ceph-source-dist: derive canonical BRANCH from SHA1

When SHA1 is provided, the checkout prefers it over BRANCH, but
dist/branch echoed the BRANCH parameter verbatim.  If the two
disagreed, downstream consumers of dist/branch (ceph-dev-pipeline and,
through it, chacra/shaman) would record the build under the wrong ref.

Ask the remote which branch tips the built commit and reset BRANCH
accordingly.  The given BRANCH is kept when it already tips the commit
(including when several branches share the tip) and, with a warning,
when the commit is not the tip of any branch, since older commits are
contained by many branches and there is no unambiguous answer.

Fixes: https://tracker.ceph.com/issues/73658

Signed-off-by: David Galloway <david.galloway@ibm.com>

Author: djgalloway

ceph-source-dist/build/Jenkinsfile

@@ -107,6 +107,26 @@ pipeline {
                 bzip2 -dc ceph-$ceph_version_tarball.$extension | gzip > ceph-$ceph_version_tarball.tar.gz
                 popd
               else
+                if [ -n "${SHA1:-}" ]; then
+                  # The checkout preferred SHA1 over BRANCH, so the BRANCH
+                  # parameter may not match the code actually built
+                  # (https://tracker.ceph.com/issues/73658).  Keep BRANCH only
+                  # if its remote tip is the built commit; otherwise use the
+                  # branch whose tip is.
+                  sha=$(git rev-parse HEAD)
+                  git ls-remote --heads "${CEPH_REPO}" > remote_heads
+                  # Build an array of every branch whose tip is the built commit:
+                  # match ls-remote lines ("<sha> refs/heads/<branch>") on the sha
+                  # and strip the ref prefix, leaving just the branch names.
+                  mapfile -t tip_branches < <(awk -v sha="${sha}" '$1 == sha {sub("refs/heads/", "", $2); print $2}' remote_heads)
+                  rm -f remote_heads
+                  if [ "${#tip_branches[@]}" -eq 0 ]; then
+                    echo "WARNING: ${sha} is not the tip of any branch on ${CEPH_REPO}; keeping BRANCH=${BRANCH}"
+                  elif ! printf '%s\n' "${tip_branches[@]}" | grep -qxF "${BRANCH}"; then
+                    echo "BRANCH=${BRANCH} does not contain SHA1=${sha}; using ${tip_branches[0]}"
+                    BRANCH="${tip_branches[0]}"
+                  fi
+                fi
                 echo "BRANCH=${BRANCH}" > dist/branch
               fi