Python wrapper verification error when there are multiple revocation registry entries

[conanoc]

This is the reason for the error openwallet-foundation/acapy#2036

The problem is here:
https://github.com/hyperledger/indy-shared-rs/blob/v0.3.1/wrappers/python/indy_credx/types.py#L457-L464

The RevocationRegistry created at L459 will be deallocated outside the for loop, but it's handle entry.handle at L462 should be retained by the reg_entries list.
This would work if the RevocationEntry at L461 is not Structure type. RevocationEntry subclasses Structure of ctypes and this makes entry.handle deallocated before bindings.verify_presentation() starts.
I could make it work by retaining the RevocationRegistry created at L459 in a dummy list to prevent deallocation, which also prevents the deallocation of its handle.

You may wonder why it works when there is only one revocation entry. I suppose it's dependent on the GC behavior of python.
I created a sample code to see this issue.

from ctypes import c_int64, Structure, c_char_p

class Age(c_int64):
    def __del__(self):
        print(f"Age: delete called for {self.value}")

class Applicant:
    def __init__(self, name, age = None):
        self.name = name
        if not age:
            age = Age(0)
        self.age = age

    def __del__(self):
        print(f"Applicant: delete called for {self.name}")

class Candidate(Structure):
    _fields_ = [("name", c_char_p), ("age", Age)]

    @classmethod
    def create(
        cls,
        name,
        age,
    ):
        return Candidate(name=c_char_p(name.encode("utf-8")), age=age)

def print_candidates(applicants):
    candidates = []
    for applicant in applicants:
        if not isinstance(applicant, Applicant):
            applicant = Applicant(applicant)
        print("== appending candidate: ", applicant.name)
        candidates.append(Candidate.create(applicant.name, applicant.age))  # <-- applicant.age is not retained by this because Candidate is a Struct.

    print("== print candidates ==")
    for candidate in candidates:
        print(candidate.name)

print("== print multiple candidates ==")
applicants = [
    "Alice",
    Applicant("Bob", Age(30)),
]

print_candidates(applicants)

print("== print a single candidate ==")
applicants = [
    "Alice",
]

print_candidates(applicants)

You can find that the Age instance, which corresponds to ObjectHandle, is deallocated before printing the candidates when there is more than one entry.

[conanoc]

This is the output of the sample code above:

== print multiple candidates ==
== appending candidate:  Alice
Applicant: delete called for Alice
Age: delete called for 0
== appending candidate:  Bob
== print candidate ==
b'Alice'
b'Bob'
== print a single candidate ==
Applicant: delete called for Bob
Age: delete called for 30
== appending candidate:  Alice
== print candidate ==
b'Alice'
Applicant: delete called for Alice
Age: delete called for 0

[swcurran]

Heads up on this issue @andrewwhitehead @TimoGlastra @blu3beri and @whalelephant. This will need to be looked at in the anoncreds-rs for sure, perhaps retrofitted here as well.

Thanks for diving into this @conanoc .