Check submodule forward progress (#8949)

We frequently mess up our submodule references. This adds one safeguard:
it checks that the submodule references are only updated "forwards", not
to some older commit, or a commit that's not a descended of the previous 
one.

As next step, I'm thinking that we should automate things so that when
you merge a PR to the 'neon' repository that updates the submodule
references, the REL_*_STABLE_neon branches are automatically updated to
match the submodule references. That way, you never need to manually
merge PRs in the postgres repository, it's all triggered from commits in
the 'neon' repository. But that's not included here.

.github/workflows/build_and_test.yml

@@ -120,6 +120,59 @@ jobs:
       - name: Run mypy to check types
         run: poetry run mypy .
 
+  # Check that the vendor/postgres-* submodules point to the
+  # corresponding REL_*_STABLE_neon branches.
+  check-submodules:
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          submodules: true
+
+      - uses: dorny/paths-filter@v3
+        id: check-if-submodules-changed
+        with:
+          filters: |
+            vendor:
+              - 'vendor/**'
+
+      - name: Check vendor/postgres-v14 submodule reference
+        if: steps.check-if-submodules-changed.outputs.vendor == 'true'
+        uses: jtmullen/submodule-branch-check-action@v1
+        with:
+          path: "vendor/postgres-v14"
+          fetch_depth: "50"
+          sub_fetch_depth: "50"
+          pass_if_unchanged: true
+
+      - name: Check vendor/postgres-v15 submodule reference
+        if: steps.check-if-submodules-changed.outputs.vendor == 'true'
+        uses: jtmullen/submodule-branch-check-action@v1
+        with:
+          path: "vendor/postgres-v15"
+          fetch_depth: "50"
+          sub_fetch_depth: "50"
+          pass_if_unchanged: true
+
+      - name: Check vendor/postgres-v16 submodule reference
+        if: steps.check-if-submodules-changed.outputs.vendor == 'true'
+        uses: jtmullen/submodule-branch-check-action@v1
+        with:
+          path: "vendor/postgres-v16"
+          fetch_depth: "50"
+          sub_fetch_depth: "50"
+          pass_if_unchanged: true
+
+      - name: Check vendor/postgres-v17 submodule reference
+        if: steps.check-if-submodules-changed.outputs.vendor == 'true'
+        uses: jtmullen/submodule-branch-check-action@v1
+        with:
+          path: "vendor/postgres-v17"
+          fetch_depth: "50"
+          sub_fetch_depth: "50"
+          pass_if_unchanged: true
+
   check-codestyle-rust:
     needs: [ check-permissions, build-build-tools-image ]
     strategy:
@@ -1154,6 +1207,7 @@ jobs:
     # Usually we do `needs: [...]`
     needs:
       - build-and-test-locally
+      - check-submodules
       - check-codestyle-python
       - check-codestyle-rust
       - promote-images