1 files changed, 27 insertions, 0 deletions
diff --git a/.circleci/merge_base_branch.sh b/.circleci/merge_base_branch.sh
new file mode 100755
index 0000000000..2d700dbf11
--- /dev/null
+++ b/.circleci/merge_base_branch.sh
@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+set -e
+
+# CircleCI doesn't give CIRCLE_PR_NUMBER in the environment for non-forked PRs. Wonderful.
+# In this case, we just need to do some ~shell magic~ to strip it out of the PULL_REQUEST URL.
+echo 'export CIRCLE_PR_NUMBER="${CIRCLE_PR_NUMBER:-${CIRCLE_PULL_REQUEST##*/}}"' >> "$BASH_ENV"
+source $BASH_ENV
+
+if [[ -z "${CIRCLE_PR_NUMBER}" ]]
+then
+    echo "Can't figure out what the PR number is!"
+    exit 1
+fi
+
+# Get the reference, using the GitHub API
+GITBASE=`curl -q https://api.github.com/repos/matrix-org/synapse/pulls/${CIRCLE_PR_NUMBER} | jq -r '.base.ref'`
+
+# Show what we are before
+git show -s
+
+# Fetch and merge. If it doesn't work, it will raise due to set -e.
+git fetch -u origin $GITBASE
+git merge --no-edit origin/$GITBASE
+
+# Show what we are after.
+git show -s
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