40 ] traffic_server      CacheVC::callcont(int)                                                       ( P_CacheInternal.h:653 )
[ 41 ] traffic_server      CacheVC::openReadStartHead(int, Event*)                                      ( CacheRead.cc:1158 )
[ 42 ] traffic_server      Continuation::handleEvent(int, void*)                                        ( I_Continuation.h:153 )
[ 43 ] traffic_server      CacheVC::handleReadDone(int, Event*)                                         ( Cache.cc:2469 )
[ 44 ] traffic_server      Continuation::handleEvent(int, void*)                                        ( I_Continuation.h:153 )
[ 45 ] traffic_server      AIOCallbackInternal::io_complete(int, void*)                                 ( P_AIO.h:117 )
[ 46 ] traffic_server      Continuation::handleEvent(int, void*)                                        ( I_Continuation.h:153 )
[ 47 ] traffic_server      EThread::process_event(Event*, int)                                          ( UnixEThread.cc:131 )
[ 48 ] traffic_server      EThread::process_queue(Queue<Event, Event::Link_link>*, int*, int*)          ( UnixEThread.cc:170 )
[ 49 ] traffic_server      EThread::execute_regular()                                                   ( UnixEThread.cc:230 )
[ 50 ] traffic_server      EThread::execute()                                                           ( UnixEThread.cc:325 )
[ 51 ] traffic_server      spawn_thread_internal                                                        ( Thread.cc:85 )
[ 52 ] libpthread-2.12.so  start_thread                                                                 

In the stack trace, t_state.hdr_info.client_response is not yet initialized, so eventually things crash because of null heaps. Looking through the response logic, I think it is sufficient the just set t_state.client_info.keep_alive. The build_response logic will look at that flag and set the connection header appropriately. We have been running with that patch internally and it seems to have fixed the issue.

[shinrich]

Updated the PR based on autest failures. We do need to set the Connection header, but there are two cases for transform and non-transform transactions. The response headers differ. So moved the do_drain_request_header into the if-else clauses and pass in the appropriate header to update.

[bryancall]

Cherry picked to 8.1.0