tree 39bff697aee8c2b318e7e10c7a5e92f365b6f439
parent 48b2d10d8f7565173ca53bed0d0be15323512de4
author Mike Frysinger <vapier@google.com> 1610075665 -0500
committer Mike Frysinger <vapier@google.com> 1611074901 +0000

launcher: add a requirements framework to declare version dependencies

Currently we don't have a way for the checked out repo version to
declare the version of tools it needs before we start running it.
For somethings, like git, it's not a big deal as it can handle all
the asserts itself.  But for things like Python, it's impossible
to reliably check before executing.

We're in this state now:
- we've been allowing Python 3.4, so the launcher accepts it
- the repo codebase starts using Python 3.6 features
- launcher tries to import us but hits syntax errors
- user is left confused and assuming new repo is broken because
  they're seeing syntax errors

This scenario is playing out with old launchers that still accept
Python 2, and will continue to play out as time goes on and we want
to require newer versions of Python 3.

Lets create a JSON file to declare all these system requirements.
That file format is extremely stable, so loading & parsing from
even ancient versions of Python shouldn't be a problem.  Then the
launcher can read these settings and check the system state before
attempting to execute any code.  If the tools are too old, it can
clearly diagnose & display information to the user as to the real
problem (and not emit tracebacks or syntax errors).

We have a couple of different tool version checks already (git,
python, ssh) and can harmonize them in a single place.

This also allows us to assert a reverse dependency if the need
ever comes up: force the user to upgrade their `repo` launcher
before we'll let them run us.  Even though the launcher warns
whenever a newer release is available, some users seem to ignore
that, or they don't use repo that often (on the scale of years),
and their upgrade jump is so dramatic that they fall back into
the syntax error pit.

Hopefully by the end of the year we can assume enough people
have upgraded their launcher such that we can delete all of the
duplicate version checks in the codebase.  But until then, we'll
keep them to maintain coverage.

Change-Id: I5c12bbffdfd0a8ce978f39aa7f4674026fe9f4f8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/293003
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>