Allow `mason install` to work without scripts?

[springmeyer]

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I'm pondering the best, most lightweight, method to recommend installing mason packages on CI systems.

Installing mason with git clone -b master --single-branch https://github.com/mapbox/mason.git brings down > 2.7 MB of data and this will likely grow over time.

That is not a lot, but much more than is needed to install a package, which can currently be done as simply as:

PACKAGE_URL="https://mason-binaries.s3.amazonaws.com/${TRAVIS_OS_NAME}-x86_64/${MASON_NAME}/${MASON_VERSION}.tar.gz"
travis_retry wget --quiet -O - ${PACKAGE_URL} | tar -xz

For packages representing libraries most usecases will want/need the main mason command in order to query for cflags and ldflags. But for command line tools these features are unnecessary.

Two ideas (with command line tools in mind):

• Write a very small mason-install script that CI systems could download and use to install a package: ./mason-install clang 3.8.0. It would not work for header-only libs (would put them in the wrong location)
• Modify the existing mason binary to work for installs without needing the script/ metadata around.
  • This would require disabling the checks for MASON_LIB_FILE among other modifications. Hmmmm...

/cc @ericfischer @TheMarex (maintainers on two repos that are using the manual method to install, but this is brittle)

[kkaefer]

I think we could support header only libaries by just using two different commands; We know when a library is header only, and it's very unlikely that they change from header-only to pre-compiled.

Another thing to consider is supporting downloads from CMake, along the lines of mason.cmake: Right now it runs mason as a subprocess, but ideally we can get rid of that by using something like:

mason_use_library(gtest 1.7.0)
mason_use_headers(rapidjson 1.0.2)

Once we're moving from a mason script that knows both how to build and how to use a package to a two part system with one part only building packages, and the other only downloading them, we should also include the flags in a file so we don't have to invoke parts of the build system to obtain flags.

[kkaefer]

This is partially implemented in https://github.com/mapbox/mason/pull/176:

If the package has a mason.ini file, it doesn't require mason to be present at all. You can add packages like this: mason_use(variant VERSION 1.1.1 HEADER_ONLY), then use MASON_CONFIG_variant_INCLUDE_DIRS or target_add_mason_package(target PUBLIC variant)

[kkaefer]

Implemented in GL Native in https://github.com/mapbox/mapbox-gl-native/pull/7981

[springmeyer]

Per chat: next step here is to split the mason bash client into a) light install client/consumer that download packages and parses only mason.ini and b) heavy install frontend that knows how to build packages. The install client will no longer fallback to source compiles.

[kkaefer]

Bash client first draft is in https://github.com/mapbox/mason/tree/client. More tasks are in https://github.com/mapbox/mason/projects/1