Fix Paho C build

[laitingsheng]

This PR is mainly for fixing the CMake error when PAHO_WITH_MQTT_C is enabled.

I also changed the Git submodule URL to a relative path so that it can follow the same protocol used to clone the main repository.

Fixes #469.

[fpagliughi]

I was waiting to see if this PR would land in the Paho C repo before making any further changes to the build here.
eclipse/paho.mqtt.c#1428

Some things in the master/develop branches assume it will land.

[laitingsheng]

OK sure.

[Zelif]

@laitingsheng @fpagliughi
This still seems to fail for the first clean cmake build with the same error in the issue.

Can repeat it in VS code by issuing
CMake: Delete cache and Reconfigure

Ends up causing issues inside github actions as it is a clean build every time. Will try to figure out why the C lib is not found on the alias and install step for the first time.

[laitingsheng]

It works fine on Ubuntu (with or without OpenSSL). What error output did you get?

FYI, I am using this command to generate the build folder:

cmake --fresh -G Ninja -B build -S . -DPAHO_WITH_SSL:BOOL=TRUE -DPAHO_WITH_MQTT_C:BOOL=TRUE

[Zelif]

@laitingsheng
Sorry for my derp, this seems to be a result of how I was setting the SSL variable in the cmake
Having the top level cmake set this:

SET(PAHO_WITH_SSL					ON)

Worked for the mqtt Cpp cmake but the C lib cmake seemed to keep seeing it as false, having the cache worked.

SET(PAHO_WITH_SSL					ON CACHE BOOL "Enable SSL build")

[laitingsheng]

@Zelif

Maybe you can have a look at this policy: https://cmake.org/cmake/help/latest/policy/CMP0077.html

And you can probably set this policy variable prior to the PAHO_WITH_SSL: https://cmake.org/cmake/help/latest/variable/CMAKE_POLICY_DEFAULT_CMPNNNN.html

[Zelif]

Does this break forks for people who have only the CPP version forked?
The solution to it is just to fork the c lib yourself. It was trying to pull down my own fork which I had not done.

[laitingsheng]

I changed it to ../../eclipse/paho.mqtt.c.git instead.

[fpagliughi]

@Zelif is correct. The main point of using the submodule is so that people can just grab the Paho C++ code and build it without having to think or worry about the C lib.

That would mean grabbing the C lib directly off of GitHub... not from a local directory! It would be wrong to assume that the C library sources have been downloaded locally.

[laitingsheng]

No. ../.. refers to github.com, and it will use the same protocol as the parent module. This is to avoid using https submodule even if you clone the parent repository with ssh.

[laitingsheng]

The reason .. doesn't work when you fork it is that the forked repository is in your own namespace, the full repository path of the forked repository will be github.com/{username}/paho.mqtt.cpp, and .. will refer to github.com/{username}.

[laitingsheng]

https://git-scm.com/docs/git-submodule#_commands
You can have a look at the doc about submodule relative URL.

[fpagliughi]

@laitingsheng , thanks so much for this PR. I gave up on waiting for the upstream library release, and started hacking on the existing build, forgetting that this was already here. I think I covered most of this in my changes, but sometimes in slightly different ways, like I meant to push externals/ up to the top-level directory, and I definitely want to keep the submodule referring to GitHub, etc.

My one question is... what's the purpose of adding EXCLUDE_FROM_ALL to the add_subdirectory() call for the C lib code?

And if you notice that I missed anything, please let me know.

[laitingsheng]

My one question is... what's the purpose of adding EXCLUDE_FROM_ALL to the add_subdirectory() call for the C lib code?

This can avoid any unwanted targets in the subdirectory to be included in the implicit all target.

[fpagliughi]

Thanks, @laitingsheng !
I will read up on all of this.