ci: treat an empty changed-files.txt on a PR as a full-list fallback

.github/workflows/ci.yml

@@ -463,6 +463,32 @@ jobs:
  --changed "$RUNNER_TEMP/changed-files.txt"; then
  echo "::warning::Could not union cross-package scans into the affected set; falling back to the full package list for this shard."
  pnpm exec turbo ls --output=json > "$RUNNER_TEMP/turbo-ls.json"
+ # A fourth signal the failure branches do not cover: a producer that
+ # exits 0 with a wrong, plausible, EMPTY answer. Only that `git
+ # diff`'s exit STATUS was checked, never its emptiness, so a
+ # merge-base resolving to something wrong-but-valid gave an empty
+ # changed-file list -> zero affected packages -> a green shard that
+ # tested nothing, with every log line reading like a normal quiet PR
+ # (#10057). The shard attestation (#6082) does not cover it: it
+ # attests "shard N ran and every step passed", which is exactly what
+ # a shard that tested nothing does.
+ #
+ # Empty is decidable as BROKEN here, and only here: a pull_request
+ # always differs from its merge-base. At the partitioner zero is
+ # frequently the CORRECT answer (a docs-only PR genuinely affects no
+ # package, and with 6 shards a small change legitimately leaves
+ # shards empty), so a blanket "red on empty" belongs there least of
+ # all -- this is the one place selection-failed and nothing-selected
+ # can be told apart.
+ #
+ # Scoping this to pull_request needs no `github.event_name` test:
+ # SCM_BASE is assigned only inside the `pull_request` guard above, so
+ # this whole `[ -n "$SCM_BASE" ]` branch is unreachable on push and
+ # merge_group. Those keep taking the full-list path in the `else`
+ # below, by design.
+ elif [ ! -s "$RUNNER_TEMP/changed-files.txt" ]; then
+ echo "::warning::The diff against merge-base $SCM_BASE listed no changed files, which a pull_request cannot legitimately produce; falling back to the full package list for this shard rather than selecting nothing (#10057)."
+ pnpm exec turbo ls --output=json > "$RUNNER_TEMP/turbo-ls.json"
  fi
  else
  # Falling back to the FULL package list, never to the frozen