select-e2e: a per-suite path naming no runnable suite has its escalation swallowed by any other selection #3816

hack/select-e2e.sh decides escalation per changed path everywhere except one arm. The per-suite Chainsaw rule at hack/select-e2e.sh:265-269 derives a suite name from the directory and appends it without checking that the name is a suite that exists:

hack/e2e-chainsaw/*/*)
  app=$(echo "$file" | sed -nE 's,^hack/e2e-chainsaw/([^/]+)/.*,\1,p')
  selected_apps="$selected_apps $app"
  trigger_any=1
  continue ;;

A name that no suite carries is dropped later by intersect_suites, so whether the path escalates depends on what else is in the diff. Alone it reaches the backstop at :413 and runs everything. Together with any path that contributes a real suite, the final selection is non-empty, the backstop never fires, and the unresolvable path is gone from the answer with nothing in the output to record it.

hack/e2e-chainsaw/backup/ is the one directory in the tree that holds no chainsaw-test.yaml, since its suite is parked as chainsaw-test.yaml.disabled. Using a hypothetical file in it against origin/main:

This is the shape #3330 fixed for graph-resolved paths, where reading escalation off the merged selection let one contributing path swallow another's. That fix made coverage a property of the changed path. This arm still decides after the merge.

It is latent rather than live. backup/ contains only chainsaw-test.yaml.disabled, matched by the *.disabled rule before this one, and README.md, dropped by the documentation rule first, so no file in the tree today reaches it. It becomes reachable on the first non-.md file added under backup/, on any suite nested deeper than the depth-2 scan all_apps performs, or on a new directory added beside a suite before its chainsaw-test.yaml exists.

The fix is the one the sibling hack/e2e-apps/*.bats arm now uses: membership-test the derived name against all_apps before appending it, and escalate with a named reason when it is not a suite, so the decision stays a property of the path. A test belongs with it, because a rule that only misbehaves in a multi-path diff is invisible to a single-path test, which is how this survived.

Two things worth knowing for whoever picks it up. The backstop is the only thing that catches this class in a single-path diff, so its reason line is load-bearing rather than decorative. And an empty selection is not a safe default here: both e2e lanes read it as "skip Chainsaw" and then post the required E2E Tests status green, which is the failure #3392 exists to prevent.