From cf8533e744229fe5b12a21063d62c3e1944c619d Mon Sep 17 00:00:00 2001
From: hauntsaninja <>
Date: Fri, 26 Nov 2021 15:45:30 -0800
Subject: [PATCH] [mypy_primer] use merge base as base commit

Solves https://github.com/hauntsaninja/mypy_primer/issues/21
---
 .github/workflows/mypy_primer.yml | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/.github/workflows/mypy_primer.yml b/.github/workflows/mypy_primer.yml
index 769cd57e9caa2..fcd8b938f2959 100644
--- a/.github/workflows/mypy_primer.yml
+++ b/.github/workflows/mypy_primer.yml
@@ -37,16 +37,19 @@ jobs:
           cd mypy_to_test
           echo "new commit"
           git rev-list --format=%s --max-count=1 $GITHUB_SHA
-          git checkout -b upstream_master origin/master
+
+          MERGE_BASE=$(git merge-base $GITHUB_SHA origin/master)
+          git checkout -b base_commit $MERGE_BASE
           echo "base commit"
-          git rev-list --format=%s --max-count=1 upstream_master
+          git rev-list --format=%s --max-count=1 base_commit
+
           echo ''
           cd ..
           # fail action if exit code isn't zero or one
           (
             mypy_primer \
             --repo mypy_to_test \
-            --new $GITHUB_SHA --old upstream_master \
+            --new $GITHUB_SHA --old base_commit \
             --num-shards 3 --shard-index ${{ matrix.shard-index }} \
             --debug \
             --output concise \