Add example for using subscriptions

This one's a little odd because basically all the other examples already
use a subscription to wait for jobs to finish, but I figure that
subscriptions should have their own distinct example showing their
various idiosyncrasies

example_subscription_test.go

@@ -0,0 +1,137 @@
+package river_test
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/riverqueue/river"
+	"github.com/riverqueue/river/internal/rivercommon"
+	"github.com/riverqueue/river/internal/riverinternaltest"
+	"github.com/riverqueue/river/internal/util/slogutil"
+	"github.com/riverqueue/river/riverdriver/riverpgxv5"
+)
+
+type SubscriptionArgs struct {
+	Cancel bool `json:"cancel"`
+	Fail   bool `json:"fail"`
+}
+
+func (SubscriptionArgs) Kind() string { return "subscription" }
+
+type SubscriptionWorker struct {
+	river.WorkerDefaults[SubscriptionArgs]
+}
+
+func (w *SubscriptionWorker) Work(ctx context.Context, job *river.Job[SubscriptionArgs]) error {
+	switch {
+	case job.Args.Cancel:
+		return river.JobCancel(fmt.Errorf("cancelling job"))
+	case job.Args.Fail:
+		return fmt.Errorf("failing job")
+	}
+	return nil
+}
+
+// Example_subscription demonstrates the use of client subscriptions to receive
+// events containing information about worked jobs.
+func Example_subscription() {
+	ctx := context.Background()
+
+	dbPool, err := pgxpool.NewWithConfig(ctx, riverinternaltest.DatabaseConfig("river_testdb_example"))
+	if err != nil {
+		panic(err)
+	}
+	defer dbPool.Close()
+
+	// Required for the purpose of this test, but not necessary in real usage.
+	if err := riverinternaltest.TruncateRiverTables(ctx, dbPool); err != nil {
+		panic(err)
+	}
+
+	workers := river.NewWorkers()
+	river.AddWorker(workers, &SubscriptionWorker{})
+
+	riverClient, err := river.NewClient(riverpgxv5.New(dbPool), &river.Config{
+		Logger: slog.New(&slogutil.SlogMessageOnlyHandler{Level: 9}), // Suppress logging so example output is cleaner (9 > slog.LevelError).
+		Queues: map[string]river.QueueConfig{
+			river.DefaultQueue: {MaxWorkers: 100},
+		},
+		Workers: workers,
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	// Subscribers tell the River client the kinds of events they'd like to receive.
+	completedChan, completedSubscribeCancel := riverClient.Subscribe(river.EventKindJobCompleted)
+	defer completedSubscribeCancel()
+
+	// Multiple simultaneous subscriptions are allowed.
+	failedChan, failedSubscribeCancel := riverClient.Subscribe(river.EventKindJobFailed)
+	defer failedSubscribeCancel()
+
+	otherChan, otherSubscribeCancel := riverClient.Subscribe(river.EventKindJobCancelled, river.EventKindJobSnoozed)
+	defer otherSubscribeCancel()
+
+	if err := riverClient.Start(ctx); err != nil {
+		panic(err)
+	}
+
+	// Insert one job for each subscription above: one to succeed, one to fail,
+	// and one that's cancelled that'll arrive on the "other" channel.
+	_, err = riverClient.Insert(ctx, SubscriptionArgs{}, nil)
+	if err != nil {
+		panic(err)
+	}
+	_, err = riverClient.Insert(ctx, SubscriptionArgs{Fail: true}, nil)
+	if err != nil {
+		panic(err)
+	}
+	_, err = riverClient.Insert(ctx, SubscriptionArgs{Cancel: true}, nil)
+	if err != nil {
+		panic(err)
+	}
+
+	waitForJob := func(subscribeChan <-chan *river.Event) {
+		select {
+		case event := <-subscribeChan:
+			if event == nil {
+				fmt.Printf("Channel is closed\n")
+				return
+			}
+
+			fmt.Printf("Got job with state: %s\n", event.Job.State)
+		case <-time.After(rivercommon.WaitTimeout()):
+			panic("timed out waiting for job")
+		}
+	}
+
+	waitForJob(completedChan)
+	waitForJob(failedChan)
+	waitForJob(otherChan)
+
+	if err := riverClient.Stop(ctx); err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("Client stopped\n")
+
+	// Try waiting again, but none of these work because stopping the client
+	// closed all subscription channels automatically.
+	waitForJob(completedChan)
+	waitForJob(failedChan)
+	waitForJob(otherChan)
+
+	// Output:
+	// Got job with state: completed
+	// Got job with state: retryable
+	// Got job with state: cancelled
+	// Client stopped
+	// Channel is closed
+	// Channel is closed
+	// Channel is closed
+}