Buildpacks CLI (pack) downloader

[manusa]

Description

Part of #439

As a JKube developer, I want to be able to let JKube automatically download a Buildpacks CLI (pack) binary compatible with my platform and system architecture, so that I can invoke a consistent version of the CLI tool (see #2453).

TODO:

• Define the binary source URLs or method to infer them
• Define the target location for the downloaded binaries
• Define what happens if the downloaded binary is corrupt (recover, force update)
• Define what happens in an air-gapped environment (should we fallback to existent CLI?)

[rohanKanojia]

• Define the binary source URLs or method to infer them
  As agreed internally at standup meeting, we decided that binary source download URL, version and other attributes could be inferred from parent pom via properties:

      # BuildPack related properties in Parent Pom
    <version.cli.pack.upstream.github>v0.32.1</version.cli.pack.upstream.github>
    <download-link.cli.pack.upstream.github>https://github.com/buildpacks/pack/releases/download</download-link.cli.pack.upstream.github>
    <binary-prefix.cli.pack.upstream.github>pack</binary-prefix.cli.pack.upstream.github>
    <archive-suffix.unix.cli.pack.upstream.github>tgz</archive-suffix.unix.cli.pack.upstream.github>
    <archive-suffix.windows.cli.pack.upstream.github>zip</archive-suffix.windows.cli.pack.upstream.github>

• Define the target location for the downloaded binaries
  .jkube

• Define what happens if the downloaded binary is corrupt (recover, force update)
  When JKube detects a pack binary already present on system, it checks whether it's in expected state by running pack --version command. If these conditions are satisfied, binary is considered to be in a usable state:

  • Command completes successfully
  • pack CLI version is equal to expected version by JKube

Otherwise, download is triggered.

• Define what happens in an air-gapped environment (should we fallback to existent CLI?)

  If download fails, JKube would look for pack CLI already present on machine. It would check whether it's valid or not by checking (pack --version) command. If previous step is successful, then we use the binary already present on User's system.

  • Shall we enforce user to have pack CLI only above certain version? Shall we throw error if user has pack CLI with old version?
  • What if download is done via proxy? Shall we pass maven proxy settings while doing download?