From 5e420ff63ba2997a37bf5dfbfaa73c5f05225f9d Mon Sep 17 00:00:00 2001
From: Kyle Edwards <kyedwards@nvidia.com>
Date: Fri, 30 Aug 2024 17:44:03 -0400
Subject: [PATCH] Use merge base when calculating changed files (#16709)

`get-pr-info.outputs.base.sha` does not actually give the merge base, but merely the tip of the target branch. Calculate the merge base and pass it to the `changed-files` step.

Authors:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)

Approvers:
  - Ray Douglass (https://github.com/raydouglass)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: https://github.com/rapidsai/cudf/pull/16709
---
 .github/workflows/pr.yaml | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/.github/workflows/pr.yaml b/.github/workflows/pr.yaml
index 35c7e3d95b6..0d79568f589 100644
--- a/.github/workflows/pr.yaml
+++ b/.github/workflows/pr.yaml
@@ -56,14 +56,21 @@ jobs:
       - name: Checkout code repo
         uses: actions/checkout@v4
         with:
-          ref: ${{ inputs.sha }}
-          fetch-depth: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).commits }}
+          fetch-depth: 0
           persist-credentials: false
+      - name: Calculate merge base
+        id: calculate-merge-base
+        env:
+          PR_SHA: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).head.sha }}
+          BASE_SHA: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).base.sha }}
+        run: |
+          (echo -n "merge-base="; git merge-base "$BASE_SHA" "$PR_SHA") > "$GITHUB_OUTPUT"
       - name: Get changed files
         id: changed-files
         uses: tj-actions/changed-files@v45
         with:
-          base_sha: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).base.sha }}
+          base_sha: ${{ steps.calculate-merge-base.outputs.merge-base }}
+          sha: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).head.sha }}
           files_yaml: |
             cpp:
               - '**'