update for opendnp3 version 3.1.0

[thewetzel]

This compiles and runs for me against the release branch of opendnp3, but I made some changes I'm not totally pleased with.

1. It looks like the old openpal/container/Buffer.h used to allocate its own storage and the new opendnp3/util/Buffer.h does not, so I replaced m_buffer in HexData.h with a std::array of fixed length 4096. This is probably not the most correct, but it was the shortest path I found to getting it working.
2. LogMacros.h is no longer installed with opendnp3, so the FORMAT_HEX_BLOCK macro no longer displays the original hex of the message to be decoded.
3. CMakeLists.txt in this project isn't very accommodating of having the opendnp3 library installed in non-standard directories. I don't know how to make it better.

[emgre]

1. openpal/container/Buffer.h is now ser4cpp/container/Buffer.h. This introduces another dependency (although it is already a private dependency of OpenDNP3), so sticking to a fixed-size array might be ok.
2. FORMAT_HEX_BLOCK is still in OpenDNP3, but it's private. But the hex output isn't printed by the log system?
   https://github.com/dnp3/opendnp3/blob/097b6ddcb012a54c9e8e10db03fec98ddc898341/cpp/lib/src/logging/LogMacros.h#L69-L73
3. We should probably use the FetchContent system of CMake to get OpenDNP3, similar to what we did here: https://github.com/dnp3/opendnp3/blob/release/deps/ser4cpp.cmake

[thewetzel]

1. So if dnp3cgi wants to make use of private headers from opendnp3, should dnp3cgi be included in opendnp3/cpp/examples with an optional CMAKE flag, like -DDNP3_CGI=ON?
2. It doesn't appear so. In DecoderImpl.cpp, the DecodeTPDU and DecodeAPDU functions have a FORMAT_HEX_BLOCK macro, but DecodeLPDU does not.
3. But I already built and installed opendnp3, I just installed it to a non-standard place. Is there something like pkg-config for cmake? It seems like we should be able to put find_package(opendnp3) in the CMakeLists.txt file and add -DCMAKE_PREFIX_PATH=/home/myuser/local to the cmake command and it should work, but it doesn't.

[jadamcrain]

I'm of the opinion that making this work with the latest version of opendnp3 is not worth it. I don't believe any new parsing functionality was added from OpenDNP3 2.1 -> current. No parsing bugs fixed, etc.

@thewetzel what's your end goal here? Do you want to have something local that you can run or do you just want this hosted again?

[thewetzel]

@jadamcrain Sorry, I didn't see your edit until now. My end goal is the latter. When I needed a decoder (either when Wireshark choked on some huge DNP3 response, or when I was trying to decode some serial DNP3), it was very convenient. I briefly put my "fixed" dnp3cgi on a webserver on a VPS, but I've since left the VPS world, and only have a Raspberry Pi hosting a small webserver now. I have a feeling that crosscompiling OpenDNP3 and dnp3cgi is going to require more effort than I'm willing to put in.