Drop libthemis-src support

[iamnotacake]

RustThemis has a libthemis-src crate which can be enabled with
vendored feature. It will build Themis library on-the-fly, during
themis build. It was a nice idea in the beginning, aimed at making
developers' lives easier. However, it also has some issues and
maintenance overhead:

• This is yet another configuration. There are tests for it, but they
  don't cover actual themis execution in this configuration, only
  libthemis-src build itself.
• Those tests may cause false positives because they are run from the
  repository root. In fact, currently published libthemis-src = 0.13.0
  does not build because some files are missing from it.
• We hijack a lot of decisions from the developers: how to link against
  Themis and what cryptography backend to use. libthemis-src does not
  offer a choice of the backend, it's always system OpenSSL.
• Building libthemis-src requires a tricky repository layout with
  symlinks that point to Themis source code. Every time a new build
  dependency is added, maintainers need to make sure that all
  dependencies are symlinked.
• cargo publish does not handle symlinks well and it may break
  publishing at times, when symlinks are included into the crate, not
  the files they point to.

Checklist

• Change is covered by automated tests
• The coding guidelines are followed
• Changelog is updated (in case of notable or breaking changes)

[ilammy]

Overall the PR looks good, but I believe there is one use case – building API docs on docs.rs – which will be broken if the changes are merged as is. However, there seems to be fix that needs to be tested. @iamnotacake, could you please look into it?

[ilammy]

This is one of the original reasons why libthemis-src was added (see #349 and ilammy/rust-themis#9). The thing is, docs.rs does not have Themis installed so if the build requires Themis to be installed, then docs.rs will not be able to build API documentation.

I suspect that it's no longer necessary given that libthemis-sys now does not use Bindgen to generate bindings on-the-fly. However, I have checked and it seems that we still (kinda) need Themis to be installed because build.rs of libthemis-sys checks that Themis is installed, even when just cargo doc is run.

We can't remove libthemis-src without fixing up docs.rs generation. I have looked around and it seems that they set DOCS_RS environment variable when generating documentation. We can use it in libthemis-sys/build.rs: look at that variable, if it is 1 then do not detect Themis installation and exit right away.

That way docs.rs will be able to run cargo doc without Themis being installed. In fact, we can remove this entire section because we don't need OpenSSL to be installed if we aren't going to build Themis.

It would also be nice to test this on CI somehow, but I don't see an immediate approach to that.

[iamnotacake]

Just checked, with

    if std::env::var("DOCS_RS") == Ok("1".to_string()) {
        return;
    }

in libthemis-sys/build.rs -> main(), DOCS_RS=1 cargo doc really works w/o libthemis installed, while running only cargo doc fails as expected. BTW, do we still need

[package.metadata.docs.rs]
dependencies = ["libssl-dev"]

?

[ilammy]

DOCS_RS=1 cargo doc really works w/o libthemis installed

Amazing! I believe that should do the trick.

do we still need dependencies = ["libssl-dev"]

No, I believe the entire [package.metadata.docs.rs] block can be removed. libssl-dev was required for Themis to compile, but if it's not going to be compiled then we don't need OpenSSL either.

[iamnotacake]

It would also be nice to test this on CI somehow, but I don't see an immediate approach to that.

Tried passing different PKG_CONFIG_* env vars (described in man pkg-config) to cargo build to make the compilation fail while the libs are installed. No success. If so, I would add something like
cargo clean --doc && DOCS_RS=1 PKG_CONFIG_SEARCH_ONLY_IN=/nonexistingdir cargo doc --workspace --no-deps
to run_tests.sh

[ilammy]

Another issue with CI is that libthemis-sys will try the system installation if pkg-config fails to find Themis Code. CI builds and installs Themis Core into the system before running RustThemis tests. We will need to somehow uninstall it for the docs.rs tests and then install it back for other tests. Maybe a better idea will present itself later. For now, I think, we can leave it like that. It's enough that you have tested it manually.

[ilammy]

Good job! 👍

[vixentael]

If you say that it works, I trust you :)