Doing synchronous cache filling for the .info endpoint

This patch shows how we could synchronously fill in storage when the proxy or registry receive a request to '.../version.info' and the package isn't in storage.

Requires gomods#291

Part of gomods#290

pkg/download/version_info.go

@@ -8,6 +8,7 @@ import (
 	"github.com/bketelsen/buffet"
 	"github.com/gobuffalo/buffalo"
 	"github.com/gobuffalo/buffalo/render"
+	"github.com/gomods/athens/pkg/module"
 	"github.com/gomods/athens/pkg/paths"
 	"github.com/gomods/athens/pkg/storage"
 )
@@ -16,26 +17,48 @@ import (
 const PathVersionInfo = "/{module:.+}/@v/{version}.info"
 
 // VersionInfoHandler implements GET baseURL/module/@v/version.info
-func VersionInfoHandler(getter storage.Getter, eng *render.Engine) func(c buffalo.Context) error {
+func VersionInfoHandler(
+	getterSaver storage.GetterSaver,
+	fetcher module.Fetcher,
+	eng *render.Engine,
+) func(c buffalo.Context) error {
 	return func(c buffalo.Context) error {
 		sp := buffet.SpanFromContext(c)
 		sp.SetOperationName("versionInfoHandler")
 		params, err := paths.GetAllParams(c)
 		if err != nil {
 			return err
 		}
-		version, err := getter.Get(params.Module, params.Version)
+		version, err := getterSaver.Get(params.Module, params.Version)
 		if storage.IsNotFoundError(err) {
-			msg := fmt.Sprintf("%s@%s not found", params.Module, params.Version)
-			return c.Render(http.StatusNotFound, eng.JSON(msg))
+			// TODO: serialize cache fills (https://github.com/gomods/athens/issues/308)
+			ref, err := fetcher.Fetch(params.Module, params.Version)
+			if err != nil {
+				// TODO: some way to figure out whether the package actually doesn't exist
+				return err
+			}
+			defer ref.Clear()
+			ver, err := ref.Read()
+			if err != nil {
+				return err
+			}
+			getterSaver.Save(c, params.Module, params.Version, ver.Mod, ver.Zip, ver.Info)
+			version = ver
 		} else if err != nil {
 			return err
 		}
-		var revInfo storage.RevInfo
-		err = json.Unmarshal(version.Info, &revInfo)
+		revInfo, err := parseRevInfo(version.Info)
 		if err != nil {
 			return err
 		}
 		return c.Render(http.StatusOK, eng.JSON(revInfo))
 	}
 }
+
+func parseRevInfo(b []byte) (*storage.RevInfo, error) {
+	revInfo := &storage.RevInfo{}
+	if err := json.Unmarshal(b, revInfo); err != nil {
+		return nil, err
+	}
+	return revInfo, nil
+}

pkg/storage/getter_saver.go

@@ -0,0 +1,7 @@
+package storage
+
+// GetterSaver is a Getter and a Saver in one
+type GetterSaver interface {
+	Getter
+	Saver
+}