Feat: Android support

[jaoleal]

This pr contain 3 commits:

1. an old version of Add MSRV Cargo-minimal.lock and Cargo-recent.lock files #150 to make CI work again flawlessly.
2. A patch to the projects flake adding a nix-shell that give us the necessary setup for the android build, which includes the android toolchain and some environment variables. The second commit also introduces a new job on the CI that executes the building for android.
3. Android building instructions for build.rs

This is just a PoC, the main commit is the third one and the CI job containing the android building at that point should run fine.
For more context read #156

[sedited]

This looks nice, can you rebase this?

[jaoleal]

Changelog:

• Removed some PoC code
• Included android native building in ci (whitout nix).
• A fix regarding building on macos was included to.
• Rebased on top of Chore: remove bindgen and replace with hand-rolled FFI bindings #163 and included a fix regarding pointer sizes.

[jaoleal]

I reduced a lot the code and i think this is the concise version to support android

[alexanderwiederin]

I don't think android is a supported target for libbitcoinkernel. Doesn't this require upstream work on bitcoin core first?

[alexanderwiederin]

From what I understand the ubuntu boost packages don't ship with the cmake config that core needs when cross-compiling.

Am I right that you got it working on nix locally? If so, can we stick with with only supporting the nix build for android?

[jaoleal]

Am I right that you got it working on nix locally? If so, can we stick with with only supporting the nix build for android?

Yes, i worked all this on nix but Im not sure about leaving android building exclusively for nix, this would make it a direct dependency of the project and its not something really special that only nix can do. Besides the dependency problem, nix can drift versions because of nixpkgs releases and that can raise problems only by itself.

Therefore, i still would like for CI to keep the runner as ubuntu. Its easier for the general developer audience to replicate the build on their machine if they need

Edit: see the comment below

[jaoleal]

Correction, I changed my mind... The idea is that the build.rs file is already bloated because of the android support additions, adding support for ubuntu bloated it even more... It looks like leaving to nix to cross-compile to android is actually a good choice, the changes stay minimal on build.rs side and it have broader support running on linux and macos machines... Ill make sure to document that somewhere and state how to make the android build.

[jaoleal]

Okay the android build being exclusive for nix made things easier for this PR, the changes on build.rs are minimal... I included some inline doc comments so the reviewer can follow why each line were added.

I added a section on the readme explaining the android build but its not that explanatory in a level that one can learn how to build this lib to android, do you guys think thats needed ?

I can extend the docs to include instructions for one to reproduce it on a downstream build.rs, just tell me where to put it.
AFAIK is trivial for those using Kotlin, as mandacaru is using, to pull an already compiled shared object during build time. Thats what ill do on floresta-nix for mandacaru, ideally the CI would also offer the compiled objects too for those who might need it.

[alexanderwiederin]

Great to see this working.

If we land the hand-written bindings PR (#163) first, the changes in build.rs should become simpler. I would suggest we wait for it.

[sedited]

@jaoleal I'm interested in getting this change through. Can you rebase it and address Alex' comments?

[jaoleal]

@jaoleal I'm interested in getting this change through. Can you rebase it and address Alex' comments?

Yes, sorry for taking long... I was waiting for the bindgen pr to be merged so we had sure what that would imply here without rebasing on a pr