Add example test for ErrorHandler (#16)

I was writing docs on how to handle errors and panics in Go and realized
that we didn't have an example demonstrating the use of `ErrorHandler`
that I could copy/link to. Here, add one in.

example_error_handler_test.go

@@ -0,0 +1,117 @@
+package river_test
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/riverqueue/river"
+	"github.com/riverqueue/river/internal/riverinternaltest"
+	"github.com/riverqueue/river/internal/util/slogutil"
+	"github.com/riverqueue/river/riverdriver/riverpgxv5"
+)
+
+type CustomErrorHandler struct{}
+
+func (*CustomErrorHandler) HandleError(ctx context.Context, job *river.JobRow, err error) *river.ErrorHandlerResult {
+	fmt.Printf("Job errored with: %s\n", err)
+	return nil
+}
+
+func (*CustomErrorHandler) HandlePanic(ctx context.Context, job *river.JobRow, panicVal any) *river.ErrorHandlerResult {
+	fmt.Printf("Job panicked with: %v\n", panicVal)
+
+	// Either function can also set the job to be immediately cancelled.
+	return &river.ErrorHandlerResult{SetCancelled: true}
+}
+
+type ErroringArgs struct {
+	ShouldError bool
+	ShouldPanic bool
+}
+
+func (ErroringArgs) Kind() string { return "erroring" }
+
+// Here to make sure our jobs are never accidentally retried which would add
+// additional output and fail the example.
+func (ErroringArgs) InsertOpts() river.InsertOpts {
+	return river.InsertOpts{MaxAttempts: 1}
+}
+
+type ErroringWorker struct {
+	river.WorkerDefaults[ErroringArgs]
+}
+
+func (w *ErroringWorker) Work(ctx context.Context, j *river.Job[ErroringArgs]) error {
+	switch {
+	case j.Args.ShouldError:
+		return fmt.Errorf("this job errored")
+	case j.Args.ShouldPanic:
+		panic("this job panicked")
+	}
+	return nil
+}
+
+// Example_errorHandler demonstrates how to use the ErrorHandler interface for
+// custom application telemetry.
+func Example_errorHandler() {
+	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, &ErroringWorker{})
+
+	riverClient, err := river.NewClient(riverpgxv5.New(dbPool), &river.Config{
+		ErrorHandler: &CustomErrorHandler{},
+		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: 10},
+		},
+		Workers: workers,
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	// Not strictly needed, but used to help this test wait until job is worked.
+	subscribeChan, subscribeCancel := riverClient.Subscribe(river.EventKindJobCancelled, river.EventKindJobFailed)
+	defer subscribeCancel()
+
+	if err := riverClient.Start(ctx); err != nil {
+		panic(err)
+	}
+
+	if _, err = riverClient.Insert(ctx, ErroringArgs{ShouldError: true}, nil); err != nil {
+		panic(err)
+	}
+
+	// Wait for the first job before inserting another to guarantee test output
+	// is ordered correctly.
+	waitForNJobs(subscribeChan, 1)
+
+	if _, err = riverClient.Insert(ctx, ErroringArgs{ShouldPanic: true}, nil); err != nil {
+		panic(err)
+	}
+
+	waitForNJobs(subscribeChan, 1)
+
+	if err := riverClient.Stop(ctx); err != nil {
+		panic(err)
+	}
+
+	// Output:
+	// Job errored with: this job errored
+	// Job panicked with: this job panicked
+}