Conditional substructure fields do not work

[mikewadsten]

I would expect to be able to have a conditional SubstructureField, but that doesn't seem to actually work.

Example:

from suitcase.structure import Structure
from suitcase.fields import SubstructureField, ConditionalField, UBInt8


class Substruct1(Structure):
    cond = UBInt8()


class Substruct2(Structure):
    f1 = UBInt8()
    f2 = UBInt8()


class Substruct3(Structure):
    f31 = UBInt8()
    f32 = UBInt8()


class Struct(Structure):
    header = SubstructureField(Substruct1)
    s2 = ConditionalField(SubstructureField(Substruct2),
                          condition=lambda m: m.header.cond == 0)
    s3 = ConditionalField(SubstructureField(Substruct3),
                          condition=lambda m: m.header.cond == 0)


print("If the field(s) are not present...")
print(Struct.from_data('\x01'))

print("\nIf the field(s) are present...")
print(Struct.from_data('\x00\x11\x22\x33\x44'))

Output:

If the field(s) are not present...
Struct (
  header=Substruct1 (
  cond=1,
),
  s2=<ConditionalField: not included>,
  s3=<ConditionalField: not included>,
)

If the field(s) are present...
Traceback (most recent call last):
  File "condsubstruct.py", line 31, in <module>
    print(Struct.from_data('\x00\x11\x22\x33\x44'))
  File "/home/mike/.envs/suitcase/lib/python2.7/site-packages/suitcase/structure.py", line 247, in from_data
    m.unpack(data)
  File "/home/mike/.envs/suitcase/lib/python2.7/site-packages/suitcase/structure.py", line 300, in unpack
    return self._packer.unpack(data, trailing)
  File "/home/mike/.envs/suitcase/lib/python2.7/site-packages/suitcase/structure.py", line 61, in unpack
    self.unpack_stream(stream)
  File "/home/mike/.envs/suitcase/lib/python2.7/site-packages/suitcase/structure.py", line 138, in unpack_stream
    (name, length, len(data)))
suitcase.exceptions.SuitcaseParseError: While attempting to parse field 's2' we tried to read None bytes but we were only able to read 4.

If I had to guess, it might have something to do with SubstructureField returning None from bytes_required. But I'm not sure. I might dig into this more later tonight.

[mikewadsten]

I did some more digging, and it looks like the problem is that Packer doesn't see ConditionalField as equivalent to s SubstructureField, and so it assumes it to be greedy.

I have a working fix, but haven't written the unit tests for it yet.