workflows: add README

This introduces some basic concepts used in these workflows and a common
terminology.

At the same time we remove some of the comments from various workflow
files, because they are assumed to be "general knowledge" through the
README.

(cherry picked from commit 9ea74225cc)

.github/workflows/README.md

@@ -0,0 +1,20 @@
+# GitHub Actions Workflows
+
+Some architectural notes about key decisions and concepts in our workflows:
+
+- Instead of `pull_request` we use [`pull_request_target`](https://docs.github.com/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#pull_request_target) for all PR-related workflows. This has the advantage that those workflows will run without prior approval for external contributors.
+
+- Running on `pull_request_target` also optionally provides us with a GH_TOKEN with elevated privileges (write access), which we need to do things like adding labels, requesting reviewers or pushing branches. **Note about security:** We need to be careful to limit the scope of elevated privileges as much as possible. Thus they should be lowered to the minimum with `permissions: {}` in every workflow by default.
+
+- By definition `pull_request_target` runs in the context of the **base** of the pull request. This means, that the workflow files to run will be taken from the base branch, not the PR, and actions/checkout will not checkout the PR, but the base branch, by default. To protect our secrets, we need to make sure to **never execute code** from the pull request and always evaluate or build nix code from the pull request with the **sandbox enabled**.
+
+- To test the pull request's contents, we checkout the "test merge commit". This is a temporary commit that GitHub creates automatically as "what would happen, if this PR was merged into the base branch now?". The checkout could be done via the virtual branch `refs/pull/<pr-number>/merge`, but doing so would cause failures when this virtual branch doesn't exist (anymore). This can happen when the PR has conflicts, in which case the virtual branch is not created, or when the PR is getting merged while workflows are still running, in which case the branch won't exist anymore at the time of checkout. Thus, we use the `get-merge-commit.yml` workflow to check whether the PR is mergeable and the test merge commit exists and only then run the relevant jobs.
+
+- Various workflows need to make comparisons against the base branch. In this case, we checkout the parent of the "test merge commit" for best results. Note, that this is not necessarily the same as the default commit that actions/checkout would use, which is also a commit from the base branch (see above), but might be older.
+
+## Terminology
+
+- **base commit**: The pull_request_target event's context commit, i.e. the base commit given by GitHub Actions. Same as `github.event.pull_request.base.sha`.
+- **head commit**: The HEAD commit in the pull request's branch. Same as `github.event.pull_request.head.sha`.
+- **merge commit**: The temporary "test merge commit" that GitHub Actions creates and updates for the pull request. Same as `refs/pull/${{ github.event.pull_request.number }}/merge`.
+- **target commit**: The base branch's parent of the "test merge commit" to compare against.

.github/workflows/check-maintainers-sorted.yml

@@ -5,8 +5,7 @@ on:
     paths:
       - 'maintainers/maintainer-list.nix'
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   nixos:
@@ -15,7 +14,6 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
           # Only these directories to perform the check
           sparse-checkout: |
@@ -24,7 +22,6 @@ jobs:
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
         with:
-          # explicitly enable sandbox
           extra_nix_config: sandbox = true
 
       - name: Check that maintainer-list.nix is sorted

.github/workflows/check-nix-format.yml

@@ -8,11 +8,9 @@ name: Check that Nix files are formatted
 
 on:
   pull_request_target:
-    # See the comment at the same location in ./nixpkgs-vet.yml
     types: [opened, synchronize, reopened, edited]
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   get-merge-commit:
@@ -26,7 +24,6 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
           # Fetches the merge commit and its parents
           fetch-depth: 2
@@ -49,7 +46,6 @@ jobs:
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
         with:
-          # explicitly enable sandbox
           extra_nix_config: sandbox = true
           nix_path: nixpkgs=${{ env.url }}
 

.github/workflows/check-nixf-tidy.yml

@@ -4,8 +4,7 @@ on:
   pull_request_target:
     types: [opened, synchronize, reopened, edited]
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   nixos:
@@ -15,7 +14,6 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
           # Fetches the merge commit and its parents
           fetch-depth: 2
@@ -38,7 +36,6 @@ jobs:
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
         with:
-          # explicitly enable sandbox
           extra_nix_config: sandbox = true
           nix_path: nixpkgs=${{ env.url }}
 

.github/workflows/check-shell.yml

@@ -25,7 +25,6 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30

.github/workflows/codeowners-v2.yml

@@ -26,7 +26,6 @@ on:
   pull_request_target:
     types: [opened, ready_for_review, synchronize, reopened, edited]
 
-# We don't need any default GitHub token
 permissions: {}
 
 env:

