kaizen: tier step dies on a non-JSON GitHub reply — a 60-min merge group lost at step 7; retry, then fall back to the full tier #5399

Loss

Merge-group run 38738529476, shard 3 (job 116433916212, yoga-build3) died at step 7 "Decide the test tier (BSE-17, PMAT-1077)" after 1.5 s with invalid character 'S' looking for beginning of value. That killed a ~60-min merge group for #5377 (cop report 13:3xZ; log read 13:5xZ).

Where

ci/sections.yml (step source, merge_group) arm, lines ~392-396 on origin/main): two bare gh api ... --jq calls, the PR head sha (pulls/${pr}) and the workspace-test check-run conclusion. The step runs under set -euo pipefail, so a non-JSON reply (an S... body, most likely "Service Unavailable" or "Server error") makes --jq fail and the whole step exits non-zero. The log shows the line before the error ("first parent available") and no Decide output after it. I did not read the HTTP status itself; the 'S' reply body is inferred from the error text.

Five whys

1. Why did the MG die? Step 7 exited non-zero.
2. Why? gh api ... --jq could not parse a non-JSON reply.
3. Why does a bad reply fail the step? set -e plus no status check and no retry on a call whose only job is to refine the tier.
4. Why is that call allowed to be fatal? The push arm already treats a failed lookup as "never a red step" (ci_mg_workspace_result.sh ... || mg=""); the merge_group arm was written without that rule.
5. Why was it not caught? No test feeds the step a non-JSON API reply.

Countermeasure

In the merge_group arm, check the HTTP status and retry (3 tries with backoff) and, if the lookup still fails, fall back to tier=full (the script's own fail-closed answer, --pr-head omitted), never fail the step. Same shape as the push arm. A test or self-test case with a non-JSON reply.

Owner

aprender-a3. This is a .github/workflows / ci/sections.yml change, so it goes to the cop before any PR (per the cop's note).

Done when

A forced non-JSON reply in the step yields tier=full and a green step in a dry run, and no MG dies at step 7 for 7 days.