cmd/gitops-pusher: add new GitOps assistant (#4893)

This is for an upcoming blogpost on how to manage Tailscale ACLs using a
GitOps flow. This tool is intended to be used in CI and will allow users
to have a git repository be the ultimate source of truth for their ACL
file. This enables ACL changes to be proposed, approved and discussed
before they are applied.

Signed-off-by: Xe <xe@tailscale.com>

cmd/gitops-pusher/README.md

@@ -0,0 +1,48 @@
+# gitops-pusher
+
+This is a small tool to help people achieve a
+[GitOps](https://about.gitlab.com/topics/gitops/) workflow with Tailscale ACL
+changes. This tool is intended to be used in a CI flow that looks like this:
+
+```yaml
+name: Tailscale ACL syncing
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  acls:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+      
+      - name: Setup Go environment
+        uses: actions/setup-go@v3.2.0
+        
+      - name: Install gitops-pusher
+        run: go install tailscale.com/cmd/gitops-pusher@latest
+              
+      - name: Deploy ACL
+        if: github.event_name == 'push'
+        env:
+          TS_API_KEY: ${{ secrets.TS_API_KEY }}
+          TS_TAILNET: ${{ secrets.TS_TAILNET }}
+        run: |
+          ~/go/bin/gitops-pusher --policy-file ./policy.hujson apply
+
+      - name: ACL tests
+        if: github.event_name == 'pull_request'
+        env:
+          TS_API_KEY: ${{ secrets.TS_API_KEY }}
+          TS_TAILNET: ${{ secrets.TS_TAILNET }}
+        run: |
+          ~/go/bin/gitops-pusher --policy-file ./policy.hujson test
+```
+
+Change the value of the `--policy-file` flag to point to the policy file on
+disk. Policy files should be in [HuJSON](https://github.com/tailscale/hujson)
+format.

cmd/gitops-pusher/gitops-pusher.go

@@ -0,0 +1,223 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Command gitops-pusher allows users to use a GitOps flow for managing Tailscale ACLs.
+//
+// See README.md for more details.
+package main
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+)
+
+var (
+	policyFname = flag.String("policy-file", "./policy.hujson", "filename for policy file")
+	timeout     = flag.Duration("timeout", 5*time.Minute, "timeout for the entire CI run")
+)
+
+func main() {
+	flag.Parse()
+	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
+	defer cancel()
+
+	tailnet, ok := os.LookupEnv("TS_TAILNET")
+	if !ok {
+		log.Fatal("set envvar TS_TAILNET to your tailnet's name")
+	}
+	apiKey, ok := os.LookupEnv("TS_API_KEY")
+	if !ok {
+		log.Fatal("set envvar TS_API_KEY to your Tailscale API key")
+	}
+
+	switch flag.Arg(0) {
+	case "apply":
+		controlEtag, err := getACLETag(ctx, tailnet, apiKey)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		localEtag, err := sumFile(*policyFname)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		log.Printf("control: %s", controlEtag)
+		log.Printf("local:   %s", localEtag)
+
+		if controlEtag == localEtag {
+			log.Println("no update needed, doing nothing")
+			os.Exit(0)
+		}
+
+		if err := applyNewACL(ctx, tailnet, apiKey, *policyFname, controlEtag); err != nil {
+			log.Fatal(err)
+		}
+
+	case "test":
+		controlEtag, err := getACLETag(ctx, tailnet, apiKey)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		localEtag, err := sumFile(*policyFname)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		log.Printf("control: %s", controlEtag)
+		log.Printf("local:   %s", localEtag)
+
+		if controlEtag == localEtag {
+			log.Println("no updates found, doing nothing")
+			os.Exit(0)
+		}
+
+		if err := testNewACLs(ctx, tailnet, apiKey, *policyFname); err != nil {
+			log.Fatal(err)
+		}
+	default:
+		log.Fatalf("usage: %s [options] <test|apply>", os.Args[0])
+	}
+}
+
+func sumFile(fname string) (string, error) {
+	fin, err := os.Open(fname)
+	if err != nil {
+		return "", err
+	}
+	defer fin.Close()
+
+	h := sha256.New()
+	_, err = io.Copy(h, fin)
+	if err != nil {
+		return "", err
+	}
+
+	return fmt.Sprintf("\"%x\"", h.Sum(nil)), nil
+}
+
+func applyNewACL(ctx context.Context, tailnet, apiKey, policyFname, oldEtag string) error {
+	fin, err := os.Open(policyFname)
+	if err != nil {
+		return err
+	}
+	defer fin.Close()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, fmt.Sprintf("https://api.tailscale.com/api/v2/tailnet/%s/acl", tailnet), fin)
+	if err != nil {
+		return err
+	}
+
+	req.SetBasicAuth(apiKey, "")
+	req.Header.Set("Content-Type", "application/hujson")
+	req.Header.Set("If-Match", oldEtag)
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	got := resp.StatusCode
+	want := http.StatusOK
+	if got != want {
+		return fmt.Errorf("wanted HTTP status code %d but got %d", want, got)
+	}
+
+	return nil
+}
+
+func testNewACLs(ctx context.Context, tailnet, apiKey, policyFname string) error {
+	fin, err := os.Open(policyFname)
+	if err != nil {
+		return err
+	}
+	defer fin.Close()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, fmt.Sprintf("https://api.tailscale.com/api/v2/tailnet/%s/acl/validate", tailnet), fin)
+	if err != nil {
+		return err
+	}
+
+	req.SetBasicAuth(apiKey, "")
+	req.Header.Set("Content-Type", "application/hujson")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		var ate ACLTestError
+		err := json.NewDecoder(resp.Body).Decode(&ate)
+		if err != nil {
+			return err
+		}
+
+		return ate
+	}
+
+	return nil
+}
+
+type ACLTestError struct {
+	Message string               `json:"message"`
+	Data    []ACLTestErrorDetail `json:"data"`
+}
+
+func (ate ACLTestError) Error() string {
+	var sb strings.Builder
+
+	fmt.Fprintln(&sb, ate.Message)
+	fmt.Fprintln(&sb)
+
+	for _, data := range ate.Data {
+		fmt.Fprintf(&sb, "For user %s:\n", data.User)
+		for _, err := range data.Errors {
+			fmt.Fprintf(&sb, "- %s\n", err)
+		}
+	}
+
+	return sb.String()
+}
+
+type ACLTestErrorDetail struct {
+	User   string   `json:"user"`
+	Errors []string `json:"errors"`
+}
+
+func getACLETag(ctx context.Context, tailnet, apiKey string) (string, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, fmt.Sprintf("https://api.tailscale.com/api/v2/tailnet/%s/acl", tailnet), nil)
+	if err != nil {
+		return "", err
+	}
+
+	req.SetBasicAuth(apiKey, "")
+	req.Header.Set("Accept", "application/hujson")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	got := resp.StatusCode
+	want := http.StatusOK
+	if got != want {
+		return "", fmt.Errorf("wanted HTTP status code %d but got %d", want, got)
+	}
+
+	return resp.Header.Get("ETag"), nil
+}