add test binary, observability

Signed-off-by: Jason Hall <imjasonh@gmail.com>

cmd/gcscat/main.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"sync"
+	"time"
+
+	"cloud.google.com/go/storage"
+	buf "github.com/imjasonh/gcsbuf"
+)
+
+func main() {
+	if len(os.Args) != 3 {
+		fmt.Fprintf(os.Stderr, "usage: gcscat <bucket> <object>\n")
+		os.Exit(1)
+	}
+	bucket, object := os.Args[1], os.Args[2]
+
+	ctx := context.Background()
+	client, err := storage.NewClient(ctx)
+	if err != nil {
+		slog.Error("creating storage client", "err", err)
+		os.Exit(1)
+	}
+	defer client.Close()
+
+	onOp := buf.OnOp(func(op string, d time.Duration, err error) {
+		if err != nil {
+			slog.Error("gcs", "op", op, "dur", d, "err", err)
+		} else {
+			slog.Info("gcs", "op", op, "dur", d)
+		}
+	})
+
+	w := buf.NewWriter(ctx, buf.WriterOptions{
+		Client: client,
+		Bucket: bucket,
+		Object: object,
+		OnOp:   onOp,
+	})
+
+	r := buf.NewReader(ctx, buf.ReaderOptions{
+		Client: client,
+		Bucket: bucket,
+		Object: object,
+		OnOp:   onOp,
+	})
+
+	// Read concurrently, printing to stdout.
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		if _, err := io.Copy(os.Stdout, r); err != nil {
+			slog.Error("read", "err", err)
+		}
+	})
+
+	// Write stdin to GCS.
+	if _, err := io.Copy(w, os.Stdin); err != nil {
+		slog.Error("write", "err", err)
+		os.Exit(1)
+	}
+	if err := w.Close(); err != nil {
+		slog.Error("close writer", "err", err)
+		os.Exit(1)
+	}
+
+	wg.Wait()
+}