Move CI jobs for draft PRs to contributor's personal forks of Mill (#…

…3543)

This PR aims to reduce the bottleneck of contributors waiting for CI
jobs on the shared Mill repo by moving CI for draft/WIP pull requests to
each contributor's own fork, each of which has a separate concurrency
budget (~20 concurrent workers) from the `com-lihaoyi` organization.

Once a PR is marked ready-for-review, `com-lihaoyi` CI begins, so the
author effectively has a choice of where they want to run their CI
workloads. Mill's main PR validation CI jobs are portable, do not need
any special secrets or environment, and can run just fine in
contributor's GH Actions environments

To do this:

* Made the upstream `build-linux`/`build-windows`/`test-docs` CI jobs
filter on `github.event.pull_request.draft == false`, and

* Removed the `branches: - main` filter on the `run-tests` jobs so they
begin running in contributor GH Actions when commits are pushed to a
branch

* Add a new `Draft CI` workflow that pushes a github status linking to
the PR's branch in the contributor's fork, as an easy way to see the CI
history for the PR there. This needs to run on `pull_request_target`
rather than just `pull_request` to have the suitable permissions to
update statuses

* Some subtleties around handling `ready_for_review` events such that
when a contributor marks a PR as ready for review, CI is re-triggered
and handled appropriately without needing to subsequently push a new
commit.

* Substituted `github.head_ref` with `github.ref_name` in the
`concurrency.group` key since the former only works on PR events while
the latter also works on push events

This is a bit of an unusual setup AFAIK for Github Actions, but I've
tested it manually a bunch and it seems to work, and should mitigate the
bottleneck of multiple people updating their WIP PRs and having everyone
blocked waiting on everyone else. Worst come to worst, if you mark a PR
as `ready_for_review` it falls back to the old behavior, but this setup
gives you an option to run draft PR validation on your own fork's GH
Actions where it won't get blocked by other people's PRs. This is
something you can actually already do today, but it requires some
tedious fiddling that PR makes seamless

Tested on #3540 via `git commit
-am . --allow-empty && git commit -am . --allow-empty && git push origin
head && git push upstream head~1:test-non-draft && git commit -am .
--allow-empty && git push origin head`, and targeting this PR against
the `test-non-draft` branch in the Mill repo

.github/ISSUE_TEMPLATE.md

@@ -2,3 +2,6 @@ Please don't open issues for questions, but ask in our Discussions forum at http
 
 Mill installations via `coursier` or `cs` are unsupported.
 
+Please open all PRs as drafts to avoid being bottlenecked by Mill CI, and only
+convert to ready for review once CI on your own fork is green. There will be a 
+PR status check linking your fork's commit/CI history for convenient viewing 

.github/workflows/draft-ci.yml

@@ -0,0 +1,34 @@
+name: Draft CI
+
+permissions: write-all
+on:
+  pull_request_target:
+    types:
+      - opened
+      - reopened
+      - synchronize
+      - ready_for_review
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    permissions: write-all
+    steps:
+      - name: Debug Echos
+        run: |
+          echo ${{ github.event.action }}
+          echo ${{ github.event.action == 'ready_for_review' }}
+          echo "${{ github.event.pull_request.head.repo.html_url }}/commits/${{github.event.pull_request.head.ref}}"
+      - name: Create status
+        run: |
+          curl --request POST \
+          --url ${{ github.event.pull_request.statuses_url }} \
+          --header 'authorization: Bearer ${{ secrets.GITHUB_TOKEN }}' \
+          --header 'content-type: application/json' \
+          --data '{
+            "state": "${{(github.event.action != 'ready_for_review' && github.event.pull_request.draft) && 'pending' || 'success'}}",
+            "context": "Draft CI / link",
+            "target_url": ${{(github.event.action != 'ready_for_review' && github.event.pull_request.draft) && format('"{0}/commits/{1}"', github.event.pull_request.head.repo.html_url, github.event.pull_request.head.ref) || 'null'}},
+            "description": "${{(github.event.action != 'ready_for_review' && github.event.pull_request.draft) && 'use CI on your repo fork (link on right) until this PR is ready for review' || 'PR is ready for review, running CI in Mill repo'}}"
+            }' \
+          --fail-with-body

.github/workflows/publish-artifacts.yml

@@ -14,12 +14,6 @@ on:
       - '**'
   workflow_dispatch:
 
-# cancel older runs of a pull request;
-# this will not cancel anything for normal git pushes
-concurrency:
-  group: cancel-old-pr-runs-${{ github.workflow }}-${{ github.head_ref || github.run_id }}
-  cancel-in-progress: true
-
 jobs:
   build-artifacts:
     # when in master repo, publish all tags and manual runs on main

.github/workflows/run-tests.yml

@@ -32,28 +32,36 @@ name: Run Tests
 on:
   push:
   pull_request:
+    types:
+      - opened
+      - reopened
+      - synchronize
+      - ready_for_review
   workflow_dispatch:
 
 # cancel older runs of a pull request;
 # this will not cancel anything for normal git pushes
 concurrency:
-  group: cancel-old-pr-runs-${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  group: cancel-old-pr-runs-${{ github.workflow }}-${{ github.ref_name }}
   cancel-in-progress: true
 
 jobs:
   # Jobs are listed in rough order of priority: if multiple jobs fail, the first job
   # in the list should be the one that's most worth looking into
   build-linux:
+    if: (github.event.action == 'ready_for_review') || (github.event.pull_request.draft == false)
     uses: ./.github/workflows/pre-build.yml
     with:
       os: ubuntu-latest
 
   build-windows:
+    if: (github.event.action == 'ready_for_review') || (github.event.pull_request.draft == false)
     uses: ./.github/workflows/pre-build.yml
     with:
       os: windows-latest
 
   test-docs:
+    if: (github.event.action == 'ready_for_review') || (github.event.pull_request.draft == false)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -62,6 +70,7 @@ jobs:
       - run: ./mill -i docs.githubPages + docs.checkBrokenLinks
 
   mac:
+    if: (github.event.action == 'ready_for_review') || (github.event.pull_request.draft == false)
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v4