Move app initialization code to a weaver.Main method. (#359)

* Move app initialization code to a weaver.Main method.

Previously, application startup code was wrapped up in a function
passed to weaver.Run. However this made it hard to test startup
code. We now have a `Main` method in `weaver.Main` and place the
startup code in the implementation of this method.

Refactored startup the internal API used by weavertest to separate out
starting the application from running its startup code.

* Disallow "weaver.Ref[weaver.Main]" component fields.

* Drop TODO and add comment for Main

component.go

@@ -15,6 +15,7 @@
 package weaver
 
 import (
+	"context"
 	"crypto/tls"
 	"net"
 	"sync"
@@ -116,8 +117,13 @@ type component struct {
 var _ Instance = &componentImpl{}
 
 // Main is interface implemented by an application's main component.
-// This component is created automatically by `weaver.Run`.
-type Main interface{}
+// This component is instantiated and its Main() method called by
+// `weaver.Run`.
+type Main interface {
+	// Main contains the application main. It typically loops
+	// forever, e.g., inside http.Serve.
+	Main(context.Context) error
+}
 
 // Implements[T] is a type that can be embedded inside a component implementation
 // struct to indicate that the struct implements a component of type T. E.g.,
@@ -174,17 +180,15 @@ func (r Ref[T]) isRef() {}
 // A Listener implements the net.Listener interface, so you can use a Listener
 // wherever you use a net.Listener. For example,
 //
-//	weaver.Run(..., func(..., s *server) {
-//	    lis, err := s.Listener("hello", weaver.ListenerOptions{})
-//	    if err != nil {
-//	        log.Fatal(err)
-//	    }
-//	    s.Logger().Info("Listener available at %v", lis)
-//	    http.HandleFunc("/a", func(http.ResponseWriter, *http.Request) {...})
-//	    http.HandleFunc("/b", func(http.ResponseWriter, *http.Request) {...})
-//	    http.HandleFunc("/c", func(http.ResponseWriter, *http.Request) {...})
-//	    http.Serve(lis, nil)
-//	})
+//	lis, err := s.Listener("hello", weaver.ListenerOptions{})
+//	if err != nil {
+//	    log.Fatal(err)
+//	}
+//	s.Logger().Info("Listener available at %v", lis)
+//	http.HandleFunc("/a", func(http.ResponseWriter, *http.Request) {...})
+//	http.HandleFunc("/b", func(http.ResponseWriter, *http.Request) {...})
+//	http.HandleFunc("/c", func(http.ResponseWriter, *http.Request) {...})
+//	http.Serve(lis, nil)
 type Listener struct {
 	net.Listener        // underlying listener
 	proxyAddr    string // address of proxy that forwards to the listener

examples/chat/main.go

@@ -25,5 +25,5 @@ import (
 
 func main() {
 	flag.Parse()
-	weaver.Run(context.Background(), serve)
+	weaver.Run(context.Background())
 }

examples/chat/server.go

@@ -45,7 +45,7 @@ type server struct {
 	cache      weaver.Ref[LocalCache]
 }
 
-func serve(ctx context.Context, s *server) error {
+func (s *server) Main(ctx context.Context) error {
 	s.httpServer.Handler = instrument(s.label, s)
 	lis, err := s.Listener("chat", weaver.ListenerOptions{LocalAddress: *flagAddress})
 	if err != nil {

examples/collatz/main.go

@@ -59,7 +59,7 @@ var localAddr = flag.String("local_addr", "localhost:9000", "Collatz server loca
 
 func main() {
 	flag.Parse()
-	if err := weaver.Run(context.Background(), serve); err != nil {
+	if err := weaver.Run(context.Background()); err != nil {
 		log.Fatal(err)
 	}
 }

examples/collatz/server.go

@@ -32,7 +32,7 @@ type server struct {
 	even weaver.Ref[Even]
 }
 
-func serve(ctx context.Context, s *server) error {
+func (s *server) Main(ctx context.Context) error {
 	s.mux.Handle("/", weaver.InstrumentHandlerFunc("collatz", s.handle))
 	s.mux.HandleFunc(weaver.HealthzURL, weaver.HealthzHandler)
 	lis, err := s.Listener("collatz", weaver.ListenerOptions{LocalAddress: *localAddr})

examples/factors/main.go

@@ -31,7 +31,7 @@ var localAddr = flag.String("local_addr", "localhost:9000", "Local address")
 func main() {
 	flag.Parse()
 	ctx := context.Background()
-	if err := weaver.Run(ctx, serve); err != nil {
+	if err := weaver.Run(ctx); err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}

examples/factors/server.go

@@ -28,7 +28,7 @@ type server struct {
 	factorer weaver.Ref[Factorer]
 }
 
-func serve(ctx context.Context, s *server) error {
+func (s *server) Main(ctx context.Context) error {
 	http.Handle("/", weaver.InstrumentHandlerFunc("/", s.handleFactors))
 
 	lis, err := s.Listener("factors", weaver.ListenerOptions{LocalAddress: *localAddr})