Introduce interruptible_select

We have places where we call a blocking gdb_select expecting that a
Ctrl-C will unblock it.  However, if the Ctrl-C is pressed just before
gdb_select, the SIGINT handler runs before gdb_select, and thus
gdb_select won't return.

For example gdb_readline_no_editing:

       QUIT;

       /* Wait until at least one byte of data is available.  Control-C
          can interrupt gdb_select, but not fgetc.  */
       FD_ZERO (&readfds);
       FD_SET (fd, &readfds);
       if (gdb_select (fd + 1, &readfds, NULL, NULL, NULL) == -1)

and stdio_file_read:

     /* For the benefit of Windows, call gdb_select before reading from
	the file.  Wait until at least one byte of data is available.
	Control-C can interrupt gdb_select, but not read.  */
     {
       fd_set readfds;
       FD_ZERO (&readfds);
       FD_SET (stdio->fd, &readfds);
       if (gdb_select (stdio->fd + 1, &readfds, NULL, NULL, NULL) == -1)
	 return -1;
     }
     return read (stdio->fd, buf, length_buf);


This is a race classically fixed with either the self-pipe trick, or
by blocking SIGINT and then using pselect instead of select.

Blocking SIGINT most of the time would mean that check_quit_flag (and
thus QUIT) would need to do a syscall every time it is called, which
sounds best avoided, since QUIT is called in many loops.  Thus we take
the self-pipe trick route (wrapped in a serial event).

Instead of having all places that need this manually add an extra file
descriptor to the set of gdb_select's watched file descriptors, we
introduce a wrapper, interruptible_select, that does that.

The Windows version of gdb_select actually does not suffer from this,
because mingw-hdep.c:gdb_call_async_signal_handler sets a Windows
event that gdb_select always waits on.  So this patch can be seen as
generalization of that technique.  We can't remove that extra event
from mingw-hdep.c until we get rid of immediate_quit though.

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

	* defs.h: Extend QUIT-related comments to mention
	interruptible_select.
	(quit_serial_event_set, quit_serial_event_clear): Declare.
	* event-top.c: Include "ser-event.h" and "gdb_select.h".
	(quit_serial_event): New global.
	(async_init_signals): Make quit_serial_event.
	(quit_serial_event_set, quit_serial_event_clear)
	(quit_serial_event_fd, interruptible_select): New functions.
	* extension.c (set_quit_flag): Set the quit serial event.
	(check_quit_flag): Clear the quit serial event.
	* gdb_select.h (interruptible_select): New declaration.
	* guile/scm-ports.c (ioscm_input_waiting): Use
	interruptible_select instead of gdb_select.
	* top.c (gdb_readline_no_editing): Likewise.
	* ui-file.c (stdio_file_read): Likewise.

1 files changed, 14 insertions, 1 deletions

diff --git a/gdb/extension.c b/gdb/extension.c
index d2c5669..c00db47 100644
--- a/gdb/extension.c
+++ b/gdb/extension.c
@@ -828,7 +828,16 @@ set_quit_flag (void)
       && active_ext_lang->ops->set_quit_flag != NULL)
     active_ext_lang->ops->set_quit_flag (active_ext_lang);
   else
-    quit_flag = 1;
+    {
+      quit_flag = 1;
+
+      /* Now wake up the event loop, or any interruptible_select.  Do
+	 this after setting the flag, because signals on Windows
+	 actually run on a separate thread, and thus otherwise the
+	 main code could be woken up and find quit_flag still
+	 clear.  */
+      quit_serial_event_set ();
+    }
 }
 
 /* Return true if the quit flag has been set, false otherwise.
@@ -852,6 +861,10 @@ check_quit_flag (void)
   /* This is written in a particular way to avoid races.  */
   if (quit_flag)
     {
+      /* No longer need to wake up the event loop or any
+	 interruptible_select.  The caller handles the quit
+	 request.  */
+      quit_serial_event_clear ();
       quit_flag = 0;
       result = 1;
     }