feat(besadii): Adapt into a Gerrit ref-updated-hook r/931

Besadii was previously invoked as a git post-update hook, but Gerrit
does not use these hooks and instead has its own concept of hooks.

This change adapts besadii to be compatible with the way Gerrit hooks
are invoked (arguments being passed as flags, rather than via stdin).

Change-Id: I487b3a9e15810583bc5442fdc024ee2771c580cb

2 files changed, 60 insertions, 65 deletions

diff --git a/ops/besadii/default.nix b/ops/besadii/default.nix
index 31f2705d7327..fc7b2286c7ee 100644
--- a/ops/besadii/default.nix
+++ b/ops/besadii/default.nix
@@ -5,8 +5,4 @@
 depot.buildGo.program {
   name = "besadii";
   srcs = [ ./main.go ];
-
-  x_defs = {
-    "main.gitBin" = "${depot.third_party.git}/bin/git";
-  };
 }
diff --git a/ops/besadii/main.go b/ops/besadii/main.go
index b035dc0a39bc..43059565242e 100644
--- a/ops/besadii/main.go
+++ b/ops/besadii/main.go
@@ -1,40 +1,35 @@
-// Copyright 2019 Google LLC.
+// Copyright 2019-2020 Google LLC.
 // SPDX-License-Identifier: Apache-2.0
 //
 // besadii is a small CLI tool that triggers depot builds on
 // builds.sr.ht
 //
-// It is designed to run as a post-update git hook on the server
-// hosting the depot.
+// It is designed to run as a Gerrit hook (ref-updated).
 package main
 
 import (
-	"bufio"
 	"bytes"
 	"encoding/json"
+	"flag"
 	"fmt"
 	"io/ioutil"
 	"log/syslog"
 	"net/http"
 	"os"
-	"os/exec"
+	"path"
 	"strings"
 )
 
-var gitBin = "git"
 var branchPrefix = "refs/heads/"
 
-// This value is set by the git hook invocation when a branch is
-// removed, builds should not be triggered in that case.
-var deletedBranch = "0000000000000000000000000000000000000000"
-
-// Represents an updated reference as passed to besadii by git
+// Represents an updated branch, as passed to besadii by Gerrit.
 //
-// https://git-scm.com/docs/githooks#pre-receive
-type refUpdate struct {
-	name string
-	old  string
-	new  string
+// https://gerrit.googlesource.com/plugins/hooks/+/HEAD/src/main/resources/Documentation/hooks.md#ref_updated
+type branchUpdate struct {
+	project   string
+	branch    string
+	commit    string
+	submitter string
 }
 
 // Represents a builds.sr.ht build object as described on
@@ -45,8 +40,8 @@ type Build struct {
 	Tags     []string `json:"tags"`
 }
 
-// Represents a build trigger object as described on <the docs for
-// this are currently down>
+// Represents a build trigger object as described on
+// https://man.sr.ht/builds.sr.ht/triggers.md
 type Trigger struct {
 	Action    string `json:"action"`
 	Condition string `json:"condition"`
@@ -96,14 +91,14 @@ cat built-paths | cachix push tazjin`},
 }
 
 // Trigger a build of a given branch & commit on builds.sr.ht
-func triggerBuild(log *syslog.Writer, token, branch, commit string) {
+func triggerBuild(log *syslog.Writer, token string, update *branchUpdate) {
 	build := Build{
-		Manifest: prepareManifest(commit),
-		Note:     fmt.Sprintf("Build of '%s' at '%s'", branch, commit),
+		Manifest: prepareManifest(update.commit),
+		Note:     fmt.Sprintf("build of %q at %q, submitted by %q", update.branch, update.commit, update.submitter),
 		Tags: []string{
 			// my branch names tend to contain slashes, which are not valid
 			// identifiers in sourcehut.
-			"depot", strings.ReplaceAll(branch, "/", "_"),
+			"depot", strings.ReplaceAll(update.branch, "/", "_"),
 		},
 	}
 
