pkg/stash: Add GCS implementation (#1124)

* pkg/stash: Add GCS implementation

* fix docs

pkg/stash/with_gcs_test.go

@@ -0,0 +1,127 @@
+package stash
+
+import (
+	"bytes"
+	"context"
+	"io/ioutil"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gomods/athens/pkg/config"
+	"github.com/gomods/athens/pkg/errors"
+	"github.com/gomods/athens/pkg/storage"
+	"github.com/gomods/athens/pkg/storage/gcp"
+	"github.com/google/uuid"
+	"golang.org/x/sync/errgroup"
+)
+
+// TestWithGCS requires a real GCP backend implementation
+// and it will ensure that saving to modules at the same time
+// is done synchronously so that only the first module gets saved.
+func TestWithGCS(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute*5)
+	defer cancel()
+	const (
+		mod = "stashmod"
+		ver = "v1.0.0"
+	)
+	strg := getStorage(t)
+	strg.Delete(ctx, mod, ver)
+	defer strg.Delete(ctx, mod, ver)
+
+	// sanity check
+	_, err := strg.GoMod(ctx, mod, ver)
+	if !errors.Is(err, errors.KindNotFound) {
+		t.Fatalf("expected the stash bucket to return a NotFound error but got: %v", err)
+	}
+
+	var eg errgroup.Group
+	for i := 0; i < 5; i++ {
+		content := uuid.New().String()
+		ms := &mockGCPStasher{strg, content}
+		s := WithGCSLock(ms)
+		eg.Go(func() error {
+			ctx, cancel := context.WithTimeout(context.Background(), time.Second*10)
+			defer cancel()
+			_, err := s.Stash(ctx, "stashmod", "v1.0.0")
+			return err
+		})
+	}
+
+	err = eg.Wait()
+	if err != nil {
+		t.Fatal(err)
+	}
+	info, err := strg.Info(ctx, mod, ver)
+	if err != nil {
+		t.Fatal(err)
+	}
+	modContent, err := strg.GoMod(ctx, mod, ver)
+	if err != nil {
+		t.Fatal(err)
+	}
+	zip, err := strg.Zip(ctx, mod, ver)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer zip.Close()
+	zipContent, err := ioutil.ReadAll(zip)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(info, modContent) {
+		t.Fatalf("expected info and go.mod to be equal but info was {%v} and content was {%v}", string(info), string(modContent))
+	}
+	if !bytes.Equal(info, zipContent) {
+		t.Fatalf("expected info and zip to be equal but info was {%v} and content was {%v}", string(info), string(zipContent))
+	}
+}
+
+// mockGCPStasher is like mockStasher
+// but leverages in memory storage
+// so that redis can determine
+// whether to call the underlying stasher or not.
+type mockGCPStasher struct {
+	strg    storage.Backend
+	content string
+}
+
+func (ms *mockGCPStasher) Stash(ctx context.Context, mod, ver string) (string, error) {
+	err := ms.strg.Save(
+		ctx,
+		mod,
+		ver,
+		[]byte(ms.content),
+		strings.NewReader(ms.content),
+		[]byte(ms.content),
+	)
+	return "", err
+}
+
+func getStorage(t *testing.T) *gcp.Storage {
+	t.Helper()
+	cfg := getTestConfig()
+	if cfg == nil {
+		t.SkipNow()
+	}
+
+	s, err := gcp.New(context.Background(), cfg, config.GetTimeoutDuration(30))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return s
+}
+
+func getTestConfig() *config.GCPConfig {
+	creds := os.Getenv("GCS_SERVICE_ACCOUNT")
+	if creds == "" {
+		return nil
+	}
+	return &config.GCPConfig{
+		Bucket:  "athens_drone_stash_bucket",
+		JSONKey: creds,
+	}
+}