With this patch, when an inferior, thread or frame is explicitly
selected by the user, notifications will appear on all CLI and MI UIs.
When a GDB console is integrated in a front-end, this allows the
front-end to follow a selection made by the user ont he CLI, and it
informs the user about selection changes made behind the scenes by the
front-end.

This patch addresses PR gdb/20487.

In order to communicate frame changes to the front-end, this patch adds
a new field to the =thread-selected event for the selected frame.  The
idea is that since inferior/thread/frame can be seen as a composition,
it makes sense to send them together in the same event.  The vision
would be to eventually send the inferior information as well, if we find
that it's needed, although the "=thread-selected" event would be
ill-named for that job.

Front-ends need to handle this new field if they want to follow the
frame selection changes that originate from the console.  The format of
the frame attribute is the same as what is found in the *stopped events.

Here's a detailed example for each command and the events they generate:

thread
------

1. CLI command:

     thread 1.3

   MI event:

     =thread-selected,id="3",frame={...}

2. MI command:

     -thread-select 3

   CLI event:

     [Switching to thread 1.3 ...]

3. MI command (CLI-in-MI):

     thread 1.3

   MI event/reply:

     &"thread 1.3\n"
     ~"#0  child_sub_function () ...
     =thread-selected,id="3",frame={level="0",...}
     ^done

frame
-----

1. CLI command:

     frame 1

   MI event:

     =thread-selected,id="3",frame={level="1",...}

2. MI command:

     -stack-select-frame 1

   CLI event:

     #1  0x00000000004007f0 in child_function...

3. MI command (CLI-in-MI):

     frame 1

   MI event/reply:

     &"frame 1\n"
     ~"#1  0x00000000004007f9 in ..."
     =thread-selected,id="3",frame={level="1"...}
     ^done

inferior
--------

Inferior selection events only go from the console to MI, since there's
no way to select the inferior in pure MI.

1. CLI command:

     inferior 2

   MI event:

     =thread-selected,id="3"

Note that if the user selects an inferior that is not started or exited,
the MI doesn't receive a notification.  Since there is no threads to
select, the =thread-selected event does not apply...

2. MI command (CLI-in-MI):

     inferior 2

   MI event/reply:

     &"inferior 2\n"
     ~"[Switching to inferior 2 ...]"
     =thread-selected,id="4",frame={level="0"...}
     ^done

Internal implementation detail: this patch makes it possible to suppress
notifications caused by a CLI command, like what is done in mi-interp.c.
This means that it's now possible to use the
add_com_suppress_notification function to register a command with some
event suppressed.  It is used to implement the select-frame command in
this patch.

The function command_notifies_uscc_observer was added to extract
the rather complicated logical expression from the if statement.  It is
also now clearer what that logic does: if the command used by the user
already notifies the user_selected_context_changed observer, there is
not need to notify it again.  It therefore protects again emitting the
event twice.

No regressions, tested on ubuntu 14.04 x86 with target boards unix and
native-extended-gdbserver.

gdb/ChangeLog:

YYYY-MM-DD  Antoine Tremblay  <antoine.tremblay@ericsson.com>
YYYY-MM-DD  Simon Marchi  <simon.marchi@ericsson.com>

	PR gdb/20487
	* NEWS: Mention new frame field of =thread-selected event.
	* cli/cli-decode.c (add_cmd): Initialize c->suppress_notification.
	(add_com_suppress_notification): New function definition.
	(cmd_func): Set and restore the suppress_notification flag.
	* cli/cli-deicode.h (struct cmd_list_element)
	<suppress_notification>: New field.
	* cli/cli-interp.c (cli_suppress_notification): New global variable.
	(cli_on_user_selected_context_changed): New function.
	(_initialize_cli_interp): Attach to user_selected_context_changed
	observer.
	* command.h (struct cli_suppress_notification): New structure.
	(cli_suppress_notification): New global variable declaration.
	(add_com_suppress_notification): New function declaration.
	* defs.h (enum user_selected_what_flag): New enum.
	(user_selected_what): New enum flag type.
	* frame.h (print_stack_frame_to_uiout): New function declaration.
	* gdbthread.h (print_selected_thread_frame): New function declaration.
	* inferior.c (print_selected_inferior): New function definition.
	(inferior_command): Remove printing of inferior/thread/frame switch
	notifications, notify user_selected_context_changed observer.
	* inferior.h (print_selected_inferior): New function declaration.
	* mi/mi-cmds.c (struct mi_cmd): Add user_selected_context
	suppression to stack-select-frame and thread-select commands.
	* mi/mi-interp.c (struct mi_suppress_notification)
	<user_selected_context>: Initialize.
	(mi_user_selected_context_changed): New function definition.
	(_initialize_mi_interp): Attach to user_selected_context_changed.
	* mi/mi-main.c (mi_cmd_thread_select): Print thread selection reply.
	(mi_execute_command): Handle notification suppression.  Notify
	user_selected_context_changed observer on thread change instead of printing
	event directly.  Don't send it if command already sends the notification.
	(command_notifies_uscc_observer): New function.
	(mi_cmd_execute): Don't handle notification suppression.
	* mi/mi-main.h (struct mi_suppress_notification)
	<user_selected_context>: New field.
	* stack.c (print_stack_frame_to_uiout): New function definition.
	(select_frame_command): Notify user_selected_context_changed
	observer.
	(frame_command): Call print_selected_thread_frame if there's no frame
	change or notify user_selected_context_changed observer if there is.
	(up_command): Notify user_selected_context_changed observer.
	(down_command): Likewise.
	(_initialize_stack): Suppress user_selected_context notification for
	command select-frame.
	* thread.c (thread_command): Notify
	user_selected_context_changed if the thread has changed, print
	thread info directly if it hasn't.
	(do_captured_thread_select): Do not print thread switch event.
	(print_selected_thread_frame): New function definition.
	* tui/tui-interp.c (tui_on_user_selected_context_changed):
	New function definition.
	(_initialize_tui_interp): Attach to user_selected_context_changed
	observer.

gdb/doc/ChangeLog:

	PR gdb/20487
	* gdb.texinfo (Context management): Update mention of frame
	change notifications.
	(gdb/mi Async Records): Document frame field in
	=thread-select event.
	* observer.texi (GDB Observers): New user_selected_context_changed
	observer.

gdb/testsuite/ChangeLog:

	PR gdb/20487
	* gdb.mi/mi-pthreads.exp (check_mi_thread_command_set): Adapt
	=thread-select-event check.