Diameter.__bytes__ has side effects (messes up len and cmd)

[pfirsich]

Describe the bug
After parsing a Diameter message, calling __bytes__ (e.g. by printing it)

To Reproduce
Steps to reproduce the behavior:

diam = dpkt.diameter.Diameter(data)
print(diam.len) # prints a number
print(diam)
print(diam.len) # prints a bytes object

Expected behavior
No fields should be affected by printing the object. len and cmd should stay the same.

Details(please complete the following information):

• OS: Red Hat Enterprise Linux Server 7.9 (Maipo)
• Python Version: 3.6.6

Sadly I cannot upload any pcap files (legal reasons), but the issue is very obvious.

Additional context
The cause of the problem is that __bytes__ calls pack_hdr which changes the member variables. Ideally it would not touch them or at the very least restore them: https://github.com/kbandla/dpkt/blob/master/dpkt/diameter.py#L106

[obormot]

Good catch. Assigning len and cmd in both the unpacking and packing functions definitely seems wrong.