[BuildStream] Element's variables in sources declarations

[Benjamin Schubert <contact benschubert me>]

Hey,

TLDR: I'm proposing to give sources access to their element's variables

I have been playing a bit more with BuildStream and encountered a few cases

where I would have the same variable repeated in multiple places.

It would be useful for users to be able to use element's variables in their

sources to remove that need. Moreover, as the 'variables' and the 'sources'

often live in the same file, it is natural to try using those variables.

Example use case:

Let's assume I want to build OpenJDK 8. A BST element currently would be like:

-------------

kind: manual

sources:

  - kind: tar

    url: openjdk:jdk8u/jdk8u/archive/jdk8u232-b05.tar.bz2

    ref: ed87467e24c623396eae0bb2057bc2a8b8fd0ed30843e9413e578c7b3e5c689f

  - kind: tar

    url: openjdk:jdk8u/jdk8u/langtools/archive/jdk8u232-b05.tar.bz2

    ref: 812c70931157f5ba41e178b6e394a8552f15f53d0171d2f1b66537c9a76d5cc9

    directory: langtools

  - kind: tar

    url: openjdk:jdk8u/jdk8u/hotspot/archive/jdk8u232-b05.tar.bz2

    directory: hotspot

    ref: 820da1b5dcfece8f249395b45983984c6c82886d78cbd1183faa98ad5497e8b8

< snip: 8 more sources with the same pattern are needed>

--------------

Notice the '232' and the '05' in each source. They represent the version and

build number of the java runtime you want to build. It is also needed in the

configure step.

Modifying the version or build number becomes annoying, whereas, if we could

define a 'version' and 'build' variable, updating would be a single line

change.

Does someone see any problem with allowing sources to access the element's

variable? Or did the use case never came up before?

Cheers,

Ben