.github/workflows/editorconfig-v2.yml

@@ -1,12 +1,9 @@
 name: "Checking EditorConfig v2"
 
 on:
-  # avoids approving first time contributors
   pull_request_target:
 
-permissions:
-  pull-requests: read
-  contents: read
+permissions: {}
 
 jobs:
   get-merge-commit:
@@ -33,7 +30,6 @@ jobs:
 
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30

.github/workflows/eval-lib-tests.yml

@@ -5,8 +5,7 @@ on:
     paths:
       - 'lib/**'
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   get-merge-commit:
@@ -20,12 +19,10 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
         with:
-          # explicitly enable sandbox
           extra_nix_config: sandbox = true
 
       - name: Building Nixpkgs lib-tests

.github/workflows/eval.yml

@@ -12,8 +12,7 @@ on:
       - haskell-updates
       - python-updates
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   get-merge-commit:
@@ -23,7 +22,6 @@ jobs:
     name: Attributes
     runs-on: ubuntu-24.04
     needs: get-merge-commit
-    # Skip this and dependent steps if the PR can't be merged
     if: needs.get-merge-commit.outputs.mergedSha
     outputs:
       targetSha: ${{ steps.targetSha.outputs.targetSha }}
@@ -45,6 +43,8 @@ jobs:
 
       - name: Install Nix
         uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+        with:
+          extra_nix_config: sandbox = true
 
       - name: Evaluate the list of all attributes and get the systems matrix
         id: systems
@@ -71,6 +71,8 @@ jobs:
 
       - name: Install Nix
         uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+        with:
+          extra_nix_config: sandbox = true
 
       - name: Query nixpkgs with aliases enabled to check for basic syntax errors
         run: |
@@ -106,6 +108,8 @@ jobs:
 
       - name: Install Nix
         uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+        with:
+          extra_nix_config: sandbox = true
 
       - name: Evaluate the ${{ matrix.system }} output paths for all derivation attributes
         env:
@@ -145,6 +149,8 @@ jobs:
 
       - name: Install Nix
         uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+        with:
+          extra_nix_config: sandbox = true
 
       - name: Combine all output paths and eval stats
         run: |

.github/workflows/get-merge-commit.yml

@@ -7,7 +7,6 @@ on:
         description: "The merge commit SHA"
         value: ${{ jobs.resolve-merge-commit.outputs.mergedSha }}
 
-# We need a token to query the API, but it doesn't need any special permissions
 permissions: {}
 
 jobs:

.github/workflows/manual-nixos-v2.yml

@@ -7,8 +7,7 @@ on:
     paths:
       - 'nixos/**'
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   nixos:
@@ -17,12 +16,10 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
         with:
-          # explicitly enable sandbox
           extra_nix_config: sandbox = true
 
       - uses: cachix/cachix-action@ad2ddac53f961de1989924296a1f236fcfbaa4fc # v15

.github/workflows/manual-nixpkgs-v2.yml

@@ -9,8 +9,7 @@ on:
       - 'lib/**'
       - 'pkgs/tools/nix/nixdoc/**'
 
-permissions:
-  contents: read
+permissions: {}
 
 jobs:
   nixpkgs:
@@ -19,12 +18,10 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
         with:
-          # explicitly enable sandbox
           extra_nix_config: sandbox = true
 
       - uses: cachix/cachix-action@ad2ddac53f961de1989924296a1f236fcfbaa4fc # v15

.github/workflows/nix-parse-v2.yml

@@ -1,12 +1,9 @@
 name: "Check whether nix files are parseable v2"
 
 on:
-  # avoids approving first time contributors
   pull_request_target:
 
-permissions:
-  pull-requests: read
-  contents: read
+permissions: {}
 
 jobs:
   get-merge-commit:
@@ -32,12 +29,12 @@ jobs:
 
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
         if: ${{ env.CHANGED_FILES && env.CHANGED_FILES != '' }}
 
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
         with:
+          extra_nix_config: sandbox = true
           nix_path: nixpkgs=channel:nixpkgs-unstable
 
       - name: Parse all changed or added nix files

.github/workflows/nixpkgs-vet.yml

@@ -6,7 +6,6 @@
 name: Vet nixpkgs
 
 on:
-  # Using pull_request_target instead of pull_request avoids having to approve first time contributors.
   pull_request_target:
     # This workflow depends on the base branch of the PR, but changing the base branch is not included in the default trigger events, which would be `opened`, `synchronize` or `reopened`.
     # Instead it causes an `edited` event, so we need to add it explicitly here.
@@ -34,7 +33,6 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          # pull_request_target checks out the base branch by default
           ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
           # Fetches the merge commit and its parents
           fetch-depth: 2