ci: Add review request scripts

Also post a comment in case base branch is wrong
  This guides newcomers in how to smoothly handle the potentially scary
  situation of having thousands of commits listed in a PR.

  While CI shows the same, people might not even look at CI if the PR
  looks botched.

ci/default.nix

@@ -24,4 +24,5 @@ let
 in
 {
   inherit pkgs;
+  requestReviews = pkgs.callPackage ./request-reviews { };
 }

ci/request-reviews/default.nix

@@ -0,0 +1,43 @@
+{
+  lib,
+  stdenvNoCC,
+  makeWrapper,
+  coreutils,
+  codeowners,
+  jq,
+  curl,
+  github-cli,
+  gitMinimal,
+}:
+stdenvNoCC.mkDerivation {
+  name = "request-reviews";
+  src = lib.fileset.toSource {
+    root = ./.;
+    fileset = lib.fileset.unions [
+      ./get-reviewers.sh
+      ./request-reviews.sh
+      ./verify-base-branch.sh
+      ./dev-branches.txt
+    ];
+  };
+  nativeBuildInputs = [ makeWrapper ];
+  dontBuild = true;
+  installPhase = ''
+    mkdir -p $out/bin
+    mv dev-branches.txt $out/bin
+    for bin in *.sh; do
+      mv "$bin" "$out/bin"
+      wrapProgram "$out/bin/$bin" \
+        --set PATH ${
+          lib.makeBinPath [
+            coreutils
+            codeowners
+            jq
+            curl
+            github-cli
+            gitMinimal
+          ]
+        }
+    done
+  '';
+}

ci/request-reviews/dev-branches.txt

@@ -0,0 +1,7 @@
+# Trusted development branches:
+# These generally require PRs to update and are built by Hydra.
+master
+staging
+release-*
+staging-*
+haskell-updates

ci/request-reviews/get-reviewers.sh

@@ -0,0 +1,87 @@
+#!/usr/bin/env bash
+
+# Get the code owners of the files changed by a PR,
+# suitable to be consumed by the API endpoint to request reviews:
+# https://docs.github.com/en/rest/pulls/review-requests?apiVersion=2022-11-28#request-reviewers-for-a-pull-request
+
+set -euo pipefail
+
+log() {
+    echo "$@" >&2
+}
+
+if (( "$#" < 5 )); then
+    log "Usage: $0 GIT_REPO BASE_REF HEAD_REF OWNERS_FILE PR_AUTHOR"
+    exit 1
+fi
+
+gitRepo=$1
+baseRef=$2
+headRef=$3
+ownersFile=$4
+prAuthor=$5
+
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' exit
+
+git -C "$gitRepo" diff --name-only --merge-base "$baseRef" "$headRef" > "$tmp/touched-files"
+readarray -t touchedFiles < "$tmp/touched-files"
+log "This PR touches ${#touchedFiles[@]} files"
+
+# Get the owners file from the base, because we don't want to allow PRs to
+# remove code owners to avoid pinging them
+git -C "$gitRepo" show "$baseRef":"$ownersFile" > "$tmp"/codeowners
+
+# Associative arrays with the team/user as the key for easy deduplication
+declare -A teams users
+
+for file in "${touchedFiles[@]}"; do
+    result=$(codeowners --file "$tmp"/codeowners "$file")
+
+    read -r file owners <<< "$result"
+    if [[ "$owners" == "(unowned)" ]]; then
+        log "File $file is unowned"
+        continue
+    fi
+    log "File $file is owned by $owners"
+
+    # Split up multiple owners, separated by arbitrary amounts of spaces
+    IFS=" " read -r -a entries <<< "$owners"
+
+    for entry in "${entries[@]}"; do
+        # GitHub technically also supports Emails as code owners,
+        # but we can't easily support that, so let's not
+        if [[ ! "$entry" =~ @(.*) ]]; then
+            warn -e "\e[33mCodeowner \"$entry\" for file $file is not valid: Must start with \"@\"\e[0m" >&2
+            # Don't fail, because the PR for which this script runs can't fix it,
+            # it has to be fixed in the base branch
+            continue
+        fi
+        # The first regex match is everything after the @
+        entry=${BASH_REMATCH[1]}
+        if [[ "$entry" =~ .*/(.*) ]]; then
+            # Teams look like $org/$team, where we only need $team for the API
+            # call to request reviews from teams
+            teams[${BASH_REMATCH[1]}]=
+        else
+            # Everything else is a user
+            users[$entry]=
+        fi
+    done
+
+done
+
+# Cannot request a review from the author
+if [[ -v users[$prAuthor] ]]; then
+    log "One or more files are owned by the PR author, ignoring"
+    unset 'users[$prAuthor]'
+fi
+
+# Turn it into a JSON for the GitHub API call to request PR reviewers
+jq -n \
+    --arg users "${!users[*]}" \
+    --arg teams "${!teams[*]}" \
+    '{
+      reviewers: $users | split(" "),
+      team_reviewers: $teams | split(" ")
+    }'

