server: a port that cannot be bound stops startup (closes #266)

The start hook now binds the listen port itself and serves on it in a
goroutine. A bind failure, such as a port already in use or a
privileged port without root, is returned to fx, which stops startup
and exits 1 with an error like "starting http server: listen tcp
:8080: bind: address already in use". Before, the bind happened inside
the goroutine and its error was only logged while the watcher kept
running without the dashboard, API or health check.

The http.Server is now built before that goroutine starts, so Shutdown
no longer reads a field another goroutine may still be writing.

TestPortInUseStopsStartup holds a port and checks that starting the
server on it fails with an error naming it.

Model: opus-5-5

internal/server/server_test.go

@@ -1,6 +1,8 @@
 package server_test
 
 import (
+	"net"
+	"strings"
 	"testing"
 
 	"github.com/spf13/viper"
@@ -58,15 +60,13 @@ func buildServer(t *testing.T) *server.Server {
 }
 
 // TestRunWiresSocketTimeouts pins that the http.Server the running
-// server actually serves — the one Run builds and hands to
-// ListenAndServe — carries every socket-level timeout, plus the two
+// server actually serves — the one Run builds and serves on the port
+// it binds — carries every socket-level timeout, plus the two
 // relationships the values must satisfy.
 //
-// Run is driven to completion with an unbindable port: it builds and
-// stores s.httpServer, then ListenAndServe fails at once and Run
-// returns without ever listening. The assertions run in the same
-// goroutine after Run returns, so reading s.httpServer is free of any
-// data race. Nothing here measures elapsed time.
+// Run is given an unbindable port: it builds and stores s.httpServer,
+// then the bind fails and Run returns the error without ever
+// listening. Nothing here measures elapsed time.
 //
 // ReadTimeout must be at least ReadHeaderTimeout. net/http reads the
 // headers under ReadHeaderTimeout, then sets the read deadline for the
@@ -84,7 +84,10 @@ func TestRunWiresSocketTimeouts(t *testing.T) {
 	srv := buildServer(t)
 	server.SetListenPort(srv, -1)
 
-	srv.Run()
+	err := srv.Run(t.Context())
+	if err == nil {
+		t.Fatal("Run bound port -1")
+	}
 
 	hs := server.HTTPServerOf(srv)
 	if hs == nil {
@@ -133,3 +136,38 @@ func TestRunWiresSocketTimeouts(t *testing.T) {
 		)
 	}
 }
+
+func TestPortInUseStopsStartup(t *testing.T) {
+	// Bound on every address, as Run binds, so that Run's bind fails.
+	lc := &net.ListenConfig{}
+
+	taken, err := lc.Listen(t.Context(), "tcp", ":0")
+	if err != nil {
+		t.Fatalf("binding a port for the test: %v", err)
+	}
+
+	defer func() { _ = taken.Close() }()
+
+	_, port, err := net.SplitHostPort(taken.Addr().String())
+	if err != nil {
+		t.Fatalf("reading the bound port: %v", err)
+	}
+
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_DATA_DIR", t.TempDir())
+	t.Setenv("PORT", port)
+
+	app := newServerApp(fx.Invoke(func(*server.Server) {}))
+
+	err = app.Start(t.Context())
+	if err == nil {
+		_ = app.Stop(t.Context())
+
+		t.Fatal("startup succeeded on a port that is already bound")
+	}
+
+	if !strings.Contains(err.Error(), ":"+port) {
+		t.Errorf("startup error does not name port %s: %v", port, err)
+	}
+}