buildkite: Add dynamic pipeline generator for conditional skipping

Use Buildkite's dynamic pipelines feature to dynamically choose between
Code/Code-skip pipelines depending on whether the build was triggered
for a pull request and the list of changes files.

.buildkite/code-skip.pipeline.yml

@@ -0,0 +1,10 @@
+##
+# Code-skip pipeline
+##
+#
+# Buildkite pipeline for skipping running code-related linters and tests.
+#
+
+steps:
+  - label: No code-related changes detected, skipping Code pipeline
+    command: exit 0

.buildkite/pipeline.yml → .buildkite/code.pipeline.yml

@@ -1,6 +1,10 @@
-################
-# Build pipeline
-################
+##
+# Code pipeline
+##
+#
+# Main Buildkite pipeline for running code-related linters and tests.
+#
+
 docker_plugin_default_config: &docker_plugin_default_config
   image: "oasislabs/testing:0.3.0"
   always_pull: true

.buildkite/pipeline.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+##
+# Dynamic Buildkite pipeline generator.
+##
+#
+# It outputs valid Buildkite pipeline in YAML format.
+#
+# To use it, define the following Steps under your Buildkite's Pipeline Settings:
+#
+# steps:
+#   - command: .buildkite/pipeline.sh | buildkite-agent pipeline upload
+#     label: ":pipeline: Upload"
+#
+# For more details, see:
+# https://buildkite.com/docs/pipelines/defining-steps#dynamic-pipelines.
+#
+
+set -eux
+
+# Helper that ensures the build is triggered for a pull request and that there
+# are no code-related changes compared to the pull request's base branch.
+pr_and_no_code_related_changes() {
+  # Check if the build was triggered for a pull request.
+  if [[ -z $BUILDKITE_PULL_REQUEST_BASE_BRANCH ]]; then
+    return 1
+  fi
+  # Get the list of changes files, excluding changes unrelated to code.
+  # NOTE: The exclude patterns below use git's pathspec syntax:
+  # https://git-scm.com/docs/gitglossary#Documentation/gitglossary.txt-aiddefpathspecapathspec.
+  git diff --name-only --exit-code "refs/remotes/origin/$BUILDKITE_PULL_REQUEST_BASE_BRANCH.." 1>&2 -- \
+      ':(exclude)*.md' \
+      ':(exclude).changelog/' \
+      ':(exclude).github/' \
+      ':(exclude).gitlint' \
+      ':(exclude).markdownlint.yml' \
+      ':(exclude).punch_config.py' \
+      ':(exclude)docs/' \
+      ':(exclude)towncrier.toml'
+}
+
+if pr_and_no_code_related_changes; then
+    cat .buildkite/code-skip.pipeline.yml
+else
+    cat .buildkite/code.pipeline.yml
+fi

.changelog/2702.internal.md

@@ -0,0 +1,6 @@
+ci: Skip some steps for non-code changes
+
+When one makes a pull request that e.g. only adds documentation or
+assembles the Change Log from fragments, all the *heavy* Buildkite
+pipeline steps (e.g. Go/Rust building, Go tests, E2E tests) should be
+skipped.