SL-20546: Try harder to infer the branch corresponding to build tag.

(cherry picked from commit 59eeaed1)

.github/workflows/build.yaml

@@ -178,11 +178,9 @@ jobs:
           then viewer_channel="${GITHUB_REF_NAME%#*}"
                export viewer_channel="${viewer_channel//_/ }"
                # Since GITHUB_REF_NAME is a tag rather than a branch, we need
-               # to discover to what branch this tag corresponds. Get the tip
-               # commit (for the tag) and then ask for branches containing it.
-               # Assume GitHub cloned only this tag and its containing branch.
-               viewer_branch="$(git branch --contains "$(git log -n 1 --format=%h)" |
-                                grep -v '(HEAD')"
+               # to discover to what branch this tag corresponds.
+               viewer_branch="$(python3 .github/workflows/which_branch.py \
+                                --token "${{ github.token }}" ${{ github.workflow_sha }})"
           else export viewer_channel="Second Life Test"
                viewer_branch="${GITHUB_REF_NAME}"
           fi

.github/workflows/which_branch.py

+#!/usr/bin/env python3
+"""\
+@file   which_branch.py
+@author Nat Goodspeed
+@date   2023-11-14
+@brief  Discover which git branch(es) correspond to a given commit hash.
+
+$LicenseInfo:firstyear=2023&license=viewerlgpl$
+Copyright (c) 2023, Linden Research, Inc.
+$/LicenseInfo$
+"""
+
+import github
+import re
+import sys
+import subprocess
+
+class Error(Exception):
+    pass
+
+def branches_for(token, commit, repo=None):
+    """
+    Use the GitHub REST API to discover which branch(es) correspond to the
+    passed commit hash. The commit string can actually be any of the ways git
+    permits to identify a commit:
+
+    https://git-scm.com/docs/gitrevisions#_specifying_revisions
+
+    branches_for() generates a (possibly empty) sequence of all the branches
+    of the specified repo for which the specified commit is the tip.
+
+    If repo is omitted or None, assume the current directory is a local clone
+    whose 'origin' remote is the GitHub repository of interest.
+    """
+    if not repo:
+        url = subprocess.check_output(['git', 'remote', 'get-url', 'origin'],
+                                      text=True)
+        parts = re.split(r'[:/]', url.rstrip())
+        repo = '/'.join(parts[-2:]).removesuffix('.git')
+
+    gh = github.MainClass.Github(token)
+    grepo = gh.get_repo(repo)
+    for branch in grepo.get_branches():
+        try:
+            delta = grepo.compare(base=commit, head=branch.name)
+        except github.GithubException:
+            continue
+
+        if delta.ahead_by == 0 and delta.behind_by == 0:
+            yield branch
+
+def main(*raw_args):
+    from argparse import ArgumentParser
+    parser = ArgumentParser(description=
+"%(prog)s reports the branch(es) for which the specified commit hash is the tip.",
+                            epilog="""\
+When GitHub Actions launches a tag build, it checks out the specific changeset
+identified by the tag, and so 'git branch' reports detached HEAD. But we use
+tag builds to build a GitHub 'release' of the tip of a particular branch, and
+it's useful to be able to identify which branch that is.
+""")
+    parser.add_argument('-t', '--token', required=True,
+                        help="""GitHub REST API access token""")
+    parser.add_argument('-r', '--repo',
+                        help="""GitHub repository name, in the form OWNER/REPOSITORY""")
+    parser.add_argument('commit',
+                        help="""commit hash at the tip of the sought branch""")
+
+    args = parser.parse_args(raw_args)
+    for branch in branches_for(token=args.token, commit=args.commit, repo=args.repo):
+        print(branch.name)
+
+if __name__ == "__main__":
+    try:
+        sys.exit(main(*sys.argv[1:]))
+    except Error as err:
+        sys.exit(str(err))