Mini Shell

"""
Debugging utilities for constructs
"""
import sys
import traceback
import pdb
import inspect
from construct.core import Construct, Subconstruct
from construct.lib import HexString, Container, ListContainer


class Probe(Construct):
    """
    A probe: dumps the context, stack frames, and stream content to the screen
    to aid the debugging process.

    .. seealso:: :class:`Debugger`.

    :param name: the display name
    :param show_stream: whether or not to show stream contents. default is True. the stream must be seekable.
    :param show_context: whether or not to show the context. default is True.
    :param show_stack: whether or not to show the upper stack frames. default is True.
    :param stream_lookahead: the number of bytes to dump when show_stack is set. default is 100.
    
    Example::
    
        Struct("foo",
            UBInt8("a"),
            Probe("between a and b"),
            UBInt8("b"),
        )
    """
    __slots__ = [
        "printname", "show_stream", "show_context", "show_stack", 
        "stream_lookahead"
    ]
    counter = 0
    
    def __init__(self, name = None, show_stream = True, 
                 show_context = True, show_stack = True, 
                 stream_lookahead = 100):
        Construct.__init__(self, None)
        if name is None:
            Probe.counter += 1
            name = "<unnamed %d>" % (Probe.counter,)
        self.printname = name
        self.show_stream = show_stream
        self.show_context = show_context
        self.show_stack = show_stack
        self.stream_lookahead = stream_lookahead
    def __repr__(self):
        return "%s(%r)" % (self.__class__.__name__, self.printname)
    def _parse(self, stream, context):
        self.printout(stream, context)
    def _build(self, obj, stream, context):
        self.printout(stream, context)
    def _sizeof(self, context):
        return 0
    
    def printout(self, stream, context):
        obj = Container()
        if self.show_stream:
            obj.stream_position = stream.tell()
            follows = stream.read(self.stream_lookahead)
            if not follows:
                obj.following_stream_data = "EOF reached"
            else:
                stream.seek(-len(follows), 1)
                obj.following_stream_data = HexString(follows)
            print("")
        
        if self.show_context:
            obj.context = context
        
        if self.show_stack:
            obj.stack = ListContainer()
            frames = [s[0] for s in inspect.stack()][1:-1]
            frames.reverse()
            for f in frames:
                a = Container()
                a.__update__(f.f_locals)
                obj.stack.append(a)
        
        print("=" * 80)
        print("Probe %s" % (self.printname,))
        print(obj)
        print("=" * 80)

class Debugger(Subconstruct):
    """
    A pdb-based debugger. When an exception occurs in the subcon, a debugger
    will appear and allow you to debug the error (and even fix on-the-fly).
    
    :param subcon: the subcon to debug
    
    Example::
    
        Debugger(
            Enum(UBInt8("foo"),
                a = 1,
                b = 2,
                c = 3
            )
        )
    """
    __slots__ = ["retval"]
    def _parse(self, stream, context):
        try:
            return self.subcon._parse(stream, context)
        except Exception:
            self.retval = NotImplemented
            self.handle_exc("(you can set the value of 'self.retval', "
                "which will be returned)")
            if self.retval is NotImplemented:
                raise
            else:
                return self.retval
    def _build(self, obj, stream, context):
        try:
            self.subcon._build(obj, stream, context)
        except Exception:
            self.handle_exc()
    def handle_exc(self, msg = None):
        print("=" * 80)
        print("Debugging exception of %s:" % (self.subcon,))
        print("".join(traceback.format_exception(*sys.exc_info())[1:]))
        if msg:
            print(msg)
        pdb.post_mortem(sys.exc_info()[2])
        print("=" * 80)