benchmarks: split off script, emit markdown table

Follow-up to #8811.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

build/bench-comment.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# Run benchlab comparisons and post (or print) a markdown PR comment.
+#
+# Required env vars:
+#   BEFORE_SHA   base commit
+#   AFTER_SHA    head commit
+#   BENCH_PKGS   JSON array of Go package paths, e.g. '["./topdown","./ast"]'
+#
+# Optional (needed to post a PR comment):
+#   GH_TOKEN     GitHub token with pull-requests:write
+#   COMMIT_SHA   the merge commit SHA used to look up the PR
+set -euo pipefail
+
+: "${BEFORE_SHA:?}"
+: "${AFTER_SHA:?}"
+: "${BENCH_PKGS:?}"
+
+for pkg in $(echo "$BENCH_PKGS" | jq -r '.[]'); do
+  benchlab \
+    -commit "$BEFORE_SHA","$AFTER_SHA" \
+    -pkg "$pkg" \
+    -host local:tags=opa_wasm \
+    -reps 3 \
+    -benchtime 300ms \
+    -run '^$'
+done
+
+body() {
+  echo "<details><summary>Benchmark Comparison (\`${BEFORE_SHA}\` vs \`${AFTER_SHA}\`)</summary>"
+  echo ""
+  for f in .benchlab/benchstat.*.txt; do
+    awk '
+      /^benchmark \\ host/ { found=1; print "| benchmark | delta |"; print "| --- | --- |"; next }
+      found && $2 ~ /^[+-]/ {
+        val = $2; for (i = 3; i <= NF; i++) val = val " " $i
+        print "| " $1 " | " val " |"
+      }
+    ' "$f"
+    echo ""
+  done
+  echo "</details>"
+  echo ""
+  echo "_This comment was automatically generated by the benchmarks workflow._"
+}
+
+if [ -n "${GH_TOKEN:-}" ] && [ -n "${COMMIT_SHA:-}" ]; then
+  PR_NUMBER=$(gh pr list --search "${COMMIT_SHA}" --state merged --json number --jq '.[0].number')
+  if [ -z "$PR_NUMBER" ] || [ "$PR_NUMBER" = "null" ]; then
+    echo "Could not find originating PR for commit ${COMMIT_SHA}"
+    exit 0
+  fi
+  body > body.md
+  gh pr comment "${PR_NUMBER}" --body-file body.md
+else
+  body
+fi