StrFixedLenField with incomplete packet

Brief description

Interactions between StrFixedLenField and raw packet cache cause some inconsistencies when some packet field is or is not modified. This should probably be fixed by a modification of StrFixedLenField but a choice needs to be made between two possible behaviours.

Scapy version

all (at least from 2.3.1 to 2.6.0rc1)

Python version

all

Operating system

all

Additional environment information

No response

How to reproduce

from scapy.packet import Packet
from scapy.fields import StrFixedLenField

class P(Packet):
    fields_desc = [
        StrFixedLenField('a', None, length=1),
        StrFixedLenField('b', None, length=1),
        ]

p = P(b'0')
s0 = p.build()
p.a = b'1'
s1 = p.build()
print(s0, s1)

Actual result

b'0' b'1\x00'

Expected result

Either b'0\x00' b'1\x00' (if StrFixedLenField shall always be of the given length) or b'0' b'1' (if incomplete StrFixedLenField are to be remembered as incomplete)

Related resources

No response

Start by reproducing the interaction between Packet.build, the raw packet cache, and the two StrFixedLenField instances using the example in the issue. Decide which incomplete-field behavior is intended, then make the resulting builds consistent and verify both builds against the selected expected output.