Re: [RFA/win32] Improve C-c handling when process in different console

[Joel Brobecker <brobecker at adacore dot com>]

Hi Chris,

> 2008-10-02  Nicolas Roche  <roche@adacore.com>
> 
>         * win32-nat.c (check_for_DebugBreakProcess): New function.
>         (ctrl_c_handler): New function.
>         (win32_wait): Register ctrl_c_handler as Ctrl-C handler if the inferior
>         is run in a separate console.
>         (_initialize_win32_nat): Check for DebugBreakProcess in kernel32.dll.

Here is a new version of the patch. I had to change a couple of things:

  1. Remove the WINBASEAPI when declaring kernel32_DebugBreakProcess;
     My understanding is that this attribute doesn't make sense at all
     for a static variable.

  2. Cast the result of GetProcAddress to "void *" in order to avoid
     a compiler warning. This is consistent with what we do in the other
     GetProcAddress calls.

This patch has been working very well for us over the past year.
Do you think you could take a look to see if we can have something in
before we roll 7.0 out?

Thanks,
-- 
Joel

Index: windows-nat.c
===================================================================
--- windows-nat.c	(.../branches/gdb/FSF/current/gdb/windows-nat.c)	(revision 146347)
+++ windows-nat.c	(.../trunk/gdb/gdb-head/gdb/windows-nat.c)	(revision 146347)
@@ -162,6 +162,14 @@ static int debug_memory = 0;		/* show ta
 static int debug_exceptions = 0;	/* show target exceptions */
 static int useshell = 0;		/* use shell for subprocesses */
 
+/* Set to non-zero if kernel32.dll provides the DebugBreakProcess
+   function (see the check_for_DebugBreakProcess function).  */
+static int has_DebugBreakProcess = 0;
+
+/* If has_DebugBreakProcess is non-zero then it contains the address of the
+   kernel32.dll DebugBreakProcess function.  */
+static BOOL WINAPI (*kernel32_DebugBreakProcess) (HANDLE);
+
 /* This vector maps GDB's idea of a register's number into an offset
    in the windows exception context vector.
 
@@ -211,6 +219,28 @@ windows_set_context_register_offsets (co
   mappings = offsets;
 }
 
+/* Check if system DLL kernel32.dll contains the DebugBreakProcess function.
+   If this is the case then set has_DebugBreakProcess to 1 and store the 
+   address of the function in kernel32_DebugBreakProcess.  Note that 
+   DebugBreakProcess should be available starting with Windows XP.  It gives
+   a convenient way to propagate a Ctrl-C event from GDB to the inferior when
+   they are running in separate consoles (see windows_wait).  This is the case 
+   when we attach to a process or new-console is set to 1.  */
+static void 
+check_for_DebugBreakProcess ()
+{
+  HMODULE kernel32_module_handle;
+
+  kernel32_module_handle = LoadLibrary ("kernel32.dll");
+  if (kernel32_module_handle) 
+    {
+      kernel32_DebugBreakProcess = 
+        (void *) GetProcAddress (kernel32_module_handle, "DebugBreakProcess");
+      if (kernel32_DebugBreakProcess != NULL) 
+        has_DebugBreakProcess = 1;
+    }
+}
+
 static void
 check (BOOL ok, const char *file, int line)
 {
@@ -1258,9 +1295,39 @@ windows_resume (struct target_ops *ops,
     windows_continue (continue_status, ptid_get_tid (ptid));
 }
 
