bors-ng: check linear git history before building

This blocks pull requests if they contain merge commits.

Author: cryslith

.circleci/config.yml

@@ -1,5 +1,13 @@
 version: 2.1
 jobs:
+  check-linear:
+    machine:
+      image: ubuntu-1604:201903-01
+    steps:
+      - checkout
+      - run:
+          name: Check git history of branch is linear
+          command: tools/check-linear.sh
   build:
     machine:
       image: ubuntu-1604:201903-01
@@ -34,8 +42,15 @@ jobs:
           command: echo "bazel build //upload --verbose_failures" | HOMEWORLD_CHROOT="$HOME/autobuild-chroot" USER="circleci" ./build-chroot/enter-ci.sh
 workflows:
   version: 2
-  build:
+  workflow:
     jobs:
+      - check-linear:
+          filters:
+            branches:
+              ignore:
+                - staging
+                - trying
+                - master
       - build:
           filters:
             branches:

bors.toml

@@ -2,6 +2,9 @@ status = [
   "ci/circleci: build",
   "continuous-integration/jenkins/branch",
 ]
+pr_status = [
+  "ci/circleci: check-linear",
+]
 required_approvals = 1
 timeout_sec = 10800 # three hour timeout
 cut_body_after = "---"

tools/check-linear.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -euo pipefail
+
+if [ 0 = "$(git rev-list --min-parents=2 --count "$(git merge-base origin/master HEAD)"..HEAD)" ]
+then
+    echo 'git history is linear'
+else
+    echo 'error: nonlinear branch git history'
+    echo 'merge commits:'
+    git rev-list --min-parents=2 "$(git merge-base origin/master HEAD)"..HEAD
+    exit 1
+fi