From 06efd191140a0e2fcb0c8a59c23d1ccf1d81d1c8 Mon Sep 17 00:00:00 2001
From: "Chris Rackauckas (Claude)" <accounts@chrisrackauckas.com>
Date: Sat, 13 Jun 2026 06:03:42 -0400
Subject: [PATCH] downgrade/sublibrary-downgrade: add apt-packages + container
 inputs (provision Python/R-stack downgrade legs)

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .github/workflows/downgrade.yml            | 17 +++++++++++++++++
 .github/workflows/sublibrary-downgrade.yml | 16 ++++++++++++++++
 README.md                                  |  4 ++++
 3 files changed, 37 insertions(+)

diff --git a/.github/workflows/downgrade.yml b/.github/workflows/downgrade.yml
index 9c3c86b..a2fe16c 100644
--- a/.github/workflows/downgrade.yml
+++ b/.github/workflows/downgrade.yml
@@ -43,11 +43,22 @@ on:
         default: "ubuntu-latest"
         required: false
         type: string
+      apt-packages:
+        description: "Space-separated apt packages to install before building/testing (Linux only)."
+        default: ""
+        required: false
+        type: string
+      container:
+        description: "Docker container image to run the job in, e.g. for Python-stack packages (empty string = no container)"
+        default: ""
+        required: false
+        type: string
 
 jobs:
   downgrade:
     name: "Downgrade Tests${{ inputs.group != '' && format(' - {0}', inputs.group) || '' }}"
     runs-on: "${{ inputs.self-hosted && 'self-hosted' || inputs.os }}"
+    container: ${{ inputs.container }}
     steps:
       - uses: actions/checkout@v6
 
@@ -108,6 +119,12 @@ jobs:
           projects: "${{ inputs.projects }}"
           mode: "${{ inputs.mode != '' && inputs.mode || 'deps' }}"
 
+      - name: "Install system packages"
+        if: "${{ inputs.apt-packages != '' && runner.os == 'Linux' }}"
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y ${{ inputs.apt-packages }}
+
       - uses: julia-actions/julia-buildpkg@v1
         with:
           project: "${{ inputs.project }}"
diff --git a/.github/workflows/sublibrary-downgrade.yml b/.github/workflows/sublibrary-downgrade.yml
index 3a210dc..28a6128 100644
--- a/.github/workflows/sublibrary-downgrade.yml
+++ b/.github/workflows/sublibrary-downgrade.yml
@@ -37,6 +37,16 @@ on:
         default: ""
         required: false
         type: string
+      apt-packages:
+        description: "Space-separated apt packages to install before building/testing (Linux only)."
+        default: ""
+        required: false
+        type: string
+      container:
+        description: "Docker container image to run the job in, e.g. for Python-stack packages (empty string = no container)"
+        default: ""
+        required: false
+        type: string
 
 jobs:
   discover:
@@ -81,6 +91,7 @@ jobs:
     needs: discover
     if: needs.discover.outputs.has_any == 'true'
     runs-on: ubuntu-latest
+    container: ${{ inputs.container }}
     strategy:
       fail-fast: false
       matrix:
@@ -156,6 +167,11 @@ jobs:
           projects: ${{ matrix.project }}
           skip: ${{ env.EFFECTIVE_SKIP }}
           julia_version: ${{ env.JULIA_MM }}
+      - name: "Install system packages"
+        if: "${{ inputs.apt-packages != '' && runner.os == 'Linux' }}"
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y ${{ inputs.apt-packages }}
       - uses: julia-actions/julia-buildpkg@v1
         with:
           project: ${{ matrix.project }}
diff --git a/README.md b/README.md
index 3ddc43d..5d8818b 100644
--- a/README.md
+++ b/README.md
@@ -230,6 +230,8 @@ to catch under-specified `[compat]` lower bounds.
 | `projects` | string | `"."` | Comma-separated project dirs to downgrade. |
 | `project` | string | `"@."` | `--project` for build/test (a workspace submodule or `lib/X`); default tests the repo root. |
 | `self-hosted` / `os` | | `false` / `ubuntu-latest` | Runner selection. |
+| `apt-packages` | string | `""` | Space-separated apt packages to install before building/testing (Linux only). |
+| `container` | string | `""` | Docker container image to run the job in (e.g. `cmhyett/julia-fenics:latest` for Python-stack packages). Empty = no container. |
 
 ```yaml
 jobs:
@@ -674,6 +676,8 @@ Downgrade-compat tests for each `lib/*` sublibrary.
 | `exclude` | string | `""` | Space-separated sublibrary names to exclude from auto-discovery. |
 | `group-env-name` | string | `""` | Optional group env var name (e.g. `ODEDIFFEQ_TEST_GROUP`). |
 | `group-env-value` | string | `""` | Value for `group-env-name`. |
+| `apt-packages` | string | `""` | Space-separated apt packages to install before building/testing (Linux only). |
+| `container` | string | `""` | Docker container image to run the job in (e.g. `cmhyett/julia-fenics:latest` for Python-stack packages). Empty = no container. |
 
 ```yaml
 jobs: