Don't throw quit while handling inferior events

Don't throw quit while handling inferior events

This implements what I suggested here:

 https://inbox.sourceware.org/gdb-patches/ab97c553-f406-b094-cdf3-ba031fdea925@palves.net/

Here is the current default quit_handler, a function that ends up
called by the QUIT macro:

 void
 default_quit_handler (void)
 {
   if (check_quit_flag ())
     {
       if (target_terminal::is_ours ())
 	quit ();
       else
 	target_pass_ctrlc ();
      }
 }

As we can see above, when the inferior is running in the foreground,
then a Ctrl-C is translated into a call to target_pass_ctrlc().

The target_terminal::is_ours() case above is there to handle the
scenario where GDB has the terminal, meaning it is handling some
command the user typed, like "list", or "p a + b" or some such.

However, when the inferior is running on the background, say with
"c&", GDB also has the terminal.  Run control handling is now done in
the "background".  The CLI is responsive to user commands.  If users
type Ctrl-C, they're expecting it to interrupt whatever command they
next type in the CLI, which again, could be "list", "p a + b", etc.
It's as if background run control was handled by a separate thread,
and the Ctrl-C is meant to go to the main thread, handling the CLI.

However, when handling an event, inside fetch_inferior_event &
friends, a Ctrl-C _also_ results in a Quit exception, from the same
default_quit_handler function shown above.  This quit aborts run
control handling, breakpoint condition evaluation, etc., and may even
leave run control in an inconsistent state.

The testcase added by this patch illustrates this.  The test program
just loops a number of times calling the "foo" function.

The idea is to set a breakpoint in the "foo" function with a condition
that sends SIGINT to GDB, and then evaluates to false, which results
in the program being re-resumed in the background.  The SIGINT-sending
emulates pressing Ctrl-C just while GDB was evaluating the breakpoint
condition, except, it's more deterministic.

It looks like this:

  (gdb) p $counter = 0
  $1 = 0
  (gdb) b foo if $counter++ == 10 || $_shell("kill -SIGINT `pidof gdb`") != 0
  Breakpoint 2 at 0x555555555131: file gdb.base/bg-exec-sigint-bp-cond.c, line 21.
  (gdb) c&
  Continuing.
  (gdb)

After that background continue, the breakpoint should be hit 10 times,
and we should see 10 "Quit" being printed on the screen.  As if the
user typed Ctrl-C on the prompt a number of times with no inferior
running:

  (gdb)        <<< Ctrl-C
  (gdb) Quit   <<< Ctrl-C
  (gdb) Quit   <<< Ctrl-C
  (gdb)

However, here's what you see instead:

  (gdb) c&
  Continuing.
  (gdb) Quit
  (gdb)

Just one Quit, and nothing else.  If we look at the thread's state, we see:

  (gdb) info threads
    Id   Target Id                                            Frame
  * 1    Thread 0x7ffff7d6f740 (LWP 112192) "bg-exec-sigint-" foo () at gdb.base/bg-exec-sigint-bp-cond.c:21

So the thread stopped, but we didn't report a stop...

Issuing another continue shows the same immediate-and-silent-stop:

  (gdb) c&
  Continuing.
  (gdb) Quit
  (gdb) p $counter
  $2 = 2

As mentioned, since the run control handling, and breakpoint and
watchpoint evaluation, etc. are running in the background from the
perspective of the CLI, when users type Ctrl-C in this situation,
they're thinking of aborting whatever other command they were typing
or running at the prompt, not the run control side, not the previous
"c&" command.

So I think that we should install a custom quit_handler while inside
fetch_inferior_event, where we already disable pagination and other
things for a similar reason.  This custom quit handler does nothing if
GDB has the terminal, and forwards Ctrl-C to the inferior otherwise.

With the patch implementing that, and the same testcase, here's what
you see instead:

 (gdb) p $counter = 0
 $1 = 0
 (gdb) b foo if $counter++ == 10 || $_shell("kill -SIGINT `pidof gdb`") != 0
 Breakpoint 2 at 0x555555555131: file gdb.base/bg-exec-sigint-bp-cond.c, line 21.
 (gdb) c&
 Continuing.
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb) Quit
 (gdb)
 Breakpoint 2, foo () at gdb.base/bg-exec-sigint-bp-cond.c:21
 21        return 0;

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I1f10d99496a7d67c94b258e45963e83e439e1778