Avoid creating resolved comments without an existing warning

Use a boolean --create-if-missing flag, supplied by the composite action by default and omitted for resolution updates.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

.github/actions/post-comment/action.yml

@@ -9,6 +9,10 @@ inputs:
   body:
     description: "Full comment markdown."
     required: true
+  create-if-missing:
+    description: "Create a comment if no matching marker exists. Set to 'false' to update existing comments only."
+    required: false
+    default: "true"
   issue-id:
     description: >
       The issue or pull request number to post/update the comment on.
@@ -25,11 +29,13 @@ runs:
       env:
         BODY: ${{ inputs.body }}
         MARKER: ${{ inputs.marker }}
+        CREATE_IF_MISSING: ${{ inputs.create-if-missing != 'false' && '--create-if-missing' || '' }}
         GH_TOKEN: ${{ inputs.token }}
         ISSUE_ID: ${{ inputs.issue-id || github.event.pull_request.number }}
       run: |
         npx tsx ./pr-checks/post-comment.ts \
           --body "$BODY" \
           --marker "$MARKER" \
+          $CREATE_IF_MISSING \
           --issue-id "$ISSUE_ID" \
           --repository "$GITHUB_REPOSITORY"

.github/workflows/pr-checks.yml

@@ -313,6 +313,7 @@ jobs:
         uses: ./.github/actions/post-comment
         with:
           marker: "post-comment:CHANGELOG.md"
+          create-if-missing: "false"
           body: |
             ⚠️ `CHANGELOG.md` has been modified in this PR. Please do not modify `CHANGELOG.md` directly.
             Instead, create a change-note file in `unreleased-change-notes/`. The `CHANGELOG.md`

pr-checks/post-comment.test.ts

@@ -49,11 +49,17 @@ describe("parseOptions", async () => {
     assert.deepEqual(options, {
       body: "hello",
       marker: "marker",
+      createIfMissing: false,
       issueId: 42,
       repository: { owner: "github", repo: "codeql-action" },
     });
   });
 
+  await it("enables creation when --create-if-missing is supplied", () => {
+    const options = parseOptions([...validFlags, "--create-if-missing"]);
+    assert.equal(options.createIfMissing, true);
+  });
+
   await it("rejects a missing --body", () => {
     assert.throws(
       () => parseOptions(withoutFlag("--body")),
@@ -206,6 +212,7 @@ describe("findExistingComment", async () => {
 const baseOptions: Options = {
   body: "hello",
   marker: "marker",
+  createIfMissing: true,
   issueId: 42,
   repository: { owner: "owner", repo: "repo" },
 };
@@ -220,6 +227,25 @@ function assertCommentContent(
 }
 
 describe("setComment", async () => {
+  await it("does not create a resolved comment when no comments exist", async () => {
+    const { client, comments } = fakeClient([]);
+
+    await setComment(client, { ...baseOptions, createIfMissing: false });
+
+    // Resolution does not introduce a warning that was never posted.
+    assert.deepEqual(comments, []);
+  });
+
+  await it("leaves unrelated comments unchanged when creation is disabled", async () => {
+    const originalComments = [{ id: 7, body: "unrelated" }];
+    const { client, comments } = fakeClient(originalComments);
+
+    await setComment(client, { ...baseOptions, createIfMissing: false });
+
+    // Unrelated comments neither trigger resolution nor get overwritten.
+    assert.deepEqual(comments, originalComments);
+  });
+
   await it("uses the supplied marker without affecting other comments", async () => {
     const marker = "custom-warning";
     const { client, comments } = fakeClient([
@@ -308,7 +334,7 @@ describe("setComment", async () => {
     const body =
       "`CHANGELOG.md` is not modified in this PR. No changelog action is needed.";
 
-    await setComment(client, { ...baseOptions, body });
+    await setComment(client, { ...baseOptions, body, createIfMissing: false });
 
     // The resolved state replaces the warning on the same marked comment.
     assert.equal(comments.length, 1);

pr-checks/post-comment.ts

@@ -5,10 +5,12 @@ Creates or updates a pull request comment identified by a marker. The marker
 is supplied via `--marker` (or the `.github/actions/post-comment` action's `marker` input)
 and must be non-empty. Supply a plain-text identifier; the CLI wraps it in a
 hidden HTML comment, prepends it to `--body`, and uses it to find the comment on later runs.
+Supply `--create-if-missing` to allow creation; omit it to update existing comments only.
 
 Example:
 ```sh
 GH_TOKEN=<token> npx tsx ./pr-checks/post-comment.ts \
+  --create-if-missing \
   --body "The changelog needs updating." \
   --marker "my-bot" \
   --issue-id 123 \
@@ -29,6 +31,8 @@ export interface Options {
   body: string;
   /** Plain-text identifier wrapped in a hidden HTML comment. */
   marker: string;
+  /** Whether to create a comment when no matching marker exists. */
+  createIfMissing: boolean;
   /** The issue/pull request number to post/update the comment on. */
   issueId: number;
   /** The repository to operate on. */
@@ -46,6 +50,7 @@ export function parseOptions(argv?: string[]): Options {
     options: {
       body: { type: "string" },
       marker: { type: "string" },
+      "create-if-missing": { type: "boolean", default: false },
       "issue-id": { type: "string" },
       repository: { type: "string" },
     },
@@ -76,6 +81,7 @@ export function parseOptions(argv?: string[]): Options {
   return {
     body: values.body,
     marker: values.marker,
+    createIfMissing: values["create-if-missing"],
     issueId: Number(values["issue-id"]),
     repository: { owner, repo },
   };
@@ -126,6 +132,10 @@ export async function setComment(
     marker,
   );
   if (existingCommentId === undefined) {
+    if (!options.createIfMissing) {
+      console.info("No existing comment found; skipping creation.");
+      return;
+    }
     console.info("No existing comment found; creating a new comment.");
     await client.rest.issues.createComment({
       ...repository,