ci/request-reviews/request-reviews.sh

@@ -0,0 +1,97 @@
+#!/usr/bin/env bash
+
+# Requests reviews for a PR after verifying that the base branch is correct
+
+set -euo pipefail
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' exit
+SCRIPT_DIR=$(dirname "$0")
+
+log() {
+    echo "$@" >&2
+}
+
+effect() {
+    if [[ -n "${DRY_MODE:-}" ]]; then
+        log "Skipping in dry mode:" "${@@Q}"
+    else
+        "$@"
+    fi
+}
+
+if (( $# < 3 )); then
+    log "Usage: $0 GITHUB_REPO PR_NUMBER OWNERS_FILE"
+    exit 1
+fi
+baseRepo=$1
+prNumber=$2
+ownersFile=$3
+
+log "Fetching PR info"
+prInfo=$(gh api \
+  -H "Accept: application/vnd.github+json" \
+  -H "X-GitHub-Api-Version: 2022-11-28" \
+  "/repos/$baseRepo/pulls/$prNumber")
+
+baseBranch=$(jq -r .base.ref <<< "$prInfo")
+log "Base branch: $baseBranch"
+prRepo=$(jq -r .head.repo.full_name <<< "$prInfo")
+log "PR repo: $prRepo"
+prBranch=$(jq -r .head.ref <<< "$prInfo")
+log "PR branch: $prBranch"
+prAuthor=$(jq -r .user.login <<< "$prInfo")
+log "PR author: $prAuthor"
+
+extraArgs=()
+if pwdRepo=$(git rev-parse --show-toplevel 2>/dev/null); then
+    # Speedup for local runs
+    extraArgs+=(--reference-if-able "$pwdRepo")
+fi
+
+log "Fetching Nixpkgs commit history"
+# We only need the commit history, not the contents, so we can do a tree-less clone using tree:0
+# https://github.blog/open-source/git/get-up-to-speed-with-partial-clone-and-shallow-clone/#user-content-quick-summary
+git clone --bare --filter=tree:0 --no-tags --origin upstream "${extraArgs[@]}" https://github.com/"$baseRepo".git "$tmp"/nixpkgs.git
+
+log "Fetching the PR commit history"
+# Fetch the PR
+git -C "$tmp/nixpkgs.git" remote add fork https://github.com/"$prRepo".git
+# This remote config is the same as --filter=tree:0 when cloning
+git -C "$tmp/nixpkgs.git" config remote.fork.partialclonefilter tree:0
+git -C "$tmp/nixpkgs.git" config remote.fork.promisor true
+
+# This should not conflict with any refs in Nixpkgs
+headRef=refs/remotes/fork/pr
+# Only fetch into a remote ref, because the local ref namespace is used by Nixpkgs, don't want any conflicts
+git -C "$tmp/nixpkgs.git" fetch --no-tags fork "$prBranch":"$headRef"
+
+log "Checking correctness of the base branch"
+if ! "$SCRIPT_DIR"/verify-base-branch.sh "$tmp/nixpkgs.git" "$headRef" "$baseRepo" "$baseBranch" "$prRepo" "$prBranch" | tee "$tmp/invalid-base-error" >&2; then
+    log "Posting error as comment"
+    if ! response=$(effect gh api \
+        --method POST \
+        -H "Accept: application/vnd.github+json" \
+        -H "X-GitHub-Api-Version: 2022-11-28" \
+        "/repos/$baseRepo/issues/$prNumber/comments" \
+        -F "body=@$tmp/invalid-base-error"); then
+        log "Failed to post the comment: $response"
+    fi
+    exit 1
+fi
+
+log "Getting code owners to request reviews from"
+"$SCRIPT_DIR"/get-reviewers.sh "$tmp/nixpkgs.git" "$baseBranch" "$headRef" "$ownersFile" "$prAuthor" > "$tmp/reviewers.json"
+
+log "Requesting reviews from: $(<"$tmp/reviewers.json")"
+
+if ! response=$(effect gh api \
+    --method POST \
+    -H "Accept: application/vnd.github+json" \
+    -H "X-GitHub-Api-Version: 2022-11-28" \
+    "/repos/$baseRepo/pulls/$prNumber/requested_reviewers" \
+    --input "$tmp/reviewers.json"); then
+    log "Failed to request reviews: $response"
+    exit 1
+fi
+
+log "Successfully requested reviews"

