Creating empty Channels shell (#18611)

* Creating empty Channels shell

This is the first step in moving to a product-based
isolated architecture.

For now, Channels is empty and does not contain anything.

Next step is to change App to contain Channels instead
of Server. Some of the initialization code in NewServer
would need to be moved inside NewChannels.

This would complete the full pass-through mode of accessing
everything.

The last step would be to gradually move Channels related
fields from Server into Channels, keeping Server to be
just the global level struct.

```release-note
NONE
```

* fix vet failure

```release-note
NONE
```

app/app.go

@@ -20,6 +20,9 @@ import (
 	"github.com/mattermost/mattermost-server/v6/utils"
 )
 
+// App is a pure functional component that does not have any fields, except Server.
+// It is a request-scoped struct constructed every time a request hits the server,
+// and its only purpose is to provide business logic to Server via its methods.
 type App struct {
 	srv *Server
 }

app/channels.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+// Channels contains all channels related state.
+type Channels struct {
+	s *Server
+}
+
+func init() {
+	RegisterProduct("channels", func(s *Server) (Product, error) {
+		return NewChannels(s)
+	})
+}
+
+func NewChannels(s *Server) (*Channels, error) {
+	return &Channels{
+		s: s,
+	}, nil
+}
+
+func (c *Channels) Start() error {
+	return nil
+}
+
+func (c *Channels) Stop() error {
+	return nil
+}

app/product.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+type Product interface {
+	Start() error
+	Stop() error
+}
+
+var products = make(map[string]func(*Server) (Product, error))
+
+func RegisterProduct(name string, f func(*Server) (Product, error)) {
+	products[name] = f
+}

app/server.go

@@ -209,6 +209,8 @@ type Server struct {
 
 	dndTaskMut sync.Mutex
 	dndTask    *model.ScheduledTask
+
+	products map[string]Product
 }
 
 func NewServer(options ...Option) (*Server, error) {
@@ -222,6 +224,7 @@ func NewServer(options ...Option) (*Server, error) {
 		licenseListeners:    map[string]func(*model.License, *model.License){},
 		hashSeed:            maphash.MakeSeed(),
 		uploadLockMap:       map[string]bool{},
+		products:            make(map[string]Product),
 	}
 
 	for _, option := range options {
@@ -691,6 +694,17 @@ func NewServer(options ...Option) (*Server, error) {
 			s.ShutDownPlugins()
 		}
 	})
+
+	// Initialize products
+	for name, initializer := range products {
+		prod, err := initializer(s)
+		if err != nil {
+			return nil, errors.Wrapf(err, "error initializing product: %s", name)
+		}
+
+		s.products[name] = prod
+	}
+
 	s.AddConfigListener(func(oldCfg, newCfg *model.Config) {
 		if !oldCfg.FeatureFlags.TimedDND && newCfg.FeatureFlags.TimedDND {
 			runDNDStatusExpireJob(app)
@@ -985,6 +999,15 @@ func (s *Server) Shutdown() {
 
 	defer sentry.Flush(2 * time.Second)
 
+	// Stop products.
+	// This needs to happen before because products are dependent
+	// on parent services.
+	for name, product := range s.products {
+		if err := product.Stop(); err != nil {
+			mlog.Warn("Unable to cleanly stop product", mlog.String("name", name), mlog.Err(err))
+		}
+	}
+
 	s.HubStop()
 	s.ShutDownPlugins()
 	s.RemoveLicenseListener(s.licenseListenerId)
@@ -1380,6 +1403,14 @@ func (s *Server) Start() error {
 		mlog.Error("Error starting inter-cluster services", mlog.Err(err))
 	}
 
+	// Start products.
+	// This needs to happen after the server has started.
+	for name, product := range s.products {
+		if err := product.Start(); err != nil {
+			return errors.Wrapf(err, "Unable to start %s", name)
+		}
+	}
+
 	return nil
 }