internal/labels: Labeler, constructor and config

First of several CLs implementing Labeler, akin to related.Poster.

For #64.

Change-Id: I621b882cf6a779f8e81042e6d5654c0b2a51c8fc
Reviewed-on: https://go-review.googlesource.com/c/oscar/+/637096
Reviewed-by: Tatiana Bradley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>

internal/labels/labeler.go

@@ -0,0 +1,103 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package labels
+
+import (
+	"context"
+	"errors"
+	"log/slog"
+	"time"
+
+	"golang.org/x/oscar/internal/actions"
+	"golang.org/x/oscar/internal/github"
+	"golang.org/x/oscar/internal/storage"
+	"golang.org/x/oscar/internal/storage/timed"
+)
+
+// A Labeler labels GitHub issues.
+type Labeler struct {
+	slog      *slog.Logger
+	db        storage.DB
+	github    *github.Client
+	projects  map[string]bool
+	watcher   *timed.Watcher[*github.Event]
+	name      string
+	timeLimit time.Time
+	ignores   []func(*github.Issue) bool
+	label     bool
+	// For the action log.
+	requireApproval bool
+	actionKind      string
+	logAction       actions.BeforeFunc
+}
+
+// New creates and returns a new Labeler. It logs to lg, stores state in db,
+//
+//	and watches for new GitHub issues using gh.
+//
+// For the purposes of storing its own state, it uses the given name.
+// Future calls to New with the same name will use the same state.
+//
+// Use the [Labeler] methods to configure the posting parameters
+// (especially [Labeler.EnableProject] and [Labeler.EnableLabels])
+// before calling [Labeler.Run].
+func New(lg *slog.Logger, db storage.DB, gh *github.Client, name string) *Labeler {
+	l := &Labeler{
+		slog:      lg,
+		db:        db,
+		github:    gh,
+		projects:  make(map[string]bool),
+		watcher:   gh.EventWatcher("labels.Labeler:" + name),
+		name:      name,
+		timeLimit: time.Now().Add(-defaultTooOld),
+	}
+	// TODO: Perhaps the action kind should include name, but perhaps not.
+	// This makes sure we only ever label each issue once.
+	l.actionKind = "labels.Labeler"
+	l.logAction = actions.Register(l.actionKind, &actioner{l})
+	return l
+}
+
+// SetTimeLimit controls how old an issue can be for the Labeler to label it.
+// Issues created before time t will be skipped.
+// The default is not to post to issues that are more than 48 hours old
+// at the time of the call to [New].
+func (l *Labeler) SetTimeLimit(t time.Time) {
+	l.timeLimit = t
+}
+
+const defaultTooOld = 48 * time.Hour
+
+// EnableProject enables the Labeler to post on issues in the given GitHub project (for example "golang/go").
+// See also [Labeler.EnableLabels], which must also be called to post anything to GitHub.
+func (l *Labeler) EnableProject(project string) {
+	l.projects[project] = true
+}
+
+// EnableLabels enables the Labeler to label GitHub issues.
+// If EnableLabels has not been called, [Labeler.Run] logs what it would post but does not post the messages.
+// See also [Labeler.EnableProject], which must also be called to set the projects being considered.
+func (l *Labeler) EnableLabels() {
+	l.label = true
+}
+
+// RequireApproval configures the Labeler to log actions that require approval.
+func (l *Labeler) RequireApproval() {
+	l.requireApproval = true
+}
+
+type actioner struct {
+	l *Labeler
+}
+
+func (ar *actioner) Run(ctx context.Context, data []byte) ([]byte, error) {
+	// TODO: implement
+	return nil, errors.New("unimplemented")
+}
+
+func (ar *actioner) ForDisplay(data []byte) string {
+	// TODO: implement
+	return ""
+}