Athens: introduce Single Flight (#573)

* Athens: introduce singleflight

* fix ci

* small fixes

* Remove unnecessary method

* simplify folder structure

* dcoument stasher iface

* fixes

* Fix olympus

* Remove Version from Download Protocol

* fix olympus

* refactor fetcher

* consolodate wrapping styles

* fix olympus

* Further refactoring

* fix typo

pkg/stash/stasher.go

@@ -0,0 +1,62 @@
+package stash
+
+import (
+	"context"
+	"time"
+
+	"github.com/gomods/athens/pkg/errors"
+	"github.com/gomods/athens/pkg/module"
+	"github.com/gomods/athens/pkg/storage"
+)
+
+// Stasher has the job of taking a module
+// from an upstream entity and stashing it to a Storage Backend.
+type Stasher interface {
+	Stash(string, string) error
+}
+
+// Wrapper helps extend the main stasher's functionality with addons.
+type Wrapper func(Stasher) Stasher
+
+// New returns a plain stasher that takes
+// a module from a download.Protocol and
+// stashes it into a backend.Storage.
+func New(f module.Fetcher, s storage.Backend, wrappers ...Wrapper) Stasher {
+	var st Stasher = &stasher{f, s}
+	for _, w := range wrappers {
+		st = w(st)
+	}
+
+	return st
+}
+
+type stasher struct {
+	f module.Fetcher
+	s storage.Backend
+}
+
+func (s *stasher) Stash(mod, ver string) error {
+	const op errors.Op = "stasher.Stash"
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute*10)
+	defer cancel()
+	v, err := s.fetchModule(ctx, mod, ver)
+	if err != nil {
+		return errors.E(op, err)
+	}
+	defer v.Zip.Close()
+	err = s.s.Save(ctx, mod, ver, v.Mod, v.Zip, v.Info)
+	if err != nil {
+		return errors.E(op, err)
+	}
+	return nil
+}
+
+func (s *stasher) fetchModule(ctx context.Context, mod, ver string) (*storage.Version, error) {
+	const op errors.Op = "stasher.fetchModule"
+	v, err := s.f.Fetch(ctx, mod, ver)
+	if err != nil {
+		return nil, errors.E(op, err)
+	}
+
+	return v, nil
+}