name: Merge Conflict Detector

on:
  pull_request:
    types: [opened, synchronize, reopened]
    branches: [main]
  push:
    branches: [main]

# Cancel in-progress runs when a new event lands — protects the expensive
# bulk-check path. github.ref is stable per PR (refs/pull/N/merge) AND stable
# for pushes to main (refs/heads/main), so one expression covers both triggers
# and actually cancels a stale bulk run when a newer push arrives.
concurrency:
  group: merge-conflict-${{ github.ref }}
  cancel-in-progress: true

jobs:
  detect-and-label:
    runs-on: ubuntu-latest
    permissions:
      pull-requests: write
      issues: write

    steps:
      - name: Ensure label exists
        uses: actions/github-script@v9
        with:
          script: |
            const LABEL = 'has-conflicts';
            const { data: labels } = await github.rest.issues.listLabelsForRepo({
              owner: context.repo.owner,
              repo: context.repo.repo,
              per_page: 100,
            });
            if (!labels.some(l => l.name === LABEL)) {
              try {
                await github.rest.issues.createLabel({
                  owner: context.repo.owner,
                  repo: context.repo.repo,
                  name: LABEL,
                  color: 'd73a4a',
                  description: 'This PR has merge conflicts that need to be resolved',
                });
              } catch (e) {
                // 422 = label already exists (race with another run) — fine.
                if (e.status !== 422) throw e;
              }
            }

      - name: Check PRs and label conflicts
        uses: actions/github-script@v9
        with:
          script: |
            const LABEL = 'has-conflicts';

            // GitHub computes `mergeable` asynchronously. Immediately after a push
            // it returns `null` ("still computing") and can stay null for minutes.
            // The previous version treated `null` as "leave as-is" and never
            // re-checked, so conflicting PRs were never labeled. This polls until
            // GitHub resolves the merge state (or we time out), returning the full
            // PR object so callers don't need to re-fetch for labels.
            async function getMergeable(prNumber, retries = 12, delayMs = 5000) {
              for (let i = 0; i < retries; i++) {
                const { data: pr } = await github.rest.pulls.get({
                  owner: context.repo.owner,
                  repo: context.repo.repo,
                  pull_number: prNumber,
                });
                if (pr.mergeable !== null) return pr;
                core.info(`PR #${prNumber}: mergeable still null, retrying (${i + 1}/${retries})...`);
                await new Promise(r => setTimeout(r, delayMs));
              }
              core.warning(`PR #${prNumber}: mergeable never resolved after ${retries} tries.`);
              return null;
            }

            async function checkAndLabel(pr) {
              const prNumber = pr.number;
              const resolved = await getMergeable(prNumber);

              if (resolved === null) {
                // Couldn't determine state in time — leave label as-is.
                core.info(`PR #${prNumber}: mergeable unknown, label left unchanged.`);
                return;
              }

              const hasConflicts = resolved.mergeable === false;
              const currentLabels = resolved.labels.map(l => l.name);
              const hasLabel = currentLabels.includes(LABEL);

              core.info(
                `PR #${prNumber}: mergeable=${resolved.mergeable}, ` +
                `hasLabel=${hasLabel}, conflicts=${hasConflicts}`
              );

              if (hasConflicts && !hasLabel) {
                await github.rest.issues.addLabels({
                  owner: context.repo.owner,
                  repo: context.repo.repo,
                  issue_number: prNumber,
                  labels: [LABEL],
                });
                core.info(`✅ Added "${LABEL}" to PR #${prNumber}`);
              } else if (!hasConflicts && hasLabel) {
                try {
                  await github.rest.issues.removeLabel({
                    owner: context.repo.owner,
                    repo: context.repo.repo,
                    issue_number: prNumber,
                    name: LABEL,
                  });
                  core.info(`✅ Removed "${LABEL}" from PR #${prNumber}`);
                } catch (e) {
                  // 404 = label already gone (race with another run) — fine.
                  if (e.status !== 404) throw e;
                }
              }
            }

            // ── Single PR (pull_request event) ─────────────────────────
            if (context.eventName === 'pull_request') {
              // For a single PR we already have the number; fetch resolves mergeable.
              await checkAndLabel({ number: context.issue.number });
              return;
            }

            // ── Bulk check all open PRs (push to main event) ───────────
            // GitHub usually recomputes mergeable for every open PR at once after a
            // push, so most hit the retry loop simultaneously. Batch a few at a time
            // with Promise.all to bound wall-clock without hammering the API.
            if (context.eventName === 'push') {
              const { data: prs } = await github.rest.pulls.list({
                owner: context.repo.owner,
                repo: context.repo.repo,
                state: 'open',
                base: 'main',
                per_page: 100,
              });
              core.info(`Checking ${prs.length} open PR(s) base on main...`);
              const CHUNK = 5;
              for (let i = 0; i < prs.length; i += CHUNK) {
                const batch = prs.slice(i, i + CHUNK);
                await Promise.all(batch.map(p => checkAndLabel(p)));
              }
            }