CI: Automatically apply pull request labels for generic PR actions

Generic PR actions include opening a PR, submit review comments, adding
new commits, etc. This prevents the reviewer and PR submitter from
having to manually bounce the labels back and forth in the general
case. The reviewer also may not have permission to set labels, meaning
the reviewer won't be able to update the labels accordingly themselves.

This does not handle more subjective labels such as pr-is-blocked and
pr-unclear. Unfortunately, there does not seem to be a GitHub Actions
trigger for when a PR has merge conflicts, so the pr-has-conflicts
label cannot be automatically applied.

Co-authored-by: kleines Filmröllchen <filmroellchen@serenityos.org>

2 files changed, 171 insertions, 0 deletions

diff --git a/.github/workflows/label-pull-requests.yml b/.github/workflows/label-pull-requests.yml
new file mode 100644
index 0000000000..c8f72a63dc
--- /dev/null
+++ b/.github/workflows/label-pull-requests.yml
@@ -0,0 +1,26 @@
+name: Pull request labeler
+
+# FIXME: Consider adding the `issue_comment` event to change labels based on generic, non-review pull request comments.
+#        Consider the trade off of how spammy it can be (one CI run per comment) and how useful it would be to have.
+#        Consider alternatives to `issue_comment`.
+on:
+  pull_request_target:
+    types: [opened, reopened, converted_to_draft, ready_for_review, synchronize, edited, review_requested, closed]
+  pull_request_review:
+    types: [submitted, edited, dismissed]
+
+jobs:
+  label_pull_request:
+    runs-on: ubuntu-22.04
+    if: always() && github.repository == 'SerenityOS/serenity'
+    
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Label pull request
+        uses: actions/github-script@v6
+        with:
+          github-token: ${{ secrets.BUGGIEBOT_TOKEN }}
+          script: |
+            const script = require('./Meta/label-pull-requests.js')
+            script({github, context})
diff --git a/Meta/label-pull-requests.js b/Meta/label-pull-requests.js
new file mode 100644
index 0000000000..1c203bc515
--- /dev/null
+++ b/Meta/label-pull-requests.js
@@ -0,0 +1,145 @@
+/*
+ * Copyright (c) 2022, Luke Wilde <lukew@serenityos.org>
+ * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+const Label = {
+    CommunityApproved: "✅ pr-community-approved",
+    HasConflicts: "⚠️ pr-has-conflicts",
+    IsBlocked: "⛔️ pr-is-blocked",
+    MaintainerApprovedButAwaitingCi: "✅ pr-maintainer-approved-but-awaiting-ci",
+    NeedsReview: "👀 pr-needs-review",
+    Unclear: "🤔 pr-unclear",
+    WaitingForAuthor: "⏳ pr-waiting-for-author",
+};
+
+const subjectiveLabels = [Label.IsBlocked, Label.Unclear];
+
+function removeExistingPrLabels(currentLabels, keepSubjectiveLabels) {
+    return currentLabels.filter(
+        label =>
+            !label.includes("pr-") ||
+            label === Label.HasConflicts ||
+            (keepSubjectiveLabels && subjectiveLabels.includes(label))
+    );
+}
+
+async function labelsForGenericPullRequestChange(currentLabels) {
+    const filteredLabels = removeExistingPrLabels(currentLabels, true);
+    filteredLabels.push(Label.NeedsReview);
+    return filteredLabels;
+}
+
+async function labelsForPullRequestEffectivelyClosed(currentLabels) {
+    return removeExistingPrLabels(currentLabels, false);
+}
+
+function apiErrorHandler(error) {
+    console.log(
+        "::warning::Encountered error during event handling, not updating labels. Error:",
+        error
+    );
+}
+
+module.exports = ({ github, context }) => {
+    async function labelsForPullRequestReviewSubmitted(currentLabels, { pull_request, review }) {
+        let newLabels = currentLabels;
+        const isBlocked = currentLabels.some(label => label === Label.IsBlocked);
+
+        if (review.state.toLowerCase() === "approved") {
+            const maintainers = (
+                await github.rest.teams.listMembersInOrg({
+                    org: "SerenityOS",
+                    team_slug: "maintainers",
+                })
+            ).data;
+
+            const approvedByMaintainer = maintainers.some(
+                maintainerInArray => maintainerInArray.login === review.user.login
+            );
+
+            if (approvedByMaintainer) {
+                newLabels = newLabels.filter(
+                    label => !(label === Label.NeedsReview || label === Label.WaitingForAuthor)
+                );
+
+                if (!newLabels.includes(Label.MaintainerApprovedButAwaitingCi))
+                    newLabels.push(Label.MaintainerApprovedButAwaitingCi);
+            } else {
+                if (!newLabels.includes(Label.CommunityApproved))
+                    newLabels.push(Label.CommunityApproved);
+            }
+        } else if (!isBlocked) {
+            // Remove approval labels.
+            newLabels = newLabels.filter(
+                label =>
+                    !(
+                        label === Label.CommunityApproved ||
+                        label === Label.MaintainerApprovedButAwaitingCi
+                    )
+            );
+
+            if (review.user.login === pull_request.user.login) newLabels.push(Label.NeedsReview);
+            else newLabels.push(Label.WaitingForAuthor);
+        }
+
+        return newLabels;
+    }
+
+    const eventHandlers = {
+        opened: labelsForGenericPullRequestChange,
+        reopened: labelsForGenericPullRequestChange,
+        submitted: labelsForPullRequestReviewSubmitted,
+        dismissed: labelsForGenericPullRequestChange,
+        converted_to_draft: labelsForPullRequestEffectivelyClosed,
+        ready_for_review: labelsForGenericPullRequestChange,
+        synchronize: labelsForGenericPullRequestChange, // synchronize is triggered when the branch is changed
+        edited: labelsForGenericPullRequestChange,
+        review_requested: labelsForGenericPullRequestChange,
+        closed: labelsForPullRequestEffectivelyClosed,
+    };
+
+    const eventName = context.payload.action;
+    const handlerForCurrentEvent = eventHandlers[eventName];
+
+    async function updateLabels(currentLabelsAsObjects) {
+        const currentLabels = [];
+        currentLabelsAsObjects.forEach(labelObject => currentLabels.push(labelObject.name));
+
+        const isEffectivelyClosed =
+            context.payload.pull_request.draft ||
+            context.payload.pull_request.state.toLowerCase() === "closed";
+
+        const newLabels = await (isEffectivelyClosed
+            ? labelsForPullRequestEffectivelyClosed(currentLabels, context.payload)
+            : handlerForCurrentEvent(currentLabels, context.payload));
+
+        console.log(
+            `Received '${eventName}' event for ${
+                isEffectivelyClosed ? "draft/closed" : "open"
+            } pull request, changing labels from '${currentLabels}' to '${newLabels}'`
+        );
+
+        return github.rest.issues.setLabels({
+            issue_number: context.payload.pull_request.number,
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            labels: newLabels,
+        });
+    }
+
+    if (handlerForCurrentEvent) {
+        github.rest.issues
+            .listLabelsOnIssue({
+                issue_number: context.payload.pull_request.number,
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+            })
+            .then(result => updateLabels(result.data))
+            .catch(apiErrorHandler);
+    } else {
+        console.log(`::warning::No handler for the '${eventName}' event, not updating labels.`);
+    }
+};