sync: rework selfupdate logic

The current logic has a downside in that it doesn't sync to the latest
signed version available if the latest commit itself is unsigned.  This
can come up when using the "main" branch as it is sometimes signed, but
often not as it's holding the latest merged commits.  When people use
the main branch, it's to get early testing on versions tagged but not
yet released, and we don't want them to get stuck indefinitely on that
old version of repo.

For example, this series of events:
* "stable" is at v2.12.
* "main" is tagged with v2.13.
* early testers use --repo-rev main to get v2.13.
* new commits are merged to "main".
* "main" is tagged with v2.14.
* new commits are merged to "main".
* devs who had synced in the past to test v2.13 are stuck on v2.13.
  repo sees "main" is unsigned and so doesn't try to upgrade at all.

The only way to get unwedged is to re-run `repo init --repo-rev main`,
or to manually sync once with repo verification disabled, or for us to
leave "main" signed for a while and hope devs will sync in that window.

The new logic is that whenever changes are available, we switch to the
latest signed tag.  We also replace some of the duplicated verification
code in the sync command with the newer wrapper logic.  This handles a
couple of important scenarios inaddition to above:
* rollback (e.g. v2.13.8 -> v2.13.7)
* do not trash uncommitted changes (in case of ad-hoc testing)
* switch tag histories (e.g. v2.13.8 -> v2.13.8-cr1)

Change-Id: I5b45ba1dd26a7c582700ee3711f303dc7538579b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/300122
Reviewed-by: Jonathan Nieder <jrn@google.com>
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/sync.py

@@ -20,9 +20,7 @@ import multiprocessing
 import netrc
 from optparse import SUPPRESS_HELP
 import os
-import re
 import socket
-import subprocess
 import sys
 import tempfile
 import time
@@ -46,7 +44,7 @@ except ImportError:
     return (256, 256)
 
 import event_log
-from git_command import GIT, git_require
+from git_command import git_require
 from git_config import GetUrlCookieFile
 from git_refs import R_HEADS, HEAD
 import git_superproject
@@ -956,12 +954,25 @@ def _PostRepoUpgrade(manifest, quiet=False):
 def _PostRepoFetch(rp, repo_verify=True, verbose=False):
   if rp.HasChanges:
     print('info: A new version of repo is available', file=sys.stderr)
-    print(file=sys.stderr)
-    if not repo_verify or _VerifyTag(rp):
-      syncbuf = SyncBuffer(rp.config)
-      rp.Sync_LocalHalf(syncbuf)
-      if not syncbuf.Finish():
-        sys.exit(1)
+    wrapper = Wrapper()
+    try:
+      rev = rp.bare_git.describe(rp.GetRevisionId())
+    except GitError:
+      rev = None
+    _, new_rev = wrapper.check_repo_rev(rp.gitdir, rev, repo_verify=repo_verify)
+    # See if we're held back due to missing signed tag.
+    current_revid = rp.bare_git.rev_parse('HEAD')
+    new_revid = rp.bare_git.rev_parse('--verify', new_rev)
+    if current_revid != new_revid:
+      # We want to switch to the new rev, but also not trash any uncommitted
+      # changes.  This helps with local testing/hacking.
+      # If a local change has been made, we will throw that away.
+      # We also have to make sure this will switch to an older commit if that's
+      # the latest tag in order to support release rollback.
+      try:
+        rp.work_git.reset('--keep', new_rev)
+      except GitError as e:
+        sys.exit(str(e))
       print('info: Restarting repo with latest version', file=sys.stderr)
       raise RepoChangedException(['--repo-upgraded'])
     else:
@@ -972,45 +983,6 @@ def _PostRepoFetch(rp, repo_verify=True, verbose=False):
             file=sys.stderr)
 
 
-def _VerifyTag(project):
-  gpg_dir = os.path.expanduser('~/.repoconfig/gnupg')
-  if not os.path.exists(gpg_dir):
-    print('warning: GnuPG was not available during last "repo init"\n'
-          'warning: Cannot automatically authenticate repo."""',
-          file=sys.stderr)
-    return True
-
-  try:
-    cur = project.bare_git.describe(project.GetRevisionId())
-  except GitError:
-    cur = None
-
-  if not cur \
-     or re.compile(r'^.*-[0-9]{1,}-g[0-9a-f]{1,}$').match(cur):
-    rev = project.revisionExpr
-    if rev.startswith(R_HEADS):
-      rev = rev[len(R_HEADS):]
-
-    print(file=sys.stderr)
-    print("warning: project '%s' branch '%s' is not signed"
-          % (project.name, rev), file=sys.stderr)
-    return False
-
-  env = os.environ.copy()
-  env['GIT_DIR'] = project.gitdir
-  env['GNUPGHOME'] = gpg_dir
-
-  cmd = [GIT, 'tag', '-v', cur]
-  result = subprocess.run(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
-                          env=env, check=False)
-  if result.returncode:
-    print(file=sys.stderr)
-    print(result.stdout, file=sys.stderr)
-    print(file=sys.stderr)
-    return False
-  return True
-
-
 class _FetchTimes(object):
   _ALPHA = 0.5