Python scripting prototype

[Vladimir Prus <vladimir at codesourcery dot com>]

The Python scripting prototype is now available 
from the 'python' branch of the git repository at

	git://repo.or.cz/gdb.git

and you can browse the code/changes at:

	http://repo.or.cz/w/gdb.git?a=log;h=python	

At present, it allows to customize how the 
-var-evaluate-expression gets the string, for example, if
'v' is a variable in the program, of std::vector type,
one can do:

	(gdb)
	-var-create V * v
	^done,name="V",numchild="1",value="{...}",type="std::vector<int,std::allocator<int> >"
	(gdb)
	-var-set-pretty-printer V format_vector
	^done
	(gdb)
	-var-evaluate-expression V
	^done,value="[1,2]"
	(gdb)

Here, format_vector is a name of Python function, which is defined 
in .gdbinit, like follows:

python
def format_vector (v):
    impl = gdb.value_element (v, '_M_impl')
    start = gdb.value_element (impl, '_M_start')
    finish = gdb.value_element (impl, '_M_finish')
    result = '['
    current = start
    first = 1
    while not gdb.value_equal (current, finish):
        if not first: result = result + ','
        first = 0
        result = result + gdb.value_string(gdb.value_dereference(current))
        current = gdb.value_increment (current, 1)
    result = result + ']'
    return result
def list_vector_children (v):
    return [("first", gdb.value_from_int(1)), ("second", gdb.value_from_int(15))]
end

here, 'python' is a new command that executes python code, and any function
defined remain in python's namespace, and are available for use.

The current problems:

1. The code exposing gdb values to python leaks like crazy.
2. The Python interface for values in procedural, and should
be object-oriented -- with class 'value' that one can
add, subscribe, etc. Internally, we now use integers
to represent value*, and we should use properly define a new
class in C.
3. The linking to python is basically hardcoded in makefile.in
4. The code for dynamically computing the children of varobj
is incomplete.

To build the code, you might need to change python2.5 and python25
strings hardcoded in Makefile.in to whatever version of Python you have.

I don't expect the current code to be very useful, but since I've promised
to publish it, here it goes. I intend to improve on points (1-4) above,
and then it will be suitable for submission, hopefully soon.

- Volodya