ci/request-reviews/verify-base-branch.sh

@@ -0,0 +1,103 @@
+#!/usr/bin/env bash
+
+# Check that a PR doesn't include commits from other development branches.
+# Fails with next steps if it does
+
+set -euo pipefail
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' exit
+SCRIPT_DIR=$(dirname "$0")
+
+log() {
+    echo "$@" >&2
+}
+
+# Small helper to check whether an element is in a list
+# Usage: `elementIn foo "${list[@]}"`
+elementIn() {
+    local e match=$1
+    shift
+    for e; do
+        if [[ "$e" == "$match" ]]; then
+            return 0
+        fi
+    done
+    return 1
+}
+
+if (( $# < 6 )); then
+    log "Usage: $0 LOCAL_REPO HEAD_REF BASE_REPO BASE_BRANCH PR_REPO PR_BRANCH"
+    exit 1
+fi
+localRepo=$1
+headRef=$2
+baseRepo=$3
+baseBranch=$4
+prRepo=$5
+prBranch=$6
+
+# All development branches
+devBranchPatterns=()
+while read -r pattern; do
+    if [[ "$pattern" != '#'* ]]; then
+        devBranchPatterns+=("$pattern")
+    fi
+done < "$SCRIPT_DIR/dev-branches.txt"
+
+git -C "$localRepo" branch --list --format "%(refname:short)" "${devBranchPatterns[@]}" > "$tmp/dev-branches"
+readarray -t devBranches < "$tmp/dev-branches"
+
+if [[ "$baseRepo" == "$prRepo" ]] && elementIn "$prBranch" "${devBranches[@]}"; then
+    log "This PR merges $prBranch into $baseBranch, no commit check necessary"
+    exit 0
+fi
+
+# The current merge base of the PR
+prMergeBase=$(git -C "$localRepo" merge-base "$baseBranch" "$headRef")
+log "The PR's merge base with the base branch $baseBranch is $prMergeBase"
+
+# This is purely for debugging
+git -C "$localRepo" rev-list --reverse "$baseBranch".."$headRef" > "$tmp/pr-commits"
+log "The PR includes these $(wc -l < "$tmp/pr-commits") commits:"
+cat <"$tmp/pr-commits" >&2
+
+for testBranch in "${devBranches[@]}"; do
+
+    if [[ -z "$(git -C "$localRepo" rev-list -1 --since="1 month ago" "$testBranch")" ]]; then
+        log "Not checking $testBranch, was inactive for the last month"
+        continue
+    fi
+    log "Checking if commits from $testBranch are included in the PR"
+
+    # We need to check for any commits that are in the PR which are also in the test branch.
+    # We could check each commit from the PR individually, but that's unnecessarily slow.
+    #
+    # This does _almost_ what we want: `git rev-list --count headRef testBranch ^baseBranch`,
+    # except that it includes commits that are reachable from _either_ headRef or testBranch,
+    # instead of restricting it to ones reachable by both
+
+    # Easily fixable though, because we can use `git merge-base testBranch headRef`
+    # to get the least common ancestor (aka merge base) commit reachable by both.
+    # If the branch being tested is indeed the right base branch,
+    # this is then also the commit from that branch that the PR is based on top of.
+    testMergeBase=$(git -C "$localRepo" merge-base "$testBranch" "$headRef")
+
+    # And then use the `git rev-list --count`, but replacing the non-working
+    # `headRef testBranch` with the merge base of the two.
+    extraCommits=$(git -C "$localRepo" rev-list --count "$testMergeBase" ^"$baseBranch")
+
+    if (( extraCommits != 0 )); then
+        log -e "\e[33m"
+        echo "The PR's base branch is set to $baseBranch, but $extraCommits commits from the $testBranch branch are included. Make sure you know the [right base branch for your changes](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#branch-conventions), then:"
+        echo "- If the changes should go to the $testBranch branch, [change the base branch](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-base-branch-of-a-pull-request) to $testBranch"
+        echo "- If the changes should go to the $baseBranch branch, rebase your PR onto the merge base with the $testBranch branch:"
+        echo "  \`\`\`"
+        echo "  git rebase --onto $prMergeBase $testMergeBase"
+        echo "  git push --force-with-lease"
+        echo "  \`\`\`"
+        log -e "\e[m"
+        exit 1
+    fi
+done
+
+log "Base branch is correct, no commits from development branches are included"