+/* Ctrl-C handler used when the inferior is not run in the same console.  The
+   handler is in charge of interrupting the inferior using DebugBreakProcess.
+   Note that this function is not available prior to Windows XP.  In this case
+   we emit a warning.  */
+BOOL WINAPI
+ctrl_c_handler (DWORD event_type) 
+{
+  const int attach_flag = current_inferior ()->attach_flag;
+
+  /* Only handle Ctrl-C event.  Ignore others.  */
+  if (event_type != CTRL_C_EVENT)
+    return FALSE;
+
+  /* If the inferior and the debugger share the same console, do nothing as
+     the inferior has also received the Ctrl-C event.  */
+  if (!new_console && !attach_flag)
+    return TRUE;
+
+  if (has_DebugBreakProcess)
+    kernel32_DebugBreakProcess (current_process_handle);
+  else
+    warning (_("\
+Cannot interrupt program on this version of Windows.\n\
+Press Ctrl-c in the program console."));
+
+  /* Return true to tell that Ctrl-C has been handled.  */
+  return TRUE;
+}
+
 /* Get the next event from the child.  Return 1 if the event requires
    handling by WFI (or whatever).
  */
+
 static int
 get_windows_debug_event (struct target_ops *ops,
 			 int pid, struct target_waitstatus *ourstatus)
@@ -1465,23 +1532,36 @@ windows_wait (struct target_ops *ops,
   while (1)
     {
       int retval;
-      
-      /* Ignore CTRL+C signals while waiting for a debug event.
-         FIXME: brobecker/2008-05-20: When the user presses CTRL+C while
-         the inferior is running, both the inferior and GDB receive the
-         associated signal.  If the inferior receives the signal first
-         and the delay until GDB receives that signal is sufficiently long,
-         GDB can sometimes receive the SIGINT after we have unblocked
-         the CTRL+C handler.  This would lead to the debugger to stop
-         prematurely while handling the new-thread event that comes
-         with the handling of the SIGINT inside the inferior, and then
-         stop again immediately when the user tries to resume the execution
-         in the inferior.  This is a classic race, and it would be nice
-         to find a better solution to that problem.  But in the meantime,
-         the current approach already greatly mitigate this issue.  */
-      SetConsoleCtrlHandler (NULL, TRUE);
+     
+      /* If the user presses Ctrl-c while the debugger is waiting
+         for an event, he expects the debugger to interrupt his program
+         and to get the prompt back.  There are two possible situations:
+
+           - The debugger and the program do not share the console, in
+             which case the Ctrl-c event only reached the debugger.
+             In that case, the ctrl_c handler will take care of interrupting
+             the inferior. Note that this case is working starting with
+             Windows XP. For Windows 2000, Ctrl-C should be pressed in the
+             inferior console.
+
+           - The debugger and the program share the same console, in which
+             case both debugger and inferior will receive the Ctrl-c event.
+             In that case the ctrl_c handler will ignore the event, as the
+             Ctrl-c event generated inside the inferior will trigger the
+             expected debug event.  
+
+             FIXME: brobecker/2008-05-20: If the inferior receives the
+             signal first and the delay until GDB receives that signal
+             is sufficiently long, GDB can sometimes receive the SIGINT
+             after we have unblocked the CTRL+C handler.  This would
+             lead to the debugger stopping prematurely while handling
+             the new-thread event that comes with the handling of the SIGINT
+             inside the inferior, and then stop again immediately when
+             the user tries to resume the execution in the inferior.
+             This is a classic race that we should try to fix one day.  */
+      SetConsoleCtrlHandler (&ctrl_c_handler, TRUE); 
       retval = get_windows_debug_event (ops, pid, ourstatus);
-      SetConsoleCtrlHandler (NULL, FALSE);
+      SetConsoleCtrlHandler (&ctrl_c_handler, FALSE);
 
       if (retval)
 	return ptid_build (current_event.dwProcessId, 0, retval);
@@ -2164,6 +2251,8 @@ _initialize_windows_nat (void)
 
   add_com_alias ("sharedlibrary", "dll-symbols", class_alias, 1);
 
+  check_for_DebugBreakProcess ();
+
 #ifdef __CYGWIN__
   add_setshow_boolean_cmd ("shell", class_support, &useshell, _("\
 Set use of shell to start subprocess."), _("\