Removed Main; changed weaver.Run to take lambda.

This PR reverts #359. In particular, it removes the main component's
Main method and changes `weaver.Run` to take a lambda.

We made this change for the following reason. The Main method is run by
`weaver.Run`, but not by `weavertest.Runner.Test`. We felt this was
confusing. With the lambda API, it is much clearer what is run. This
change also unifies the `weaver.Run` and `weavertest.Runner.Test` API.
Both take a lambda with component arguments.

Right now, the signature of `weaver.Run` is a bit gnarly. I think I can
simplify it a bit in a future PR to remove PointerToMain at least.

component.go

@@ -15,7 +15,6 @@
 package weaver
 
 import (
-	"context"
 	"crypto/tls"
 	"net"
 	"sync"
@@ -101,13 +100,7 @@ type component struct {
 var _ Instance = &componentImpl{}
 
 // Main is interface implemented by an application's main component.
-// 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
-}
+type Main interface{}
 
 // 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.,

examples/chat/main.go

@@ -26,7 +26,7 @@ import (
 
 func main() {
 	flag.Parse()
-	if err := weaver.Run(context.Background()); err != nil {
+	if err := weaver.Run(context.Background(), serve); err != nil {
 		log.Fatal(err)
 	}
 }

examples/chat/server.go

@@ -43,7 +43,7 @@ type server struct {
 	chat       weaver.Listener
 }
 
-func (s *server) Main(ctx context.Context) error {
+func serve(ctx context.Context, s *server) error {
 	s.httpServer.Handler = instrument(s.label, s)
 	s.Logger().Debug("Chat service available", "address", s.chat)
 	return s.httpServer.Serve(s.chat)

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()); err != nil {
+	if err := weaver.Run(context.Background(), serve); err != nil {
 		log.Fatal(err)
 	}
 }

examples/collatz/server.go

@@ -33,7 +33,7 @@ type server struct {
 	lis  weaver.Listener `weaver:"collatz"`
 }
 
-func (s *server) Main(ctx context.Context) error {
+func serve(ctx context.Context, s *server) error {
 	s.mux.Handle("/", weaver.InstrumentHandlerFunc("collatz", s.handle))
 	s.mux.HandleFunc(weaver.HealthzURL, weaver.HealthzHandler)
 	s.Logger().Debug("Collatz service available", "address", s.lis)

examples/factors/main.go

@@ -28,7 +28,7 @@ import (
 func main() {
 	flag.Parse()
 	ctx := context.Background()
-	if err := weaver.Run(ctx); err != nil {
+	if err := weaver.Run(ctx, serve); err != nil {
 		log.Fatal(err)
 	}
 }

examples/factors/server.go

@@ -29,7 +29,7 @@ type server struct {
 	lis      weaver.Listener `weaver:"factors"`
 }
 
-func (s *server) Main(ctx context.Context) error {
+func serve(ctx context.Context, s *server) error {
 	http.Handle("/", weaver.InstrumentHandlerFunc("/", s.handleFactors))
 	s.Logger().Info("factors server running", "addr", s.lis)
 	return http.Serve(s.lis, nil)