Prepare repo for use (#4)

Prepare the repo for use by:

* Add .go files currently in github.com/elastic/quark
* Adding license and notice files
* Set up submodule with github.com/elastic/quark
* Setting up build scripts

Note on the build system:
To build a usable Go module, which doesn't require users to compile C code before themselves, compiled libquark artifacts are included in the published Go module. These files must be compiled by CI machines. In order to enforce this, any commits with modified .a files will have an automated commit overwrite these changes with files compiled by CI machines, so any merged PRs only contain .a files created by build machines.

.buildkite/default-pipeline.yml

@@ -1,14 +1,31 @@
 ---
 # $yaml-language-server: $schema=https://raw.githubusercontent.com/buildkite/pipeline-schema/main/schema.json
+
 steps:
   - label: "Build"
-    command: "true"
-
-  - label: "Test"
-    command: "true"
+    key: build
+    command: "./.buildkite/scripts/build.sh"
+    artifact_paths:
+      - "libquark_big_amd64.a"
+      - "libquark_big_arm64.a"
+    agents:
+      image: family/core-ubuntu-2004
+      provider: gcp
+      machineType: n2-standard-2
 
-  - label: "Deploy"
-    command: "true"
+  - label: "Commit"
+    depends_on: build
+    key: commit
+    command: "./.buildkite/scripts/commit.sh"
+    agents:
+     image: "docker.elastic.co/ci-agent-images/sec-linux-platform/cloud-defend-builder@sha256:623a0d998dd6ebde09c7b7bbb829e3ba95b5475a0702ab35a5c6049775226f37"
+     cpu: "1"
+     memory: "1Gi"
 
-  - label: "Publish"
-    command: "true"
+  - label: "Check NOTICE.txt"
+    key: check-notice
+    command: "make check-notice"
+    agents:
+     image: "docker.elastic.co/ci-agent-images/sec-linux-platform/cloud-defend-builder@sha256:623a0d998dd6ebde09c7b7bbb829e3ba95b5475a0702ab35a5c6049775226f37"
+     cpu: "1"
+     memory: "1Gi"

.buildkite/scripts/build.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+set -euo pipefail
+
+# Build libquark_big.a for all supported architectures
+
+git submodule update --init --recursive
+
+for ARCH in amd64
+do
+	ARCH=$ARCH make -C src centos7
+	mv src/libquark_big.a libquark_big_${ARCH}.a
+done

.buildkite/scripts/commit.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+
+set -euo pipefail
+
+# Commit modified .a files to the same branch that was built
+
+BOT_NAME=${BOT_NAME:-"buildkite-bot"}
+BOT_EMAIL=${BOT_EMAIL:-"buildkite-bot@noreply.elastic.co"}
+
+function download {
+	buildkite-agent artifact download "$1" "$2"
+}
+
+for ARCH in amd64; do
+	download libquark_big_${ARCH}.a .
+done
+
+# If there are no changes, don't commit
+if ! git diff --name-only HEAD | grep -q -E '\.a$'; then
+	echo "No changes detected"
+	exit 0
+fi
+
+# Don't commit if the last commit is from the bot, to avoid infinite loop of builds
+if test "$(git log -1 --pretty=format:'%ae')" = "${BOT_EMAIL}"; then
+	echo "last commit from bot"
+	exit 0
+fi
+
+if test -z "${BUILDKITE}"; then
+	echo "This script doesn't appear to be running in buildkite; refusing to commit"
+	exit 1
+fi
+
+git config --global user.name "${BOT_NAME}"
+git config --global user.email "${BOT_EMAIL}"
+
+git config --global credential.https://git.luolix.top.username token
+git config --global credential.https://git.luolix.top.helper '!echo \"password=\$(cat /run/secrets/VAULT_GITHUB_TOKEN)\";'
+
+git add libquark_big_amd64.a
+
+git commit -m "Auto-update .a files by Buildkite"
+
+git push origin HEAD:"${BUILDKITE_BRANCH}"

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "quark"]
+	path = src
+	url = https://github.com/elastic/quark.git

LICENSE.txt

@@ -0,0 +1,2 @@
+Source code in this repository is licensed under the Apache License Version 2.0,
+an Apache compatible license.

Makefile

@@ -0,0 +1,18 @@
+.PHONY: notice
+notice:
+	@echo "Generating NOTICE"
+	go mod tidy
+	go mod download
+	go list -m -json all | go run go.elastic.co/go-licence-detector \
+		-includeIndirect \
+		-rules tools/notice/rules.json \
+		-overrides tools/notice/overrides.json \
+		-noticeTemplate tools/notice/NOTICE.txt.tmpl \
+		-noticeOut NOTICE.txt \
+		-depsOut ""
+
+.PHONY: check-notice
+check-notice: notice
+	@if git diff --name-only | grep "NOTICE.txt"; then \
+		echo "NOTICE.txt differs from committed version; regenerate and commit."; \
+	fi