Test against oldest supported version

.github/workflows/pr-checks.yml

@@ -292,7 +292,10 @@ jobs:
     outputs:
       versions: ${{ steps.compare.outputs.versions }}
       nightly-url: ${{ steps.get-url.outputs.nightly-url }}
-
+    env:
+      # URL of the oldest release that we support, if this is being bumped then the constant
+      # CODEQL_MINIMUM_VERSION in the file codeql.ts should also be bumped to match this.
+      OLDEST_URL: https://github.com/github/codeql-action/releases/download/codeql-bundle-20201028/codeql-bundle.tar.gz
     steps:
     - uses: actions/checkout@v2
     - name: Move codeql-action
@@ -336,31 +339,44 @@ jobs:
       with:
         tools: ${{ steps.get-url.outputs.nightly-url }}
         languages: javascript
+    - name: Remove empty database
+      # allows us to run init a fourth time
+      run: |
+        rm -rf "$RUNNER_TEMP/codeql_databases"
+    - name: Init with a CodeQL bundle from the oldest supported release
+      id: init-oldest
+      uses: ./../action/init
+      with:
+        tools: ${{ env.OLDEST_URL }}
+        languages: javascript
     - name: Compare CodeQL bundle versions
       id: compare
       env:
         CODEQL_DEFAULT: ${{ steps.init-default.outputs.codeql-path }}
         CODEQL_LATEST: ${{ steps.init-latest.outputs.codeql-path }}
         CODEQL_NIGHTLY: ${{ steps.init-nightly.outputs.codeql-path }}
+        CODEQL_OLDEST: ${{ steps.init-oldest.outputs.codeql-path }}
         NIGHTLY_URL: ${{ steps.get-url.outputs.nightly-url }}
       run: |
         CODEQL_VERSION_DEFAULT="$("$CODEQL_DEFAULT" version --format terse)"
         CODEQL_VERSION_LATEST="$("$CODEQL_LATEST" version --format terse)"
         CODEQL_VERSION_NIGHTLY="$("$CODEQL_NIGHTLY" version --format terse)"
+        CODEQL_VERSION_OLDEST="$("$CODEQL_OLDEST" version --format terse)"
         echo "Default CodeQL bundle version is $CODEQL_VERSION_DEFAULT"
         echo "Latest CodeQL bundle version is $CODEQL_VERSION_LATEST"
         echo "Nightly CodeQL bundle version is $CODEQL_VERSION_NIGHTLY"
+        echo "Oldest supported CodeQL bundle version is $CODEQL_VERSION_OLDEST"
 
-        # If we're running on a pull request, run each integration test with all three bundles, even
+        # If we're running on a pull request, run each integration test with all four bundles, even
         # if `tools: latest` would be the same as `tools: null`. This allows us to make the
-        # integration test job for each of the three bundles a required status check.
+        # integration test job for each of the four bundles a required status check.
         #
         # If we're running on push, then we can skip running with `tools: latest` when it would be
         # the same as running with `tools: null`.
         if [[ "$GITHUB_EVENT_NAME" != "pull_request" && "$CODEQL_VERSION_DEFAULT" == "$CODEQL_VERSION_LATEST" ]]; then
-          VERSIONS_JSON="[null, \"$NIGHTLY_URL\"]"
+          VERSIONS_JSON="[null, \"$NIGHTLY_URL\", \"$OLDEST_URL\"]"
         else
-          VERSIONS_JSON="[null, \"$NIGHTLY_URL\", \"latest\"]"
+          VERSIONS_JSON="[null, \"$NIGHTLY_URL\", \"$OLDEST_URL\", \"latest\"]"
         fi
 
         # Output a JSON-encoded list with the distinct versions to test against.