PR python/16699: GDB Python command completion with overriden complete vs. completer class

This PR came from a Red Hat bug that was filed recently.  I checked and
it still exists on HEAD, so here's a proposed fix.  Although this is
marked as a Python backend bug, this is really about the completion
mechanism used by GDB.  Since this code reminds me of my first attempt
to make a good noodle, it took me quite some time to fix it in a
non-intrusive way.

The problem is triggered when one registers a completion method inside a
class in a Python script, rather than registering the command using a
completer class directly.  For example, consider the following script:

    class MyFirstCommand(gdb.Command):
          def __init__(self):
              gdb.Command.__init__(self,'myfirstcommand',gdb.COMMAND_USER,gdb.COMPLETE_FILENAME)

              def invoke(self,argument,from_tty):
                  raise gdb.GdbError('not implemented')

    class MySecondCommand(gdb.Command):
          def __init__(self):
              gdb.Command.__init__(self,'mysecondcommand',gdb.COMMAND_USER)

              def invoke(self,argument,from_tty):
                  raise gdb.GdbError('not implemented')

                  def complete(self,text,word):
                      return gdb.COMPLETE_FILENAME

    MyFirstCommand ()
    MySecondCommand ()

When one loads this into GDB and tries to complete filenames for both
myfirstcommand and mysecondcommand, she gets:

    (gdb) myfirstcommand /hom<TAB>
    (gdb) myfirstcommand /home/
                               ^
    ...
    (gdb) mysecondcommand /hom<TAB>
    (gdb) mysecondcommand /home
                                ^

(The "^" marks the final position of the cursor after the TAB).

So we see that myfirstcommand honors the COMPLETE_FILENAME class (as
specified in the command creation), but mysecondcommand does not.  After
some investigation, I found that the problem lies with the set of word
break characters that is used for each case.  The set should be the same
for both commands, but it is not.

During the process of deciding which type of completion should be used,
the code in gdb/completer.c:complete_line_internal analyses the command
that requested the completion and tries to determine the type of
completion wanted by checking which completion function will be called
(e.g., filename_completer for filenames, location_completer for
locations, etc.).

This all works fine for myfirstcommand, because immediately after the
command registration the Python backend already sets its completion
function to filename_completer (which then causes the
complete_line_internal function to choose the right set of word break
chars).  However, for mysecondcommand, this decision is postponed to
when the completer function is evaluated, and the Python backend uses an
internal completer (called cmdpy_completer).  complete_line_internal
doesn't know about this internal completer, and can't choose the right
set of word break chars in time, which then leads to a bad decision when
completing the "/hom" word.

So, after a few attempts, I decided to create another callback in
"struct cmd_list_element" that will be responsible for handling the case
when there is an unknown completer function for complete_line_internal
to work with.  So far, only the Python backend uses this callback, and
only when the user provides a completer method instead of registering
the command directly with a completer class.  I think this is the best
option because it not very intrusive (all the other commands will still
work normally), but especially because the whole completion code is so
messy that it would be hard to fix this without having to redesign
things.

I have regtested this on Fedora 18 x86_64, without regressions.  I also
included a testcase.

gdb/ChangeLog:
2014-09-03  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR python/16699
	* cli/cli-decode.c (set_cmd_completer_handle_brkchars): New
	function.
	(add_cmd): Set "completer_handle_brkchars" to NULL.
	* cli/cli-decode.h (struct cmd_list_element)
	<completer_handle_brkchars>: New field.
	* command.h (completer_ftype_void): New typedef.
	(set_cmd_completer_handle_brkchars): New prototype.
	* completer.c (set_gdb_completion_word_break_characters): New
	function.
	(complete_line_internal): Call "completer_handle_brkchars"
	callback from command.
	* completer.h: Include "command.h".
	(set_gdb_completion_word_break_characters): New prototype.
	* python/py-cmd.c (cmdpy_completer_helper): New function.
	(cmdpy_completer_handle_brkchars): New function.
	(cmdpy_completer): Adjust to use cmdpy_completer_helper.
	(cmdpy_init): Set completer_handle_brkchars to
	cmdpy_completer_handle_brkchars.

gdb/testsuite/ChangeLog:
2014-09-03  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR python/16699
	* gdb.python/py-completion.exp: New file.
	* gdb.python/py-completion.py: Likewise.

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