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>

1 files changed, 89 insertions, 0 deletions

diff --git a/repo b/repo
index 8f13015f..8936f57b 100755
--- a/repo
+++ b/repo
@@ -246,6 +246,7 @@ GITC_FS_ROOT_DIR = '/gitc/manifest-rw/'
 
 import collections
 import errno
+import json
 import optparse
 import re
 import shutil
@@ -1035,6 +1036,90 @@ def _ParseArguments(args):
   return cmd, opt, arg
 
 
+class Requirements(object):
+  """Helper for checking repo's system requirements."""
+
+  REQUIREMENTS_NAME = 'requirements.json'
+
+  def __init__(self, requirements):
+    """Initialize.
+
+    Args:
+      requirements: A dictionary of settings.
+    """
+    self.requirements = requirements
+
+  @classmethod
+  def from_dir(cls, path):
+    return cls.from_file(os.path.join(path, cls.REQUIREMENTS_NAME))
+
+  @classmethod
+  def from_file(cls, path):
+    try:
+      with open(path, 'rb') as f:
+        data = f.read()
+    except EnvironmentError:
+      # NB: EnvironmentError is used for Python 2 & 3 compatibility.
+      # If we couldn't open the file, assume it's an old source tree.
+      return None
+
+    return cls.from_data(data)
+
+  @classmethod
+  def from_data(cls, data):
+    comment_line = re.compile(br'^ *#')
+    strip_data = b''.join(x for x in data.splitlines() if not comment_line.match(x))
+    try:
+      json_data = json.loads(strip_data)
+    except Exception:  # pylint: disable=broad-except
+      # If we couldn't parse it, assume it's incompatible.
+      return None
+
+    return cls(json_data)
+
+  def _get_soft_ver(self, pkg):
+    """Return the soft version for |pkg| if it exists."""
+    return self.requirements.get(pkg, {}).get('soft', ())
+
+  def _get_hard_ver(self, pkg):
+    """Return the hard version for |pkg| if it exists."""
+    return self.requirements.get(pkg, {}).get('hard', ())
+
+  @staticmethod
+  def _format_ver(ver):
+    """Return a dotted version from |ver|."""
+    return '.'.join(str(x) for x in ver)
+
+  def assert_ver(self, pkg, curr_ver):
+    """Verify |pkg|'s |curr_ver| is new enough."""
+    curr_ver = tuple(curr_ver)
+    soft_ver = tuple(self._get_soft_ver(pkg))
+    hard_ver = tuple(self._get_hard_ver(pkg))
+    if curr_ver < hard_ver:
+      print('repo: error: Your version of "%s" (%s) is unsupported; '
+            'Please upgrade to at least version %s to continue.' %
+            (pkg, self._format_ver(curr_ver), self._format_ver(soft_ver)),
+            file=sys.stderr)
+      sys.exit(1)
+
+    if curr_ver < soft_ver:
+      print('repo: warning: Your version of "%s" (%s) is no longer supported; '
+            'Please upgrade to at least version %s to avoid breakage.' %
+            (pkg, self._format_ver(curr_ver), self._format_ver(soft_ver)),
+            file=sys.stderr)
+
+  def assert_all(self):
+    """Assert all of the requirements are satisified."""
+    # See if we need a repo launcher upgrade first.
+    self.assert_ver('repo', VERSION)
+
+    # Check python before we try to import the repo code.
+    self.assert_ver('python', sys.version_info)
+
+    # Check git while we're at it.
+    self.assert_ver('git', ParseGitVersion())
+
+
 def _Usage():
   gitc_usage = ""
   if get_gitc_manifest_dir():
@@ -1192,6 +1277,10 @@ def main(orig_args):
     print("fatal: unable to find repo entry point", file=sys.stderr)
     sys.exit(1)
 
+  reqs = Requirements.from_dir(os.path.dirname(repo_main))
+  if reqs:
+    reqs.assert_all()
+
   ver_str = '.'.join(map(str, VERSION))
   me = [sys.executable, repo_main,
         '--repo-dir=%s' % rel_repo_dir,