Make input_fd be per UI

And with that, we can switch the current UI to the UI whose input
descriptor woke up the event loop.  IOW, if the user types in UI 2,
the event loop wakes up, switches to UI 2, and processes the input.
Next the user types in UI 3, the event loop wakes up and switches to
UI 3, etc.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* event-top.c (input_fd): Delete.
	(stdin_event_handler): Switch to the UI whose input descriptor got
	the event.  Adjust to per-UI input_fd.
	(gdb_setup_readline): Don't set the input_fd global.  Adjust to
	per-UI input_fd.
	(gdb_disable_readline): Adjust to per-UI input_fd.
	* event-top.h (input_fd): Delete declaration.
	* linux-nat.c (linux_nat_terminal_inferior): Don't remove input_fd
	from the event-loop here.
	(linux_nat_terminal_ours): Don't register input_fd in the
	event-loop here.
	* main.c (captured_main): Adjust to per-UI input_fd.
	* remote.c (remote_terminal_inferior): Don't remove input_fd from
	the event-loop here.
	(remote_terminal_ours): Don't register input_fd in the event-loop
	here.
	* target.c: Include top.h and event-top.h.
	(target_terminal_inferior): Remove input_fd from the event-loop
	here.
	(target_terminal_ours): Register input_fd in the event-loop.
	* top.h (struct ui) <input_fd>: New field.

gdb/event-top.c

@@ -94,10 +94,6 @@ int async_command_editing_p;
    asynchronous execution command.  */
 int exec_done_display_p = 0;
 
-/* This is the file descriptor for the input stream that GDB uses to
-   read commands from.  */
-int input_fd;
-
 /* Used by the stdin event handler to compensate for missed stdin events.
    Setting this to a non-zero value inside an stdin callback makes the callback
    run again.  */
@@ -503,12 +499,16 @@ get_command_line_buffer (void)
 void
 stdin_event_handler (int error, gdb_client_data client_data)
 {
-  struct ui *ui = current_ui;
+  struct ui *ui = (struct ui *) client_data;
+
+  /* Switch to the UI whose input descriptor woke up the event
+     loop.  */
+  current_ui = ui;
 
   if (error)
     {
       printf_unfiltered (_("error detected on stdin\n"));
-      delete_file_handler (input_fd);
+      delete_file_handler (ui->input_fd);
       /* If stdin died, we may as well kill gdb.  */
       quit_command ((char *) 0, stdin == ui->instream);
     }
@@ -1270,18 +1270,11 @@ gdb_setup_readline (void)
   /* Tell readline to use the same input stream that gdb uses.  */
   rl_instream = ui->instream;
 
-  /* Get a file descriptor for the input stream, so that we can
-     register it with the event loop.  */
-  input_fd = fileno (ui->instream);
-
-  /* Now we need to create the event sources for the input file
-     descriptor.  */
-  /* At this point in time, this is the only event source that we
-     register with the even loop.  Another source is going to be the
-     target program (inferior), but that must be registered only when
-     it actually exists (I.e. after we say 'run' or after we connect
-     to a remote target.  */
-  add_file_handler (input_fd, stdin_event_handler, 0);
+  /* Now create the event source for this UI's input file descriptor.
+     Another source is going to be the target program (inferior), but
+     that must be registered only when it actually exists (I.e. after
+     we say 'run' or after we connect to a remote target.  */
+  add_file_handler (ui->input_fd, stdin_event_handler, ui);
 }
 
 /* Disable command input through the standard CLI channels.  Used in
@@ -1290,6 +1283,8 @@ gdb_setup_readline (void)
 void
 gdb_disable_readline (void)
 {
+  struct ui *ui = current_ui;
+
   /* FIXME - It is too heavyweight to delete and remake these every
      time you run an interpreter that needs readline.  It is probably
      better to have the interpreters cache these, which in turn means
@@ -1304,5 +1299,5 @@ gdb_disable_readline (void)
 #endif
 
   gdb_rl_callback_handler_remove ();
-  delete_file_handler (input_fd);
+  delete_file_handler (ui->input_fd);
 }