Fix signal handler/event-loop races

GDB's core signal handling suffers from a classical signal handler /
mainline code race:

  int
  gdb_do_one_event (void)
  {
  ...
  /* First let's see if there are any asynchronous signal handlers
       that are ready.  These would be the result of invoking any of the
       signal handlers.  */
    if (invoke_async_signal_handlers ())
      return 1;
  ...
    /* Block waiting for a new event.  (...).  */

    if (gdb_wait_for_event (1) < 0)
      return -1;
  ...
  }

If a signal is delivered while gdb is blocked in the poll/select
inside gdb_wait_for_event, then the select/poll breaks with EINTR,
we'll loop back around and call invoke_async_signal_handlers.

However, if the signal handler runs between
invoke_async_signal_handlers and gdb_wait_for_event,
gdb_wait_for_event will block, until the next unrelated event...

The fix is to a struct serial_event, and register it in the set of
files that select/poll in gdb_wait_for_event waits on.  The signal
handlers that defer work to invoke_async_signal_handlers call
mark_async_signal_handler, which is adjusted to also set the new
serial event in addition to setting a flag, and is thus now is
garanteed to immediately unblock the next gdb_select/poll call, up
until invoke_async_signal_handlers is called and the event is cleared.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* event-loop.c: Include "ser-event.h".
	(async_signal_handlers_serial_event): New global.
	(async_signals_handler, initialize_async_signal_handlers): New
	functions.
	(mark_async_signal_handler): Set
	async_signal_handlers_serial_event.
	(invoke_async_signal_handlers): Clear
	async_signal_handlers_serial_event.
	* event-top.c (async_init_signals): Call
	initialize_async_signal_handlers.

gdb/event-loop.c

@@ -21,6 +21,7 @@
 #include "event-loop.h"
 #include "event-top.h"
 #include "queue.h"
+#include "ser-event.h"
 
 #ifdef HAVE_POLL
 #if defined (HAVE_POLL_H)
@@ -262,6 +263,28 @@ static int update_wait_timeout (void);
 static int poll_timers (void);
 
 
+/* This event is signalled whenever an asynchronous handler needs to
+   defer an action to the event loop.  */
+static struct serial_event *async_signal_handlers_serial_event;
+
+/* Callback registered with ASYNC_SIGNAL_HANDLERS_SERIAL_EVENT.  */
+
+static void
+async_signals_handler (int error, gdb_client_data client_data)
+{
+  /* Do nothing.  Handlers are run by invoke_async_signal_handlers
+     from instead.  */
+}
+
+void
+initialize_async_signal_handlers (void)
+{
+  async_signal_handlers_serial_event = make_serial_event ();
+
+  add_file_handler (serial_event_fd (async_signal_handlers_serial_event),
+		    async_signals_handler, NULL);
+}
+
 /* Process one high level event.  If nothing is ready at this time,
    wait for something to happen (via gdb_wait_for_event), then process
    it.  Returns >0 if something was done otherwise returns <0 (this
@@ -905,6 +928,7 @@ void
 mark_async_signal_handler (async_signal_handler * async_handler_ptr)
 {
   async_handler_ptr->ready = 1;
+  serial_event_set (async_signal_handlers_serial_event);
 }
 
 /* See event-loop.h.  */
@@ -925,13 +949,19 @@ async_signal_handler_is_marked (async_signal_handler *async_handler_ptr)
 
 /* Call all the handlers that are ready.  Returns true if any was
    indeed ready.  */
+
 static int
 invoke_async_signal_handlers (void)
 {
   async_signal_handler *async_handler_ptr;
   int any_ready = 0;
 
-  /* Invoke ready handlers.  */
+  /* We're going to handle all pending signals, so no need to wake up
+     the event loop again the next time around.  Note this must be
+     cleared _before_ calling the callbacks, to avoid races.  */
+  serial_event_clear (async_signal_handlers_serial_event);
+
+  /* Invoke all ready handlers.  */
 
   while (1)
     {