rc-docs-sync: collapse multiple unprocessed RCs to the latest per run

.github/workflows/rc-docs-sync.yml

@@ -130,7 +130,7 @@ jobs:
 
           input_product = os.environ.get("INPUT_PRODUCT") or ""
           input_rc_tag = os.environ.get("INPUT_RC_TAG") or ""
-          queue, seed_actions = [], []
+          queue, seed_actions, superseded_actions = [], [], []
 
           products_to_sweep = [input_product] if input_product else list(PRODUCTS.keys())
           for slug in products_to_sweep:
@@ -165,6 +165,26 @@ jobs:
                       continue
                   last_key = sort_key(processed_markers[-1])
                   rcs_to_process = sorted([t for t in rc_tags if sort_key(t) > last_key], key=sort_key)
+                  # Collapse multiple unprocessed RCs to just the latest. Earlier
+                  # RCs in the same sync run would otherwise fan out as separate
+                  # matrix entries and produce duplicate PRs (see GH run
+                  # 26561237947, which opened PRs #400 and #401 for the same
+                  # filter introduced in 27.8-RC1 and re-emitted in 27.8-RC2).
+                  # The latest RC's diff base is computed below as usual; the
+                  # skipped RCs get a "superseded" marker so the state machine
+                  # advances past them. Explicit per-RC backfill via
+                  # workflow_dispatch with input_rc_tag bypasses this collapse.
+                  if len(rcs_to_process) > 1:
+                      latest = rcs_to_process[-1]
+                      for skipped in rcs_to_process[:-1]:
+                          superseded_actions.append({
+                              "issue": tracking_issue,
+                              "product": slug,
+                              "rc_tag": skipped,
+                              "superseded_by": latest,
+                              "display_name": product["display_name"],
+                          })
+                      rcs_to_process = [latest]
 
               for rc_tag in rcs_to_process:
                   # Prefer the most recent already-processed RC of the same base version as
@@ -200,13 +220,14 @@ jobs:
                       "tracking_issue": tracking_issue,
                   })
 
-          print(json.dumps({"queue": queue, "seeds": seed_actions}))
+          print(json.dumps({"queue": queue, "seeds": seed_actions, "superseded": superseded_actions}))
           PY
           cat queue.json
           # Emit queue as compact JSON for matrix consumption.
           echo "queue_json=$(jq -c '.queue' queue.json)" >> "$GITHUB_OUTPUT"
           echo "count=$(jq '.queue | length' queue.json)" >> "$GITHUB_OUTPUT"
           echo "seed_count=$(jq '.seeds | length' queue.json)" >> "$GITHUB_OUTPUT"
+          echo "superseded_count=$(jq '.superseded | length' queue.json)" >> "$GITHUB_OUTPUT"
 
       - name: Seed first-run tracking issues
         if: steps.queue.outputs.seed_count != '0'
@@ -225,6 +246,31 @@ jobs:
           **First-run seed for ${display}** — RC tag \`${rc_tag}\` recorded as the baseline. No historical RCs will be processed automatically. To backfill a specific RC, use \`workflow_dispatch\` with \`product=${product}\` and the desired \`rc_tag\`."
           done
 
+      - name: Mark superseded RCs (latest-only-per-run)
+        if: steps.queue.outputs.superseded_count != '0'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_REPO: ${{ github.repository }}
+        run: |
+          set -euo pipefail
+          jq -c '.superseded[]' queue.json | while read -r entry; do
+            issue=$(echo "$entry" | jq -r .issue)
+            product=$(echo "$entry" | jq -r .product)
+            rc_tag=$(echo "$entry" | jq -r .rc_tag)
+            superseded_by=$(echo "$entry" | jq -r .superseded_by)
+            display=$(echo "$entry" | jq -r .display_name)
+            base_version="${rc_tag%-RC*}"
+            # Idempotency: skip if a marker for this RC already exists.
+            if gh issue view "$issue" --json comments --jq '.comments[].body' \
+                | grep -Eq "<!--[[:space:]]*rc-docs-sync:v1[[:space:]]+product=${product}[[:space:]]+rc_tag=${rc_tag}[[:space:]]*-->"; then
+              echo "Marker for ${product} ${rc_tag} already exists; skipping superseded marker."
+              continue
+            fi
+            gh issue comment "$issue" --body "<!-- rc-docs-sync:v1 product=${product} rc_tag=${rc_tag} -->
+
+          **${display} ${base_version}** (RC \`${rc_tag}\`) — superseded by \`${superseded_by}\` in the same sync run. The later RC was processed against the same diff base, so any net public-surface changes are covered there. This RC was not individually processed."
+          done
+
       - name: Note when queue is empty
         if: steps.queue.outputs.count == '0'
         run: echo "No new RC tags to process this run."