Let Publish Images be started by hand

.github/workflows/publish.yml

@@ -1,5 +1,12 @@
 name: Publish Images
 
+# The automatic path is CI -> Publish Images -> Deploy Delta, chained on
+# workflow_run. That chain has a single point of failure outside our control:
+# when GitHub throttles webhook delivery during an Actions incident, no
+# workflow_run event fires and nothing publishes, so Deploy Delta has no image
+# to deploy and main sits undeployed with no way to force it. workflow_dispatch
+# goes through the REST API instead, which stays up independently, so this is
+# the manual door for exactly that case.
 on:
   workflow_run:
     workflows:
@@ -8,9 +15,14 @@ on:
       - main
     types:
       - completed
+  workflow_dispatch:
+    inputs:
+      sha:
+        description: Full commit SHA on main to build (defaults to the tip of main)
+        required: false
 
 concurrency:
-  group: publish-${{ github.event.workflow_run.head_sha }}
+  group: publish-${{ github.event.workflow_run.head_sha || inputs.sha || github.sha }}
   cancel-in-progress: false
 
 permissions:
@@ -19,21 +31,53 @@ permissions:
 
 jobs:
   publish:
+    # Both paths publish only main. The automatic one gets that from the
+    # workflow_run fields; the manual one is pinned to the main ref here and
+    # then has its resolved SHA checked for ancestry below, so a dispatch
+    # cannot publish a commit that never landed on main.
     if: >
-      github.event.workflow_run.conclusion == 'success' &&
-      github.event.workflow_run.name == 'CI' &&
-      github.event.workflow_run.head_branch == 'main' &&
-      github.event.workflow_run.event == 'push'
+      (github.event_name == 'workflow_dispatch' && github.ref == 'refs/heads/main') ||
+      (
+        github.event.workflow_run.conclusion == 'success' &&
+        github.event.workflow_run.name == 'CI' &&
+        github.event.workflow_run.head_branch == 'main' &&
+        github.event.workflow_run.event == 'push'
+      )
     runs-on: ubuntu-latest
     steps:
       - name: Resolve SHA
         id: sha
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
-          sha="${{ github.event.workflow_run.head_sha }}"
+          if [[ "${{ github.event_name }}" == "workflow_dispatch" ]]; then
+            sha="${{ inputs.sha }}"
+            if [[ -z "$sha" ]]; then
+              sha="${{ github.sha }}"
+            fi
+          else
+            sha="${{ github.event.workflow_run.head_sha }}"
+          fi
+
           if [[ ! "$sha" =~ ^[0-9a-fA-F]{40}$ ]]; then
             echo "invalid full commit SHA: $sha" >&2
             exit 1
           fi
+
+          # A hand-supplied SHA is the one input here that is not already
+          # constrained to main, so confirm it is on main before we build an
+          # image Deploy Delta would treat as a production release. Comparing
+          # main to the commit reports "identical" when it is the tip and
+          # "behind" when it is an earlier commit on main; anything else means
+          # it lives on some other branch.
+          if [[ "${{ github.event_name }}" == "workflow_dispatch" && -n "${{ inputs.sha }}" ]]; then
+            status="$(gh api "repos/${GITHUB_REPOSITORY}/compare/main...${sha}" --jq .status)"
+            if [[ "$status" != "identical" && "$status" != "behind" ]]; then
+              echo "refusing to publish $sha: not a commit on main (compare status: $status)" >&2
+              exit 1
+            fi
+          fi
+
           echo "value=$sha" >> "$GITHUB_OUTPUT"
 
       - uses: actions/checkout@v4