Fix "attach" command vs user input race

On async targets, a synchronous attach is done like this:

   #1 - target_attach is called (PTRACE_ATTACH is issued)
   #2 - a continuation is installed
   #3 - we go back to the event loop
   #4 - target reports stop (SIGSTOP), event loop wakes up, and
        attach continuation is called
   #5 - among other things, the continuation calls
        target_terminal_inferior, which removes stdin from the event
        loop

Note that in #3, GDB is still processing user input.  If the user is
fast enough, e.g., with something like:

  echo -e "attach PID\nset xxx=1" | gdb

... then the "set" command is processed before the attach completes.

We get worse behavior even, if input is a tty and therefore
readline/editing is enabled, with e.g.,:

 (gdb) attach PID\nset xxx=1

we then crash readline/gdb, with:

 Attaching to program: attach-wait-input, process 14537
 readline: readline_callback_read_char() called with no handler!
 Aborted
 $

Fix this by calling target_terminal_inferior before #3 above.

The test covers both scenarios by running with editing/readline forced
to both on and off.

gdb/
2014-07-09  Pedro Alves  <palves@redhat.com>

	* infcmd.c (attach_command_post_wait): Don't call
	target_terminal_inferior here.
	(attach_command): Call it here instead.

gdb/testsuite/
2014-07-09  Pedro Alves  <palves@redhat.com>

	* gdb.base/attach-wait-input.exp: New file.
	* gdb.base/attach-wait-input.c: New file.

gdb/infcmd.c

@@ -2380,9 +2380,6 @@ attach_command_post_wait (char *args, int from_tty, int async_exec)
 
   post_create_inferior (&current_target, from_tty);
 
-  /* Install inferior's terminal modes.  */
-  target_terminal_inferior ();
-
   if (async_exec)
     {
       /* The user requested an `attach&', so be sure to leave threads
@@ -2498,6 +2495,22 @@ attach_command (char *args, int from_tty)
      based on what modes we are starting it with.  */
   target_terminal_init ();
 
+  /* Install inferior's terminal modes.  This may look like a no-op,
+     as we've just saved them above, however, this does more than
+     restore terminal settings:
+
+     - installs a SIGINT handler that forwards SIGINT to the inferior.
+       Otherwise a Ctrl-C pressed just while waiting for the initial
+       stop would end up as a spurious Quit.
+
+     - removes stdin from the event loop, which we need if attaching
+       in the foreground, otherwise on targets that report an initial
+       stop on attach (which are most) we'd process input/commands
+       while we're in the event loop waiting for that stop.  That is,
+       before the attach continuation runs and the command is really
+       finished.  */
+  target_terminal_inferior ();
+
   /* Set up execution context to know that we should return from
      wait_for_inferior as soon as the target reports a stop.  */
   init_wait_for_inferior ();