workflows/nixpkgs-vet: Make merge check script reusable

This is useful for other workflows as well. Originally I thought it
couldn't be put in the repo, but it can (just needs another checkout)

.github/workflows/nixpkgs-vet.yml

@@ -26,52 +26,22 @@ jobs:
     # This should take 1 minute at most, but let's be generous. The default of 6 hours is definitely too long.
     timeout-minutes: 10
     steps:
-      # This step has to be in this file, because it's needed to determine which revision of the repository to fetch, and we can only use other files from the repository once it's fetched.
+      # This checks out the base branch because of pull_request_target
+      - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
+        with:
+          path: base
+          sparse-checkout: ci
       - name: Resolving the merge commit
         env:
           GH_TOKEN: ${{ github.token }}
         run: |
-          # This checks for mergeability of a pull request as recommended in
-          # https://docs.github.com/en/rest/guides/using-the-rest-api-to-interact-with-your-git-database?apiVersion=2022-11-28#checking-mergeability-of-pull-requests
-
-          # Retry the API query this many times
-          retryCount=5
-          # Start with 5 seconds, but double every retry
-          retryInterval=5
-          while true; do
-            echo "Checking whether the pull request can be merged"
-            prInfo=$(gh api \
-              -H "Accept: application/vnd.github+json" \
-              -H "X-GitHub-Api-Version: 2022-11-28" \
-              /repos/"$GITHUB_REPOSITORY"/pulls/${{ github.event.pull_request.number }})
-            mergeable=$(jq -r .mergeable <<< "$prInfo")
-            mergedSha=$(jq -r .merge_commit_sha <<< "$prInfo")
-
-            if [[ "$mergeable" == "null" ]]; then
-              if (( retryCount == 0 )); then
-                echo "Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com/"
-                exit 1
-              else
-                (( retryCount -= 1 )) || true
-
-                # null indicates that GitHub is still computing whether it's mergeable
-                # Wait a couple seconds before trying again
-                echo "GitHub is still computing whether this PR can be merged, waiting $retryInterval seconds before trying again ($retryCount retries left)"
-                sleep "$retryInterval"
-
-                (( retryInterval *= 2 )) || true
-              fi
-            else
-              break
-            fi
-          done
-
-          if [[ "$mergeable" == "true" ]]; then
-            echo "The PR can be merged, checking the merge commit $mergedSha"
+          if mergedSha=$(base/ci/get-merge-commit.sh ${{ github.repository }} ${{ github.event.number }}); then
+            echo "Checking the merge commit $mergedSha"
             echo "mergedSha=$mergedSha" >> "$GITHUB_ENV"
           else
-            echo "The PR cannot be merged, it has a merge conflict, skipping the rest.."
+            echo "Skipping the rest..."
           fi
+          rm -rf base
       - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
         if: env.mergedSha
         with:

ci/get-merge-commit.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# This checks for mergeability of a pull request as recommended in
+# https://docs.github.com/en/rest/guides/using-the-rest-api-to-interact-with-your-git-database?apiVersion=2022-11-28#checking-mergeability-of-pull-requests
+
+set -euo pipefail
+
+log() {
+    echo "$@" >&2
+}
+
+if (( $# < 2 )); then
+    log "Usage: $0 GITHUB_REPO PR_NUMBER"
+    exit 99
+fi
+repo=$1
+prNumber=$2
+
+# Retry the API query this many times
+retryCount=5
+# Start with 5 seconds, but double every retry
+retryInterval=5
+
+while true; do
+    log "Checking whether the pull request can be merged"
+    prInfo=$(gh api \
+        -H "Accept: application/vnd.github+json" \
+        -H "X-GitHub-Api-Version: 2022-11-28" \
+        "/repos/$repo/pulls/$prNumber")
+    mergeable=$(jq -r .mergeable <<< "$prInfo")
+    if [[ "$mergeable" == "null" ]]; then
+        if (( retryCount == 0 )); then
+            log "Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com/"
+            exit 1
+        else
+            (( retryCount -= 1 )) || true
+
+            # null indicates that GitHub is still computing whether it's mergeable
+            # Wait a couple seconds before trying again
+            log "GitHub is still computing whether this PR can be merged, waiting $retryInterval seconds before trying again ($retryCount retries left)"
+            sleep "$retryInterval"
+
+            (( retryInterval *= 2 )) || true
+        fi
+    else
+        break
+    fi
+done
+
+if [[ "$mergeable" == "true" ]]; then
+    log "The PR can be merged"
+    jq -r .merge_commit_sha <<< "$prInfo"
+else
+    log "The PR has a merge conflict"
+    exit 1
+fi