Fix is_maintenance_branch heuristic

This was broken because Nix language's version comparison does not know
how to deal with versions like -rc1 and considers them newer, which is
in this case not desirable.

That in turn led to not tagging 2.90.0 docker images as "latest" since
the heuristic was wrong.

This commit also adds some more cross-checking and failsafes in case the
person running releng does not have a local main branch that is up to
date.

Fixes: #443
Change-Id: I537103ebab58ae978c00e06972abe14432dd9c80

package.nix

@@ -423,6 +423,7 @@ stdenv.mkDerivation (finalAttrs: {
             p.python-frontmatter
             p.requests
             p.xdg-base-dirs
+            p.packaging
             (p.toPythonModule xonsh.passthru.unwrapped)
           ]
         );

releng/gitutils.xsh

@@ -1,11 +1,24 @@
 import subprocess
-import json
+from packaging.version import Version
 
 from .version import VERSION
 
 
+def remote_is_plausible(url: str) -> bool:
+    return ('git.lix.systems' in url and 'lix-project/lix' in url) or ('gerrit.lix.systems' in url and url.endswith('lix'))
+
+
 def version_compare(v1: str, v2: str):
-    return json.loads($(nix-instantiate --eval --json --argstr v1 @(v1) --argstr v2 @(v2) --expr '{v1, v2}: builtins.compareVersions v1 v2'))
+    v1 = Version(v1)
+    v2 = Version(v2)
+    if v1 < v2:
+        return -1
+    elif v1 > v2:
+        return 1
+    elif v1 == v2:
+        return 0
+    else:
+        raise ValueError('these versions are beyond each others celestial plane')
 
 
 def latest_tag_on_branch(branch: str) -> str:
@@ -13,16 +26,18 @@ def latest_tag_on_branch(branch: str) -> str:
 
 
 def is_maintenance_branch(branch: str) -> bool:
-    try:
+    """
-        main_tag = latest_tag_on_branch('main')
+    Returns whether the given branch is probably a maintenance branch.
+
+    This uses a heuristic: `main` should have a newer tag than a given
+    maintenance branch if there has been a major release since that maintenance
+    branch.
+    """
+    assert remote_is_plausible($(git remote get-url origin).strip())
+    main_tag = latest_tag_on_branch('origin/main')
     current_tag = latest_tag_on_branch(branch)
 
     return version_compare(current_tag, main_tag) < 0
-    except subprocess.CalledProcessError:
-        # This is the case before Lix releases 2.90, since main *has* no
-        # release tag on it.
-        # FIXME: delete this case after 2.91
-        return False
 
 
 def verify_are_on_tag():