[bug] Performance issues with nested `PacketField`s

Brief description

When PacketFields are nested on several stages,
Scapy races exponential performance issues.

Scapy version

2.6.1

Python version

3.8.6

Operating system

Windows

Additional environment information

No response

How to reproduce

The following .uts test demonstrates the issue:

% Packet field performance issue

############
############
+ Test utils

= Prepare a `check_exec_time()` function.

def check_exec_time(description, t0, tf, max):
    print(f"{description}: {tf - t0:.3f} seconds")
    assert tf - t0 <= max, f"{description!r} FAILED: {tf - t0:.3f} > {max:.3f}"


############
############
+ Test data

= Define the `F` final packet class.

class F(Packet):
    fields_desc = [
        ByteField(name="value", default=0),
    ]
    def extract_padding(self, s):
        return (b'', s)

= Define the `I` intermediate packet class.

class I(Packet):
    NUMBER_OF_F_PER_I = 100
    fields_desc = [
        PacketField(name=f"f{i}", pkt_cls=F, default=F(value=i))
        for i in range(NUMBER_OF_F_PER_I)
    ]
    def extract_padding(self, s):
        return (b'', s)

= Define the `M` main packet class.

class M(Packet):
    NUMBER_OF_I_PER_M = 100
    fields_desc = [
        PacketField(name=f"i{i}", pkt_cls=I, default=I())
        for i in range(NUMBER_OF_I_PER_M)
    ]

= Define `MAX_INSTANTIATION_TIME_EXPECTED`.

MAX_INSTANTIATION_TIME_EXPECTED = 1.0

= Define `MAX_SERIALIZATION_TIME_EXPECTED`.

MAX_SERIALIZATION_TIME_EXPECTED = 0.250

= Define `MAX_PARSING_TIME_EXPECTED`.

MAX_PARSING_TIME_EXPECTED = 0.250


############
############
+ Default instantiations

= Build a default instance of `F`.

t0 = time.time()
f = F()
tf = time.time()
check_exec_time("Default instantiation of `F`", t0, tf, MAX_INSTANTIATION_TIME_EXPECTED)

= Build a default instance of `F` again.

t0 = time.time()
f = F()
tf = time.time()
check_exec_time("Default instantiation of `F`", t0, tf, MAX_INSTANTIATION_TIME_EXPECTED)

= Build a default instance of `I`.

t0 = time.time()
i = I()
tf = time.time()
check_exec_time("Default instantiation of `I`", t0, tf, MAX_INSTANTIATION_TIME_EXPECTED)

= Build a default instance of `B` again.

t0 = time.time()
i = I()
tf = time.time()
check_exec_time("Default instantiation of `I`", t0, tf, MAX_INSTANTIATION_TIME_EXPECTED)

= Build a default instance of `M`.

t0 = time.time()
m = M()
tf = time.time()
check_exec_time("Default instantiation of `M`", t0, tf, MAX_INSTANTIATION_TIME_EXPECTED)

= Build a default instance of `M` again.

t0 = time.time()
m = M()
tf = time.time()
check_exec_time("Default instantiation of `M`", t0, tf, MAX_INSTANTIATION_TIME_EXPECTED)


############
############
+ Serialization

= Launch serialization from the latest instance of `M` created.

t0 = time.time()
buf = m.build()
tf = time.time()
check_exec_time("Serialization of `M`", t0, tf, MAX_SERIALIZATION_TIME_EXPECTED)


############
############
+ Parsing

= Parse the buffer serialized before with the latest instance of `M` created.

t0 = time.time()
m.dissect(buf)
tf = time.time()
check_exec_time("Parsing of `M`", t0, tf, MAX_PARSING_TIME_EXPECTED)

Actual result

No response

Expected result

No response

Related resources

No response

Start by running the supplied .uts reproduction with Scapy 2.6.1 and inspect the nested PacketField behavior during instantiation, serialization, and parsing. Trace the relevant PacketField entry points to identify the performance regression; done means the reproduced timings stay within the stated limits for the nested F, I, and M packets.