[contrib] message-capture-parser: fix out of bounds error for empty vectors #25367

What do we want to happen in the case where val[0] is an integer and (some of) the other elements are not? Should this be an explicit error, or is it fine to implicitly fall through to to_jsonable?

Good question. On master, this scenario would lead to a (confusing) error right now, since calling ser_uint256(...) with a non-integer type fails:

diff --git a/contrib/message-capture/message-capture-parser.py b/contrib/message-capture/message-capture-parser.py
index 9988478f1b..a6bac539de 100755
--- a/contrib/message-capture/message-capture-parser.py
+++ b/contrib/message-capture/message-capture-parser.py
@@ -168,6 +168,12 @@ def process_file(path: str, messages: List[Any], recv: bool, progress_bar: Optio


 def main():
+    from test_framework.messages import msg_cfcheckpt
+    invalid_msg = msg_cfcheckpt()
+    invalid_msg.headers = [23, 42, "foobar"]
+    print(to_jsonable(invalid_msg))
+    return 1
+
     parser = argparse.ArgumentParser(
         description=__doc__,
         epilog="EXAMPLE \n\t{0} -o out.json <data-dir>/message_capture/**/*.dat".format(sys.argv[0]),

$ ./contrib/message-capture/message-capture-parser.py
Traceback (most recent call last):
  File "/home/honey/bitcoin/./contrib/message-capture/message-capture-parser.py", line 220, in <module>
    main()
  File "/home/honey/bitcoin/./contrib/message-capture/message-capture-parser.py", line 174, in main
    print(to_jsonable(invalid_msg))
  File "/home/honey/bitcoin/./contrib/message-capture/message-capture-parser.py", line 83, in to_jsonable
    ret[slot] = [ser_uint256(a).hex() for a in val]
  File "/home/honey/bitcoin/./contrib/message-capture/message-capture-parser.py", line 83, in <listcomp>
    ret[slot] = [ser_uint256(a).hex() for a in val]
  File "/home/honey/bitcoin/./contrib/message-capture/../../test/functional/test_framework/messages.py", line 117, in ser_uint256
    rs += struct.pack("<I", u & 0xFFFFFFFF)
TypeError: unsupported operand type(s) for &: 'bytes' and 'int'

On the PR it falls through to to_jsonable:

$ ./contrib/message-capture/message-capture-parser.py
{'filter_type': None, 'stop_hash': None, 'headers': [23, 42, 'foobar']}

Options that I can think of right now:

1. keep the PR as it is (being more tolerant if some internal data types are not matching, but potentially also covering bugs)
2. change the all predicate to any to closely mimic the behaviour on master
3. explicitly assert that all array elements have to be integers for the HASH_INT_VECTORS types (seems right, but could have unintended consequences that I am not aware of)

Thanks for checking! In that case, I like adding an explicit assertion best. It makes it immediately clear what the problem is.

Okay, done. 👌 Didn't check in detail, but I think this assert can't be triggered by a malformed message-capture input, it would rather only point out that something is wrong internally (e.g. our deserialization routines), which is good.