Fix control-c handling on Windows

As Hannes pointed out, the Windows target-async patches broke C-c
handling there.  Looking into this, I found a few oddities, fixed
here.

First, windows_nat_target::interrupt calls GenerateConsoleCtrlEvent.
I think this event can be ignored by the inferior, so it's not a great
way to interrupt.  Instead, using DebugBreakProcess (or a more
complicated thing for Wow64) seems better.

Second, windows_nat_target did not implement the pass_ctrlc method.
Implementing this lets us remove the special code to call
SetConsoleCtrlHandler and instead integrate into gdb's approach to C-c
handling.  I believe that this should also fix the race that's
described in the comment that's being removed.

Initially, I thought a simpler version of this patch would work.
However, I think what happens is that some other library (I'm not sure
what) calls SetConsoleCtrlHandler while gdb is running, and this
intercepts and handles C-c -- so that the gdb SIGINT handler is not
called.  C-break continues to work, presumably because whatever
handler is installed ignores it.

This patch works around this issue by ensuring that the gdb handler
always comes first.

1 files changed, 24 insertions, 0 deletions

diff --git a/gdb/posix-hdep.c b/gdb/posix-hdep.c
index 2621197..eddf73f 100644
--- a/gdb/posix-hdep.c
+++ b/gdb/posix-hdep.c
@@ -21,6 +21,7 @@
 #include "gdbsupport/event-loop.h"
 #include "gdbsupport/gdb_select.h"
 #include "inferior.h"
+#include <signal.h>
 
 /* Wrapper for select.  Nothing special needed on POSIX platforms.  */
 
@@ -56,3 +57,26 @@ sharing_input_terminal (int pid)
   return TRIBOOL_UNKNOWN;
 #endif
 }
+
+/* Current C-c handler.  */
+static c_c_handler_ftype *current_handler;
+
+/* A wrapper that reinstalls the current signal handler.  */
+static void
+handler_wrapper (int num)
+{
+  signal (num, handler_wrapper);
+  if (current_handler != SIG_IGN)
+    current_handler (num);
+}
+
+/* See inferior.h.  */
+
+c_c_handler_ftype *
+install_sigint_handler (c_c_handler_ftype *fn)
+{
+  signal (SIGINT, handler_wrapper);
+  c_c_handler_ftype *result = current_handler;
+  current_handler = fn;
+  return result;
+}