remote: allow aborting long operations (e.g., file transfers)

remote: allow aborting long operations (e.g., file transfers)

Currently, when remote debugging, if you type Ctrl-C just while the
target stopped for an internal event, and GDB is busy doing something
that takes a while (e.g., fetching chunks of a shared library off of
the target, with vFile, to process ELF headers and debug info), the
Ctrl-C is lost.

The patch hooks up the QUIT macro to a new target method that lets the
target react to the double-Ctrl-C before the event loop is reached,
which allows reacting to a double-Ctrl-C even when GDB is busy doing
some long operation and not waiting for a stop reply.  That end result
is:

 (gdb) c
 Continuing.
 ^C
 ^C
 Interrupted while waiting for the program.
 Give up waiting? (y or n) y
 Quit
 (gdb) info threads
   Id   Target Id         Frame
 * 1    Thread 11673      0x00007ffff7deb240 in _dl_debug_state () from target:/lib64/ld-linux-x86-64.so.2
 (gdb)

If, however, GDB is waiting for a stop reply (because the target has
been resumed, with e.g., vCont;c), but the target isn't responding, we
now get:

 (gdb) c
 Continuing.
 ^C
 ^C
 The target is not responding to interrupt requests.
 Stop debugging it? (y or n) y
 Disconnected from target.
 (gdb) info threads
 No threads.

This offers to disconnect, because when we're waiting for a stop
reply, there's nothing else we can send the target other than an
interrupt request.  And if that doesn't work, there's nothing else we
can do.

The Ctrl-C is presently lost because until we get to a user-visible
stop, the SIGINT handler that is installed is the one that forwards
the interrupt to the remote side, with the \003 "packet" [1].  But,
gdbserver ignores an interrupt request if the program is stopped.
Still, even if it didn't, the server can only report back a
stop-because-of-SIGINT when the program is next resumed.  And it may
take a while to actually re-resume the target.

[1] - In the old sync days, the remote target would react to a
double-Ctrl-C by asking users whether they wanted to give up waiting
and disconnect.  The code is still there, but it it isn't reacheable
on most hosts, which support serial connections in async mode
(probably only DJGPP doesn't).  Even then, in sync mode, remote.c's
SIGINT handler is only installed while the target is resumed, and is
removed as soon as the target sends back a stop reply.  That means
that a Ctrl-C just while GDB is processing an internal event can end
up with an odd "Quit" at the prompt instead of "Program stopped by
SIGINT".  In contrast, in async mode, remote.c's SIGINT handler is set
up as long as target_terminal_inferior or
target_terminal_ours_for_output are in effect (IOW, until we get a
user-visible stop and call target_terminal_ours), so the user
shouldn't get back a spurious Quit.  However, it's still desirable to
be able to interrupt a long-running GDB operation, if GDB takes a
while to re-resume the target or get back to the event loop.

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-08-24  Pedro Alves  <palves@redhat.com>

	* defs.h (maybe_quit): Declare.
	(QUIT): Now calls maybe_quit.
	* event-loop.c (clear_async_signal_handler)
	(async_signal_handler_is_marked): New functions.
	* event-loop.h (async_signal_handler_is_marked)
	(clear_async_signal_handler): New declarations.
	* remote.c (remote_check_pending_interrupt): New function.
	(interrupt_query): Use make_cleanup_restore_target_terminal.  No
	longer check whether the target is async.  If waiting for a stop
	reply, and a Ctrl-C as been sent to the target, offer to
	disconnect, and throw TARGET_CLOSE_ERROR instead of a quit.
	Otherwise do not disconnect and throw a quit.
	(_initialize_remote): Install remote_check_pending_interrupt as
	to_check_pending_interrupt.
	* target.c (target_check_pending_interrupt): New function.
	* target.h (struct target_ops) <to_check_pending_interrupt>: New
	field.
	(target_check_pending_interrupt): New declaration.
	* utils.c (maybe_quit): New function.
	* target-delegates.c: Regenerate.