Add interfaces for Image Watcher (#902)

**What this PR does / why we need it**:

**Which issue(s) this PR fixes**:

Fixes #

**Does this PR introduce a user-facing change?**:
<!--
If no, just write "NONE" in the release-note block below.
-->
```release-note
NONE
```

This PR was merged by Kapetanios.

pkg/app/piped/containerregistry/BUILD.bazel → pkg/app/piped/imageprovider/gcr/BUILD.bazel

@@ -2,7 +2,7 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library")
 
 go_library(
     name = "go_default_library",
-    srcs = ["registry.go"],
-    importpath = "github.com/pipe-cd/pipe/pkg/app/piped/containerregistry",
+    srcs = ["gcr.go"],
+    importpath = "github.com/pipe-cd/pipe/pkg/app/piped/imageprovider/gcr",
     visibility = ["//visibility:public"],
 )

pkg/app/piped/containerregistry/registry.go → pkg/app/piped/imageprovider/gcr/gcr.go

@@ -12,6 +12,4 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Package containerregistry watches the configured container registry
-// for the new images.
-package containerregistry
+package gcr

pkg/app/piped/imagewatcher/BUILD.bazel

@@ -0,0 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["watcher.go"],
+    importpath = "github.com/pipe-cd/pipe/pkg/app/piped/imagewatcher",
+    visibility = ["//visibility:public"],
+    deps = ["@org_uber_go_zap//:go_default_library"],
+)

pkg/app/piped/imagewatcher/watcher.go

@@ -0,0 +1,80 @@
+// Copyright 2020 The PipeCD Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package imagewatcher provides a piped component
+// that periodically checks the image registry and updates
+// the image if there are differences with Git.
+package imagewatcher
+
+import (
+	"context"
+	"time"
+
+	"go.uber.org/zap"
+)
+
+type Watcher interface {
+	Run(context.Context) error
+}
+
+type watcher struct {
+	timer  *time.Timer
+	logger *zap.Logger
+}
+
+type imageRepos map[string]imageRepo
+type imageRepo struct {
+}
+
+func NewWatcher(interval time.Duration, logger *zap.Logger) Watcher {
+	return &watcher{
+		timer:  time.NewTimer(interval),
+		logger: logger,
+	}
+}
+
+func (w *watcher) Run(ctx context.Context) error {
+	for {
+		select {
+		case <-ctx.Done():
+			return nil
+		case <-w.timer.C:
+			reposInReg := w.fetchFromRegistry()
+			reposInGit := w.fetchFromGit()
+			outdated := calculateChanges(reposInReg, reposInGit)
+			if err := w.update(outdated); err != nil {
+				w.logger.Error("failed to update image", zap.Error(err))
+			}
+		}
+	}
+	return nil
+}
+
+func (w *watcher) fetchFromRegistry() imageRepos {
+	return nil
+}
+
+func (w *watcher) fetchFromGit() imageRepos {
+	return nil
+}
+
+func (w *watcher) update(targets imageRepos) error {
+	return nil
+}
+
+// calculateChanges compares between image repos in the image registry and
+// image repos in git. And then gives back image repos to be updated.
+func calculateChanges(x, y imageRepos) imageRepos {
+	return nil
+}

pkg/model/BUILD.bazel

@@ -76,7 +76,6 @@ go_test(
     data = glob(["testdata/**"]),
     embed = [":go_default_library"],
     deps = [
-        "//pkg/crypto:go_default_library",
         "@com_github_stretchr_testify//assert:go_default_library",
         "@org_golang_x_crypto//bcrypt:go_default_library",
     ],