@@ -132,39 +127,52 @@ func triggerBuild(log *syslog.Writer, token, branch, commit string) {
 		respBody, _ := ioutil.ReadAll(resp.Body)
 		log.Err(fmt.Sprintf("received non-success response from builds.sr.ht: %s (%v)", respBody, resp.Status))
 	} else {
-		fmt.Fprintf(log, "triggered builds.sr.ht job for branch '%s' at commit '%s'", branch, commit)
+		fmt.Fprintf(log, "triggered builds.sr.ht job for branch %q at commit %q", update.branch, update.commit)
 	}
 }
 
-func parseRefUpdates() ([]refUpdate, error) {
-	var updates []refUpdate
-
-	scanner := bufio.NewScanner(os.Stdin)
-	for scanner.Scan() {
-		line := scanner.Text()
-		fragments := strings.Split(line, " ")
-
-		if len(fragments) != 3 {
-			return nil, fmt.Errorf("invalid ref update: '%s'", line)
-		}
+func branchUpdateFromFlags() (*branchUpdate, error) {
+	if path.Base(os.Args[0]) != "ref-updated" {
+		return nil, fmt.Errorf("besadii must be invoked as the 'ref-updated' hook")
+	}
 
-		update := refUpdate{
-			old:  fragments[0],
-			new:  fragments[1],
-			name: fragments[2],
-		}
+	var update branchUpdate
+
+	flag.StringVar(&update.project, "project", "", "Gerrit project")
+	flag.StringVar(&update.commit, "newrev", "", "new revision")
+	flag.StringVar(&update.submitter, "submitter-username", "", "Submitter username")
+	ref := flag.String("refname", "", "updated reference name")
+
+	// Gerrit passes more flags than we want, but Rob Pike decided[0] in
+	// 2013 that the Go art project will not allow users to ignore flags
+	// because he "doesn't like it". The following code ignores the
+	// flags.
+	//
+	// [0]: https://github.com/golang/go/issues/6112#issuecomment-66083768
+	var _old, _submitter string
+	flag.StringVar(&_old, "oldrev", "", "")
+	flag.StringVar(&_submitter, "submitter", "", "")
+
+	flag.Parse()
+
+	if update.project == "" || *ref == "" || update.commit == "" || update.submitter == "" {
+		// If we get here, the user is probably being a dummy and invoking
+		// this manually - but incorrectly.
+		return nil, fmt.Errorf("'ref-updated' hook invoked without required arguments")
+	}
 
-		if strings.HasPrefix(update.name, branchPrefix) && update.new != deletedBranch {
-			update.name = strings.TrimPrefix(update.name, branchPrefix)
-			updates = append(updates, update)
-		}
+	if update.project != "depot" {
+		// this is not an error, but also not something we handle.
+		return nil, nil
 	}
 
-	if err := scanner.Err(); err != nil {
-		return nil, err
+	if !strings.HasPrefix(*ref, branchPrefix) {
+		return nil, fmt.Errorf("besadii only supports branch updates at the moment")
 	}
 
-	return updates, nil
+	update.branch = strings.TrimPrefix(*ref, branchPrefix)
+
+	return &update, nil
 }
 
 func main() {
@@ -174,30 +182,21 @@ func main() {
 		os.Exit(1)
 	}
 
-	// Before triggering builds, it is important that git
-	// update-server-info is run so that cgit correctly serves the
-	// repository.
-	err = exec.Command(gitBin, "update-server-info").Run()
-	if err != nil {
-		log.Alert("failed to run 'git update-server-info' for depot!")
-		os.Exit(1)
-	}
-
 	token, err := ioutil.ReadFile("/etc/secrets/srht-token")
 	if err != nil {
 		log.Alert("sourcehot token could not be read")
 		os.Exit(1)
 	}
 
-	updates, err := parseRefUpdates()
+	update, err := branchUpdateFromFlags()
 	if err != nil {
-		log.Err(fmt.Sprintf("could not parse updated refs:", err))
+		log.Err(fmt.Sprintf("failed to parse ref update: %s", err))
 		os.Exit(1)
 	}
 
-	fmt.Fprintf(log, "triggering builds for %v refs", len(updates))
-
-	for _, update := range updates {
-		triggerBuild(log, string(token), update.name, update.new)
+	if update == nil { // the project was not 'depot'
+		os.Exit(0)
 	}
+
+	triggerBuild(log, string(token), update)
 }