ci(release): thread Slack release notices under the start notice

Result notices reply in the start notice thread via its ts. Failures
are also broadcast to the channel. If the start post never happened,
thread_ts is null and notices post to the channel as before.

Author: gorandodig

.github/workflows/publish-to-pypi.yml

@@ -14,6 +14,9 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: write
+    outputs:
+      # The notify job replies in the start notice's thread.
+      slack-ts: ${{ steps.slack-start.outputs.ts }}
 
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
@@ -23,6 +26,7 @@ jobs:
 
       # Non-fatal: Slack being down must not block the release.
       - name: Notify Slack (start)
+        id: slack-start
         if: ${{ vars.SLACK_CHANNEL_ID != '' }}
         continue-on-error: true
         uses: slackapi/slack-github-action@dcb1066f776dd043e64d0e8ba94ca15cc7e1875d # v4.0.0
@@ -93,6 +97,13 @@ jobs:
     if: ${{ !cancelled() && vars.SLACK_CHANNEL_ID != '' }}
     runs-on: ubuntu-latest
 
+    env:
+      # Notices reply in the start notice's thread, quoted so YAML keeps the ts a
+      # string. Without a start notice (its post failed or never ran) this is null,
+      # which the Slack client drops, and they post to the channel. Failures are
+      # also broadcast to the channel; success stays in the thread.
+      THREAD_TS: ${{ needs.build.outputs.slack-ts && format('"{0}"', needs.build.outputs.slack-ts) || 'null' }}
+
     steps:
       - name: Notify Slack (build failure)
         if: ${{ needs.build.result == 'failure' }}
@@ -103,6 +114,8 @@ jobs:
           errors: true
           payload: |
             channel: ${{ vars.SLACK_CHANNEL_ID }}
+            thread_ts: ${{ env.THREAD_TS }}
+            reply_broadcast: ${{ env.THREAD_TS != 'null' }}
             text: >-
               *Codeplain Client*: Build of `${{ github.ref_name }}` FAILED :x:. Nothing was published to PyPI.
               See <${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}|the run log>.
@@ -116,6 +129,8 @@ jobs:
           errors: true
           payload: |
             channel: ${{ vars.SLACK_CHANNEL_ID }}
+            thread_ts: ${{ env.THREAD_TS }}
+            reply_broadcast: ${{ env.THREAD_TS != 'null' }}
             text: >-
               *Codeplain Client*: Publishing `${{ github.ref_name }}` to PyPI FAILED :x:.
               See <${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}|the run log>.
@@ -141,6 +156,7 @@ jobs:
           errors: true
           payload: |
             channel: ${{ vars.SLACK_CHANNEL_ID }}
+            thread_ts: ${{ env.THREAD_TS }}
             text: >-
               *Codeplain Client*: pre-release `${{ github.ref_name }}` published to PyPI :test_tube:.
               Normal installs are unaffected — try it with `pip install --pre codeplain==${{ steps.version.outputs.version }}`.
@@ -154,6 +170,7 @@ jobs:
           errors: true
           payload: |
             channel: ${{ vars.SLACK_CHANNEL_ID }}
+            thread_ts: ${{ env.THREAD_TS }}
             text: >-
               *Codeplain Client*: `${{ github.ref_name }}` published to PyPI :white_check_mark:.
               Install with `pip install --upgrade codeplain`.
@@ -165,7 +182,7 @@ jobs:
   # # keeps its minimal permissions and no checkout.
   # linear-release:
   #   name: Record release in Linear
-  #   needs: publish-to-pypi
+  #   needs: [build, publish-to-pypi]
   #   runs-on: ubuntu-latest
   #
   #   permissions:
@@ -192,13 +209,17 @@ jobs:
   #     # outcome — `failure()` would never fire.
   #     - name: Notify Slack (release not recorded)
   #       if: ${{ steps.linear.outcome == 'failure' && vars.SLACK_CHANNEL_ID != '' }}
+  #       env:
+  #         # See the notify job.
+  #         THREAD_TS: ${{ needs.build.outputs.slack-ts && format('"{0}"', needs.build.outputs.slack-ts) || 'null' }}
   #       uses: slackapi/slack-github-action@dcb1066f776dd043e64d0e8ba94ca15cc7e1875d # v4.0.0
   #       with:
   #         method: chat.postMessage
   #         token: ${{ secrets.SLACK_BOT_TOKEN }}
   #         errors: true
   #         payload: |
   #           channel: ${{ vars.SLACK_CHANNEL_ID }}
+  #           thread_ts: ${{ env.THREAD_TS }}
   #           text: >-
   #             *Codeplain Client*: `${{ github.ref_name }}` is on PyPI, but recording it in Linear FAILED :warning:.
   #             See <${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}|the run log>.