discourse-release: announce published releases on the community forum

A composite action: reads the release from the event file with jq, looks up
the repo's Announcements category (release-map.json) and posts as
release-bot. One topic per repo+tag via the topic external_id, so a re-run
is a no-op. Optional Discord webhook.

discourse-release/announce.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+# Called by action.yml. Everything user-written (release name, notes) comes in
+# through the event JSON file and is only ever handled by jq -- never spliced
+# into a command line or a JSON string by the shell.
+set -euo pipefail
+
+category_path=$(jq -r --arg r "$REPO" '.[$r] // empty' "$MAP")
+if [ -z "$category_path" ]; then
+  echo "::error::$REPO is not in discourse-release/release-map.json"
+  exit 1
+fi
+
+tag=$(jq -r '.release.tag_name' "$EVENT_PATH")
+# The category id, looked up anonymously (the category is public); the
+# release-bot key is scoped to creating posts and nothing else.
+category_id=$(curl -fsS "$FORUM/c/$category_path/find_by_slug.json" | jq -r '.category.id')
+
+# One topic per repo+tag, ever: Discourse enforces external_id uniqueness.
+external_id="rel-$(printf '%s@%s' "$REPO" "$tag" | sha1sum | cut -c1-40)"
+
+payload=$(jq -n --arg repo "$REPO" --argjson category "$category_id" --arg ext "$external_id" \
+  --slurpfile ev "$EVENT_PATH" '
+  ($ev[0].release) as $r
+  | ($repo | split("/") | last) as $name
+  | ($r.name // "" | if . == "" then $r.tag_name else . end) as $title
+  | {
+      title: "\($name) \($title) released",
+      category: $category,
+      external_id: $ext,
+      raw: (
+        "**\($name) \($r.tag_name)** is out.\n\n"
+        + (if ($r.body // "") == "" then "" else "\($r.body)\n\n" end)
+        + "---\n[Release on Gitea](\($r.html_url))"
+      )
+    }')
+
+status=$(curl -sS -o /tmp/discourse-release.out -w '%{http_code}' -X POST "$FORUM/posts.json" \
+  -H "Api-Key: $API_KEY" -H "Api-Username: release-bot" \
+  -H "Content-Type: application/json" --data-binary "$payload")
+body=$(cat /tmp/discourse-release.out)
+case "$status" in
+  200)
+    url="$FORUM/t/$(jq -r '.topic_slug' <<<"$body")/$(jq -r '.topic_id' <<<"$body")"
+    echo "announced: $url" ;;
+  422)
+    if jq -e '.errors | tostring | test("External ID"; "i")' <<<"$body" >/dev/null; then
+      echo "already announced (external_id $external_id); nothing to do"
+      exit 0
+    fi
+    echo "::error::forum refused the post: $body"; exit 1 ;;
+  *)
+    echo "::error::forum returned HTTP $status: $body"; exit 1 ;;
+esac
+
+if [ -n "${DISCORD_WEBHOOK:-}" ]; then
+  jq -n --arg u "$url" --slurpfile ev "$EVENT_PATH" --arg repo "$REPO" '
+    ($ev[0].release) as $r
+    | {content: ("**\($repo | split("/") | last) \($r.tag_name)** released\n\($u)")[0:1900]}' |
+  curl -fsS -X POST "$DISCORD_WEBHOOK" -H "Content-Type: application/json" --data-binary @- >/dev/null \
+    && echo "posted to Discord" \
+    || echo "::warning::Discord webhook failed; the forum post stands"
+fi