From 12181e74c4b612af6a989e2c98dc593be7d1263a Mon Sep 17 00:00:00 2001
From: vthorsteinsson <vthorsteinsson@users.noreply.github.com>
Date: Fri, 23 Oct 2026 12:14:51 +0000
Subject: [PATCH] CI: skip the master push run for already-tested PR merges

A merged PR was tested twice: once by its pull_request run and again
by the push run on master, 8 jobs each time. A new gate job skips the
test matrix on a master push when the pushed commit is a merge whose
first parent (master's previous head) is an ancestor of the second
parent (the PR head) and the PR head has a successful pull_request
run. The merge commit then has exactly the tree that the PR run
tested. Direct pushes, squash merges and PRs that were behind master
still get the full run.

Also cancel a superseded PR run when new commits are pushed (master
runs are never cancelled), and skip CI for Markdown-only changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .github/workflows/python-package.yml | 45 ++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/.github/workflows/python-package.yml b/.github/workflows/python-package.yml
index a6d6aa7..f5f69dd 100644
--- a/.github/workflows/python-package.yml
+++ b/.github/workflows/python-package.yml
@@ -6,12 +6,57 @@ on:
   # events fire for each push to a branch with an open PR.)
   push:
     branches: [ master ]
+    paths-ignore: [ "**.md" ]
   pull_request:
     # Note: "**" is required to match branch names containing '/'
     branches: [ "**" ]
+    paths-ignore: [ "**.md" ]
+
+# A new push to a PR cancels its run in progress; master runs are never cancelled
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
+  gate:
+    # Skip the push run on master for a PR merge commit whose tree was
+    # already tested by a successful pull_request run. That holds when
+    # master's previous head (first parent) is an ancestor of the PR head
+    # (second parent): the PR run then tested a merge with an even older
+    # master commit, which also yields the PR head's tree, and so does
+    # the merge commit. Direct pushes, squash merges and PRs that were
+    # behind master get the full run.
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+    outputs:
+      skip: ${{ steps.check.outputs.skip }}
+    steps:
+    - id: check
+      env:
+        GH_TOKEN: ${{ github.token }}
+        REPO: ${{ github.repository }}
+        SHA: ${{ github.sha }}
+        EVENT: ${{ github.event_name }}
+      run: |
+        skip=false
+        if [ "$EVENT" = push ]; then
+          set -- $(gh api "repos/$REPO/commits/$SHA" --jq '[.parents[].sha] | join(" ")')
+          if [ $# -eq 2 ]; then
+            status=$(gh api "repos/$REPO/compare/$1...$2" --jq .status)
+            passed=$(gh api "repos/$REPO/actions/workflows/python-package.yml/runs?event=pull_request&head_sha=$2&status=success" --jq .total_count)
+            echo "parents: $1 $2; compare: $status; successful PR runs: $passed"
+            if { [ "$status" = ahead ] || [ "$status" = identical ]; } && [ "$passed" -gt 0 ]; then
+              skip=true
+            fi
+          fi
+        fi
+        echo "skip=$skip" | tee -a "$GITHUB_OUTPUT"
+
   build:
+    needs: gate
+    if: needs.gate.outputs.skip != 'true'
 
     runs-on: ${{ matrix.os }}
     strategy: