This commit completes porting the internals of the MiniFakeDNS server
class to Python 3. This primarily means converting the Python 2 `str`
types to Python 3's `bytes` objects.
In the process, I've also translated the variable names from their
original Spanish into English, and added explanatory comments for how
the DNS header parsing is accomplished to enhance the educational
potential of SET.
Another small change is the addition of a new core helper function,
`detect_public_ip()`, which makes a couple parts of the codebase a
little more DRY by reducing code duplication across the `set.py` and
`setcore.py` files. This change also makes it possible to parameterize
the IP address that MiniFakeDNS server responds to requests with.
This is a relatively large commit because it refactors SET's interface
to the build-in DNS server it runs. Instead of a block of code inside of
the `setcore.py` file, a new module called `minifakedns` is added, which
houses all of the DNS-related code. Note that this commit *only*
refactors the DNS interface and its internals, it does not actually fix
the exception caused by receiving some DNS query, nor does it complete
the work required to set parameters for the DNS server, such as which IP
address it should respond with. It is just intended to make these
changes easier to introduce moving forward in upcoming chunks of work.
This replaces the `core.start_dns()` function with a new helper, whose
full path is `src.core.minifakedns.start_dns_server()`.
The previous implementation assumed the DNS server would be run from the
main thread, and thus have access to SIGINT, but this was never actually
possible because a `KeyboardInterrupt` was actually intercepted by SET
itself, before the DNS server code ever got to see it. This means that
the DNS server would never shut down cleanly.
This implementation changes that by using a simplistic sentinel value
(called `stop_flag`) that can be set as an instance attribute on the new
`MiniFakeDNS` object. When the sentinel value is `True`, the DNS server
thread will exit its listening loop.
Another change this introduces is the fact that, due to this new design,
the DNS server stops when `core.cleanup_routine()` is run. And, as a
note about that, this was running twice, once upon *startup* rather than
when SET shut down. This looked like a bug to me, and it was causing
problems for this DNS server design, so I removed that extra call and
performed some simplistic tests of various SET functionality to make
sure everything still works. (It seems fine, but might warrant a second
look.)
Finally, note that this commit breaks Python 2 compatibility due to the
use of a context manager handling the UDP socket. Given that the DNS
server was not really functional for some period of time before I
started looking at it more closely, the fact that Python 2 is officially
end-of-life'd (as of January 2020), the relative complexity of writing a
Python 2 and Python 3 implementation, and the fact that many comments
elsewhere in the SET codebase all seem to be nudging towards a Python 3
compatible upgrade, I am choosing to drop support for Python 2 in my own
patches, unless there is some considerable objection.
