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, 11 insertions, 0 deletions

diff --git a/gdb/defs.h b/gdb/defs.h
index b94df30..ad9b259 100644
--- a/gdb/defs.h
+++ b/gdb/defs.h
@@ -131,6 +131,11 @@ extern char *debug_file_directory;
    take a long time, and which ought to be interruptible, checks this
    flag using the QUIT macro.
 
+   In addition to setting a flag, the SIGINT handler also marks a
+   select/poll-able file descriptor as read-ready.  That is used by
+   interruptible_select in order to support interrupting blocking I/O
+   in a race-free manner.
+
    These functions use the extension_language_ops API to allow extension
    language(s) and GDB SIGINT handling to coexist seamlessly.  */
 
@@ -159,6 +164,12 @@ extern void maybe_quit (void);
    connection.  */
 #define QUIT maybe_quit ()
 
+/* Set the serial event associated with the quit flag.  */
+extern void quit_serial_event_set (void);
+
+/* Clear the serial event associated with the quit flag.  */
+extern void quit_serial_event_clear (void);
+
 /* * Languages represented in the symbol table and elsewhere.
    This should probably be in language.h, but since enum's can't
    be forward declared to satisfy opaque references before their