[RFA 2/2][master only] gdb/windows-nat.c: Get rid of main_thread_id global

[Joel Brobecker <brobecker@adacore.com>]

Hello,

This global is meant to point to the "main" thread of execution of
the program we are debugging. It is set when attaching to a process
or when receiving a CREATE_PROCESS_DEBUG_EVENT event. The theory at
the time was that this was also going to be the thread receiving
the EXIT_PROCESS_DEBUG_EVENT event.

Unfortunately, we have discovered since then that this is actually
not guaranteed. What this means in practice is that there is moderate
risk that main_thread_id refers to a thread which no longer exists.

This global is used in 3 situations:
  - OUTPUT_DEBUG_STRING_EVENT
  - LOAD_DLL_DEBUG_EVENT
  - UNLOAD_DLL_DEBUG_EVENT

It's not clear why we would need to use the main_thread_id in those cases
instead of using the thread ID provided by the kernel events itself.
So this patch implements this approach, which then allows us to delete
the main_thread_id global.

gdb/testsuite:

	* windows-nat.c (main_thread_id): Delete.
	(handle_output_debug_string): Replace main_thread_id by
	current_event.dwThreadId.
	(fake_create_process): Likewise.
	(get_windows_debug_event) <CREATE_PROCESS_DEBUG_EVENT>:
	Do not set main_thread_id.
	<LOAD_DLL_DEBUG_EVENT>: Replace main_thread_id by
	current_event.dwThreadId.
	<UNLOAD_DLL_DEBUG_EVENT>: Likewise.

Tested on x86-windows and x86_64-windows using AdaCore's testsuite.
I'd like to observe this patch in our nightly testing environment
for at least a week before pushing it to master here. But it doesn't
preclude us from reviewing the patch, IMO.

Thoughts? OK for master (after a week of AdaCore testing)?

Thanks
-- 
Joel

---
 gdb/windows-nat.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index 039eb13..d188ed0 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -237,7 +237,6 @@ static DEBUG_EVENT current_event;	/* The current debug event from
 					   WaitForDebugEvent */
 static HANDLE current_process_handle;	/* Currently executing process */
 static windows_thread_info *current_thread;	/* Info on currently selected thread */
-static DWORD main_thread_id;		/* Thread ID of the main thread */
 
 /* Counts of things.  */
 static int exception_count = 0;
@@ -1026,7 +1025,7 @@ handle_output_debug_string (struct target_waitstatus *ourstatus)
 	  ourstatus->kind = TARGET_WAITKIND_STOPPED;
 	  retval = strtoul (p, &p, 0);
 	  if (!retval)
-	    retval = main_thread_id;
+	    retval = current_event.dwThreadId;
 	  else if ((x = (LPCVOID) (uintptr_t) strtoull (p, NULL, 0))
 		   && ReadProcessMemory (current_process_handle, x,
 					 &saved_context,
@@ -1402,13 +1401,12 @@ fake_create_process (void)
        (unsigned) GetLastError ());
       /*  We can not debug anything in that case.  */
     }
-  main_thread_id = current_event.dwThreadId;
   current_thread
     = windows_add_thread (ptid_t (current_event.dwProcessId, 0,
 				  current_event.dwThreadId),
 			  current_event.u.CreateThread.hThread,
 			  current_event.u.CreateThread.lpThreadLocalBase);
-  return main_thread_id;
+  return current_event.dwThreadId;
 }
 
 void
@@ -1604,7 +1602,6 @@ get_windows_debug_event (struct target_ops *ops,
 	break;
 
       current_process_handle = current_event.u.CreateProcessInfo.hProcess;
-      main_thread_id = current_event.dwThreadId;
       /* Add the main thread.  */
       th = windows_add_thread
         (ptid_t (current_event.dwProcessId, 0,
@@ -1648,7 +1645,7 @@ get_windows_debug_event (struct target_ops *ops,
       catch_errors (handle_load_dll);
       ourstatus->kind = TARGET_WAITKIND_LOADED;
       ourstatus->value.integer = 0;
-      thread_id = main_thread_id;
+      thread_id = current_event.dwThreadId;
       break;
 
     case UNLOAD_DLL_DEBUG_EVENT:
@@ -1661,7 +1658,7 @@ get_windows_debug_event (struct target_ops *ops,
       catch_errors (handle_unload_dll);
       ourstatus->kind = TARGET_WAITKIND_LOADED;
       ourstatus->value.integer = 0;
-      thread_id = main_thread_id;
+      thread_id = current_event.dwThreadId;
       break;
 
     case EXCEPTION_DEBUG_EVENT:
--